Flux Blog

What is a Schematic Diagram?

A schematic diagram, or schematic drawing, is a fundamental two-dimensional circuit representation showing the functionality and connectivity between different electrical components. It's composed of electronic symbols, wiring diagrams, designators, net names, and netlist. A PCB designer needs to get familiarized with the schematic symbols that represent the components on a schematic diagram. These symbols serve as a universal language understood by engineers, technicians, and electricians across different industries. They provide a concise and standardized way of communicating circuit designs.

How schematic diagram is different from a schematic block diagram?

A schematic block diagram also referred to as a block diagram or a functional diagram, is a simplified representation of a system or a process using blocks to represent its various components or stages. It provides an overview of the system's structure and the interconnections between its major functional units or subsystems.

In a schematic block diagram, each block represents a specific component, module, or function within the system. The blocks are typically labeled with descriptive text to indicate their purpose or functionality. The interconnections between the blocks are represented by lines or arrows, indicating the flow of signals, information, or energy between the different components or stages.

Schematic Component Symbols

Schematic symbols are typically designed to visually represent the physical characteristics, electrical properties, and functionality of components. They are usually simple, abstract, and intuitive, allowing quick recognition and identification.

For example, a resistor in a circuit diagram is represented by a zigzag line, which indicates its function of restricting current flow. A capacitor is depicted by two parallel lines separated by a gap, representing its ability to store electrical energy. Symbols exist for diodes, transistors, switches, connectors, IC (integrated circuits), and many other components used in electrical circuits and electronic circuits.

Reference Designators

Each component in a schematic diagram is assigned a component designator or reference designator (REFDES) to indicate its specific function within the circuit. It is a letter or combination of letters used to identify individual components within a circuit diagram uniquely. The designators serve as labels or identifiers for the components and are typically placed next to the corresponding symbols in the circuit diagram. It is important to follow a consistent and well-defined designator system within a circuit diagram to ensure clear communication and easy referencing of components.

Reference designators play a crucial role in the connection to the bill of materials (BOM). The BOM contains the part number for each component in your PCBA design and specifies where each part should be installed, based on its reference designator (REFDES).

Here are industry-standard formats for reference designators including a letter, indicating the type of component, followed by a number. For example R1, R2, R3.. etc

Are there any Standards for Schematic Symbols?

IEC 60617 and ANSI standard Y32 are two widely recognized and used standards for schematic diagram symbols and graphical representations in electrical and electronic circuit diagrams. These standards provide guidelines for the consistent depiction of components, connections, and other elements in circuit diagrams, ensuring clear communication and interoperability. 

1. IEC 60617, also known as the "Graphical Symbols for Diagrams" standard, is published by the International Electrotechnical Commission (IEC). It is based on the older standard, British Standard (BS 3939). This database includes over 1750 schematic symbols. It establishes a comprehensive set of symbols and graphical representations for various electrical, electronic, and related technologies.
2. ANSI standard Y32 is published by the American National Standards Institute (ANSI) and specifically focuses on graphical symbols originally used for aircraft applications. This standard is predominantly followed in the United States.

Both standards cover a wide range of components, including passive elements (resistors, capacitors, inductors), active devices (transistors, diodes), power sources, switches, connectors, logic gates, and more. They are periodically updated to incorporate new symbols, reflect technological advancements, and address industry needs. 

Symbol Attributes and Values

The schematic diagram should provide this additional information to ensure that appropriate components are selected. The resistor should have its resistance value expressed in ohms (Ω). The battery should state its potential difference (voltage) expressed in volts. Other components are described in different terms. For example, capacitors differ by their capacitance value expressed in farads (F), and inductors are differentiated by their inductance value expressed in Henrys (H).

Additional attributions can be given to the symbols like power ratings, values, tolerances, etc. This helps us in identifying the correct components for the circuit. Here are common attributes of a component are:

1. The symbol with its unique shape, pin names and pin numbers
2. Reference designator U1, R1, C1, L1, D1 etc.
3. Manufacturer part number (MPN)
4. Values such as resistance in ohms, capacitance, and inductance
5. Tolerances. For resistors: ±1%, ±5%
6. Component footprints or package case codes (resistors: 0402, 0805; for 8 pin IC: PDIP-8 or SOIC-8)
7. Maximum operating conditions. Like for polarized capacitors, maximum reverse voltage, max power wattage for resistors

Electric currents generate heat as they pass through resistive elements of a circuit. The higher the resistance of a conductor, the more heat will be generated as current passes through it. Therefore, addressing both electric and thermal parameters in board design is essential for long-term functionality. PCB thermal analysis plays a vital role in the design process, as it can predict thermal flaws and provide an opportunity for circuit redesign. Some key PCB design considerations for improved thermal performance include temperature-sensitive components. Components that are especially sensitive to temperature should be placed in the location with the lowest temperature, such as the bottom of the board.

The simplest way to dissipate heat would be through thermal vias to the cooling system (heat sink or heat pipes). The heat sink draws heat away from the PCB to fins that provide a larger surface area for faster heat dissipation.

PCB Thermal Analysis Basics

Thermal Equivalent Circuits

Thermal Equivalent Circuits are an analogy of electrical circuits to provide an estimation of the flow of heat in a design. They work because the underlying equations for the transfer of thermal energy and electrical energy are similar enough. With the analogies, we can calculate the heat transfer within the PCB.

Thermal Equivalent Analogy of Electrical Circuits

Equivalent Equations

I = V1-V2/R => Q = T1-T2/R_t

Where:
I is the current (A)
V is the voltage (V)
R is the electrical resistance
Q is the heat flow (W)
T is the junction temperature (°C)
R_t is the thermal resistance (°C/W)

Similarly, equivalent thermal resistance in series and parallel also follows the same equations for electrical resistance.

Component Thermal Impedance

Thermal impedance measures the sum of thermal resistance and thermal contact resistance of a material. This value can be found in the component datasheet for integrated circuits and ranges from 20 °C/W for low-power amplifiers or ICs, to as high as ~200 °C/W for powerful microprocessors. The operating temperature can be determined by multiplying the component’s power consumption by its thermal impedance.

T = Z*P

Where:
T is the component temperature (°C).
P is the power usage of the component (W)
Z is the thermal impedance (°C/W)

Thermal Management Considerations

• Materials and Temperature Ratings
• Use PCB substrate material with appropriate ratings for your operating environment.
• FR-4, the most common PCB material, is rated to operate up to 130 degrees.
• Consider alternatives like Shengyi S1000-2 or ARLON 85N for higher temperature operations.

Increasing Thermal Conductivity

When your PCB generates significant heat, choosing a substrate with superior thermal conductivity is crucial. Ceramics are an excellent option due to their high thermal conductivity and adjustable mechanical properties, which help manage mechanical stress during thermal cycling. Adding a metal core to the board or increasing copper below components, such as adding a plane layer, can also enhance heat dissipation.

When designing PCBs, the importance of effective thermal management cannot be overstated. High-power components generate significant heat, and without proper dissipation, the performance and longevity of the circuit boards can be compromised. One effective method to manage this heat is by using a heat sink. A heat sink helps to draw heat away from critical components, improving the overall thermal performance of the PCB.

Thermal relief is also design feature used in electronic circuit boards to manage heat dissipation from components that generate significant heat during operation. It helps to prevent overheating and ensures the reliability and longevity of the components.

There are two main configurations for thermal relief:

1. Relief Connect: This option creates an air gap between the thermal pad and the fill material, connecting them with conductors. This configuration allows for some thermal isolation, which can be beneficial in certain applications where you want to limit heat transfer.
2. Direct Connect: This option removes the air gap, allowing the thermal pad and the fill material to be in full contact. This configuration provides better thermal conductivity, which is ideal for components that need to dissipate heat quickly.

PCBs are the backbone of modern electronics, and ensuring they operate within safe thermal limits is crucial. Circuit boards that overheat can lead to component failure and reduced reliability. This is why thermal resistance and thermal impedance are important parameters to consider in PCB design. By carefully selecting materials and employing efficient cooling techniques, such as integrating cooling fans, the thermal performance of PCBs can be significantly enhanced.

Tips for Sensitive and High-Power Components

• Place temperature-sensitive components in cooler areas, away from high-heat producers like frequency transistors and microprocessors.
• Position passive components near the edges and high-powered ones in the center for even temperature distribution.
• Spread high-power components out to prevent hotspots.
• Use metal-plated holes and short leads for better heat transfer. Choose components with suitable temperature ratings and larger cross-sectional areas for efficient heat dissipation.
• Use mounting screws to enhance thermal distribution.
• Incorporate thermal vias with grounded polygon pours beneath heat-generating components.

Fan and Heat Sink Considerations

• Thermal Interface Materials: Utilize thermal pads and pastes to enhance heat transfer from components to heat sinks or enclosure walls.
• Fan Placement: Position fans directly on hot components for efficient cooling or mount larger fans on the enclosure for quieter and more energy-efficient heat removal.
• Heat Sink Design: Customize heat sink designs for maximum convective cooling, ensuring they are compatible with the airflow provided by the fan placement.
• Enclosure Integration: Attach the PCB directly to a metal enclosure with thermal interface material to enhance heat transfer to the surrounding environment.

Other Considerations for Routing Traces

• Trace Design: Keep power traces short and wide to reduce heat generation. Round corners and avoid right angles to minimize inductance and noise.
• Ground and Power Planes: Maximize the size of copper planes to act as heat sinks and connect them to external layers for improved heat transfer.
• Thermal Vias: Use them to dissipate heat from multi-layer boards and internal copper layers.
• Standards Compliance: Follow IPC, UL, and IEC standards for current density and conductor spacing to ensure thermal reliability.

PCB Thermal Simulation

PCB thermal simulation enables designers to predict thermal management issues, ensuring optimal heat dissipation. By simulating different layouts and thermal solutions, designers can avoid costly physical iterations and achieve a thermally efficient design from the outset.

Key advantages of PCB thermal simulation include the ability to

• Identify thermal hotspots, thermal interferences
• Analyze airflow, temperature distribution, heat dissipation and heat release patterns.
• Evaluate the configuration of thermal vias
• Assess the heat sink's performance and different cooling technique efficiency
• Optimize the PCB layout cost-effectively.

Thermally conductive materials, such as certain ceramics, can greatly improve the heat dissipation capabilities of circuit boards. Additionally, incorporating thermal vias and using thermally conductive paste can further aid in managing the heat generated by high-power components. Cooling fans are another essential element in thermal management, ensuring that heat is effectively removed from the PCB and dissipated into the surrounding environment.

PCB design requires careful consideration of thermal management techniques. By utilizing heat sinks, thermally conductive materials, and cooling fans, designers can create circuit boards that operate efficiently and reliably, even under demanding conditions. Proper thermal analysis and simulation are key to achieving optimal performance and preventing overheating issues in PCBs.

Let’s start by exploring the benefits of SMDs, as opposed to traditional through-holes:

SMD Packaging for Passive Components

Surface Mount Device (SMD) packaging for passive components, such as resistors, capacitors, and inductors, involves a variety of package types:

Chip Resistors and Capacitors (Standard Ceramic Packages)

• Packages: The most common SMD package for chip resistors and capacitors is the ceramic chip package. It comes in various standardized sizes, such as 0402, 0603, 0805, and 1206.
• Materials: Typically made of ceramic materials with conductive layers for electrodes and dielectric layers for capacitors.
• Advantages: Compact size, suitable for high-density applications.

Tantalum Capacitors

• Packages: Tantalum capacitors come in SMD packages like the molded tantalum chip (MTC) and molded tantalum leadframe (MTL) packages.
• Materials: Tantalum-based dielectric material with a metal case for encapsulation.
• Advantages: Higher capacitance values in a compact size, suitable for applications requiring stable capacitance over a wide temperature range.

Aluminum Electrolytic Capacitors

• Packages: SMD aluminum electrolytic capacitors are available in various configurations, including cylindrical and chip-type packages.
• Materials: Aluminum-based electrolyte with a metallic case.
• Advantages: Larger capacitance values compared to ceramic capacitors, suitable for applications requiring higher energy storage.

Multilayer Ceramic Capacitors (MLCC)

• Packages: MLCCs are available in various standard sizes, including 0402, 0603, 0805, 1206, and larger.
• Materials: Multilayered ceramic structure with alternating layers of ceramic and conductive material.
• Advantages: High capacitance density, suitable for high-frequency applications and decoupling.

SMD Packaging for Active Components

Challenges with Surface Mount Devices

SMDs come with their own set of challenges during assembly and use:

• Tombstoning: Tombstoning occurs when one end of a passive SMD component is vertically displaced during reflow soldering, resembling a tombstone. This anomaly may arise from uneven heating profiles, inadequacies in solder paste deposition, or irregularities in component placement precision.

• Solder Bridging: Solder bridging involves unintended connections between adjacent pads, leading to short circuits. Root causes include excessive solder paste application, suboptimal stencil design, or inadequate implementation of solder mask features.

• Skew and Misalignment: Skew and misalignment issues materialize when SMD components deviate from precise placement on PCB pads during the pick-and-place process. These discrepancies can result in compromised solder joints and consequential performance degradation.

• Insufficient Solder Joints: Incomplete or inadequate solder joints may occur due to factors such as insufficient solder paste deposition, suboptimal reflow soldering conditions, or inaccuracies in component placement. These deficient joints can yield intermittent electrical connections or outright failures.

RS485 is engineered to overcome the limitations inherent in older serial communication standards, such as RS232 and RS422. RS485's facilitates communication between more than two devices, a feature central to the concept of multidrop networking.

RS485 Electrical Characteristics

Key Characteristics

• Differential Signaling. One of the most noteworthy features of RS485 is its use of differential signaling. This method involves transmitting each bit of data as a difference in voltage across the two wires of atwisted pair. Unlike single-ended signaling, where the signal is measured against a common ground, differential signaling measures the voltage difference between the two wires,significantly mitigating the impact of external noise.
  
  The principle behind differential signaling is relatively straightforward. Any external electrical noise typically induces similar electrical disturbances on both wires of the twisted pair. Since RS485 receivers interpret signals based on the voltage difference between these wires, common-mode noise is effectively canceled out. This characteristic makes RS485 particularly suited for environments with high electromagnetic interference, ensuring reliable data transmission even under challenging conditions.
• Half-Duplex Communication. RS485 operates under a half-duplex communication mode. While the network can handle two-way communication, it cannot transmit and receive data simultaneously. In a half-duplex RS485 system, each device on the network takes turns in sending or receiving data. This mode of operation is a deliberate design choice that caters to the multidrop nature of RS485 networks.
  
  The half-duplex configuration allows for a simpler and more cost-effective network design, especially crucial in industrial applications where long cable runs and multiple devices are common. Although this means that data cannot flow in both directions simultaneously, the high-speed capabilities of RS485 ensure that the time lag in switching between sending and receiving modes is negligible for most practical applications.

Technical Specifications

Physical Layer

1. ‍Twist Pair Wiring - The physical infrastructure of RS485 relies on twisted pair wiring. In a twisted pair cable, two conductors are intertwined in a helical form, which significantly diminishes the effect of electromagnetic radiation and noise. The twist rate, or the number of twists per unit length, plays a role in determining the effectiveness of this noise cancellation.
   
   Twisted pair wiring in RS485 can be shielded or unshielded. Shielded twisted pair (STP) cables come with an additional layer of conducting material that acts as a shield against external noise. Unshielded twisted pair (UTP) cables, while more vulnerable to noise, are often preferred in less demanding environments due to their lower cost and ease of handling.
   
   The cable impedance, typically around 120 Ohms for RS485, requires matching of  the impedance of the cable with the terminating resistors at each end of the network to prevent signal reflections, which can degrade communication quality.‍
2. Voltage Levels and Signal Encoding - RS485 defines specific voltage levels for logical high (1) and logical low (0) states in digital communication as at least 200 millivolts required for a valid signal, with a range typically extending up to ±6 volts. This range allows RS485 to achieve a balance between sufficient signal strength for reliable data detection and low enough voltages to minimize power consumption and reduce the risk of circuitry damage.
   
   Signal encoding in RS485 converts digital data into electrical signals for transmission. The most common encoding techniques include Non-Return-to-Zero (NRZ) and Bi-phase encoding. NRZ is straightforward, representing a logical 1 or 0 by a high or low voltage, respectively. Bi-phase encoding, on the other hand, incorporates transitions within each bit period, thereby aiding in clock recovery and synchronization in the absence of a separate clock line.

Data Link Layer

1. ‍Master-Slave Communication - At the data link layer, RS485 commonly employs a master-slave communication protocol. In this architecture, one device (the master) initiates and controls the communication with one or more slave devices. The master device sends commands or queries to the slaves, and the slaves respond accordingly. This hierarchical structure ensures organized and collision-free communication, especially in systems with multiple devices.‍
2. Multi-Drop Configuration - RS485's ability to support multi-drop: allowing for multiple devices (up to 32, or more with repeaters) to be connected to a single communication bus without requiring additional select lines. Each device on the network has a unique address, and the master device can direct communication to a specific slave or broadcast to all devices. This topology is highly efficient for systems where centralized control and data collection from various points are necessary–i.e. systems with decentralized data sources and/or a variety of sensors and devices.‍
   1. Full-Duplex Capability - While RS485 is inherently a half-duplex standard, it can be extended to support full-duplex communication by using four-wire cabling (as opposed to the standard two-wire configuration). In a full-duplex RS485 system, two twisted pairs are used: one pair for transmitting data and the other for receiving. This allows simultaneous bi-directional communication, effectively doubling the data throughput. Full-duplex RS485 is advantageous in applications where rapid and continuous two-way data exchange is critical.

Comparison with RS232 and RS422

RS232 and RS422 are two other major serial communication protocols. While RS232 is limited in distance and device connectivity, offering point-to-point communication typically within 50 feet, RS485 extends this capability to 4000 feet and allows up to 32 devices on a single bus. RS422, similar to RS485 in terms of distance and voltage levels, differs primarily in its support for multi-drop configurations. RS422 is more suited for point-to-point or multipoint unidirectional communication, lacking the bidirectional, multi-drop capability inherent to RS485. 

Practical Applications: Industrial Use Cases

Profibus and Other Industrial Protocols

RS485 is the physical layer for several industrial protocols. 

• Profibus (Process Field Bus) is a leading standard in industrial automation and capitalizes on RS485's robustness and reliability for field bus communication, facilitating data exchange among automation systems and field devices like sensors and actuators. RS485's ability to support long cable lengths and multiple devices makes it an ideal foundation for Profibus.

• Modbus RTU and DNP3 leverage RS485's physical layer characteristics to ensure secure and efficient communication in automation systems, SCADA (Supervisory Control and Data Acquisition) systems, and other industrial control systems. 

Integration with Analog Devices

While RS485 inherently supports digital communication, it is often used in conjunction with analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) to interface with analog sensors and control devices. This is useful when analog signals from sensors (like temperature, pressure, or flow sensors) need to be digitized for processing and monitoring in digital control systems. 

Considerations for Installation and Maintenance

Proper termination of the RS485 network is essential to prevent signal reflections, which can degrade communication quality. This involves matching the characteristic impedance of the cable with terminating resistors at each end of the network.

Cable selection and layout are also critical. Factors such as the type of twisted pair cable (shielded or unshielded), the environment where the cable is installed (including potential exposure to EMI), and adherence to proper grounding practices are critical for optimal network performance.

Maintenance of RS485 networks involves regular checks for any deterioration in cable integrity, connectors, and terminations. Additionally, Ensuring network reliability involves managing device limits and adhering to cable length specifications.

RS232 Architecture and Components

Devices in RS232 Communication

• Data Terminal Equipment (DTE) - DTE devices, such as computers, terminals, or network equipment, serve as the starting or ending point in RS232 communication. They are configured to transmit and receive data in accordance with the RS232 standard.
• Data Circuit-Terminating Equipment (DCE)
  • Modems and Their Role - In RS232 communication, modems are typical DCE devices. They modulate digital data from a DTE into analog signals for transmission over telephone lines and demodulate incoming analog signals back into digital data.
  • Handshaking Protocols (RTS/CTS, DSR/DTR) - Handshaking protocols like RTS/CTS and DSR/DTR are essential for synchronized communication between DTE and DCE. These protocols help in managing the readiness of devices for communication and the flow of data, thus preventing data loss or overflow.

Serial Ports

Physical Characteristics of Connector Types (DB9, DB25)

The RS232 standard utilizes two primary types of connectors: DB9 and DB25. 

• The DB9 connector, a smaller 9-pin version, is prevalent in current applications. 
• Conversely, the DB25 connector, featuring 25 pins, was more commonly used in earlier RS232 implementations. 

Each connector type serves a specific application–modern, smaller applications or complex setups that need more control, respectively–with different pin configurations for transmitting data, receiving data, and various control signals.

Pinouts

Pinouts in RS232 connectors define the functionality of each pin. These configurations vary between DB9 and DB25 connectors. The standard pinouts include pins designated for Transmit Data (TXD), Receive Data (RXD), and several control signals like Request to Send (RTS) and Data Terminal Ready (DTR).

DB9 Pinout for RS232 Communication:

Data Transmission

1. Transmit Data (TXD) - The TXD pin is responsible for sending data from the DTE to the DCE. This data transmission occurs in a serial format, meaning data bits are sent sequentially over a single channel.
2. Receive Data (RXD) - Conversely, the RXD pin facilitates the reception of data by the DTE from the DCE. Notably, as a bidirectional, half-duplex system, RS232 allows you to send and receive data on the respective lines, but not simultaneously.
3. Control Signals (RTS, DTR) - RS232 also incorporates several control signals to manage the flow of data. The Request to Send (RTS) and Data Terminal Ready (DTR) signals are instrumental in initiating and maintaining the data communication process.

RS232 Communication Protocol

Serial Communication Basics

• ‍Start and Stop Bits. A start bit indicates the beginning of a data packet, followed by the actual data bits. Optionally, a parity bit is included for error checking, and the packet ends with one or more stop bits. This ensures that both the sender and receiver are synchronized in data transmission.‍
• Parity Bits. Parity bits are used in RS232 to detect errors in data transmission. Depending on the configuration, the system can use no parity (no additional bit is added for error-checking), even parity (an extra bit is added to ensure the total number of bits (including the parity bit) is even), or odd parity (an extra bit is added to ensure the total number of bits (including the parity bit) is odd). This bit checks the number of 1's in the data bits and adjusts accordingly to match the selected parity mode, providing a basic form of error checking.‍
• Flow Control
  • Hardware Flow Control:
    • ‍RTS/CTS Handshaking - In RTS/CTS (Request to Send/Clear to Send) handshaking, the RTS signal from the transmitting device alerts the receiver of impending data transmission. Upon readiness, the receiving device responds with the CTS signal, clearing the sender to proceed. This method effectively prevents data overflow by managing the pace of data transmission.‍
    • DTR/DSR Handshaking - Similarly, DTR/DSR (Data Terminal Ready/Data Set Ready) handshaking works on the same principle. DTR is sent by the DTE to indicate its readiness, and DSR is the DCE's response, signifying its preparedness to communicate.
  • Software Flow Control‍
    • XON/XOFF Protocol - The XON/XOFF protocol is a software-based alternative to hardware flow control. It utilizes specific control characters (XON for start and XOFF for stop) to manage data transmission. This method is particularly useful in scenarios where the hardware lacks dedicated pins for flow control, the communication setup is straightforward, or compatibility with legacy systems is essential. 

Applications and Relevance of RS232 Today

Despite the advent of more advanced communication protocols like USB and Ethernet, RS232 remains used due to its simplicity and wide compatibility. It's commonly used in industrial and scientific instruments, networking equipment, and legacy computer systems.

Troubleshooting Common RS232 Issues

Common issues in RS232 communication include misconfigured baud rates, incorrect parity settings, and improper cable connections. Troubleshooting these problems requires a systematic approach, often involving checking cable connections, verifying device configurations, and using diagnostic tools like serial port monitors.

Voltage drop in electrical circuits can lead to various issues, affecting the performance and lifespan of electrical equipment:

• Inductive loads like motors and ballasts may overheat if they operate below their voltage rating, leading to shorter equipment life and increased costs. 
• Sensitive electronic devices, such as computers and printers, can experience data loss or even failure if subjected to under-voltage conditions.
• Resistive loads, such as heaters and incandescent lighting, will not provide their expected power output when operating under voltages.

Factors Affecting Voltage Drop

1. Conductor Material: Different materials have different resistivities, which affects the voltage drop. Copper and aluminum are common materials used in electrical conductors, with copper having a lower resistivity than aluminum.
2. Wire Size: The diameter of the wire (or wire gauge) affects its resistance. Larger wire sizes have less resistance and, therefore, less voltage drop.
3. Length of the Wire: Longer wires have more resistance, leading to a greater voltage drop.
4. Current: The amount of current flowing through the wire also affects the voltage drop. Higher current leads to a greater voltage drop.

National Electrical Code (NEC) Guidelines on Voltage Drop

Here's a table summarizing the NEC guidelines on voltage drop:

Using a Voltage Drop Calculator

Voltage drop calculators typically require you to input the following parameters:

• Type of circuit (single-phase, three-phase, or DC). Single-phase AC is used in homes, three-phase AC is used in industries for efficiency, and DC is typically used in electronics and batteries.
• Conductor material (e.g., copper, aluminum)
• Size of the conductor (e.g., AWG, kcmil)
• Length of the conductor
• Current or load in amperes

The calculator then computes the voltage drop based on the inputs and provides the result in volts or as a percentage of the initial voltage. Some calculators also provide the minimum conductor size required to limit the voltage drop to a specified level.

Calculating Voltage Drop

Ohm’s Law Method – Single-Phase Only

Voltage drop of the circuit conductors can be determined by multiplying the current of the circuit by the total resistance of the circuit conductors: The Ohm’s law (V = I*R) method only applies for DC current and cannot be used for three-phase circuits.

Transforming the formula, we have:

V = 2*R*I*L

Where: 

• V ​is the voltage drop in volts (V)
• L is one way length of circuit (source to load) in thousands of feet (Kft.)
• I is the current in amperes
• R is conductive resistance in ohms/ 1000 ft.

Three-phased system

Generally, with the cable information (length, size, material, operating temperature, and cover type), we can find its resistance and inductance from the reference tables. Three-phase AC power, with three sine waves offset by 120 degrees, is used in industrial and commercial settings for more efficient power distribution and to run heavy machinery.

V=3*I*L*(R*cosθ+X*sinθ)

For balanced three-phase circuits, a simplified calculation  is:

V =3*I*Z*L/1000

Where:

• V is the voltage drop in volts 
• I is the current in amperes
• R is conductive resistance in ohms/ 1000 ft. obtained from tables.
• X is conductor inductive reactance in ohms/1000 ft. obtained from tables.
• L is one way length of circuit (source to load) in thousands of feet (Kft.)
• Z is  complex impedance in ohms/ 1000 ft. obtain from Tables.
• Cosθ = Power Factor. Power factor measures the efficiency of power usage, defined as the cosine of the phase angle between voltage and current in an AC circuit.

Conclusion

The significance of calculating voltage drop extends beyond basic electrical design; it's a key factor in the overall performance and safety of electrical systems. Excessive voltage drop is a common culprit behind inefficient system performance, equipment malfunctions, and even safety hazards, which can lead to costly repairs and downtime. Furthermore, meeting the stipulations of electrical codes and standards, which dictate maximum allowable voltage drop levels, is non-negotiable for electrical professionals. Ensuring devices operate within these guidelines means that voltage drop calculations are not just a matter of efficiency but of compliance and safety as well.