Voltage Drop in PV Circuits: A Professional Guide to NEC-Compliant Calculations

Calculating voltage drop in PV circuits is a critical skill for ensuring a solar installation operates at maximum efficiency and safety. For any journeyman electrician or master electrician working with photovoltaics, correctly performing a wire size computation is essential to prevent significant power loss. Excessive voltage drop in a PV source circuit or PV output circuit means less energy delivered, reduced system performance, and potential equipment damage. This is not merely an academic exercise; it’s a fundamental aspect of professional solar installation guided by the NEC. By using the correct voltage drop formula and proper conductor sizing techniques, you can guarantee that the energy generated by the panels is effectively delivered to the inverter and the grid, protecting the client’s investment and ensuring a reliable, high-performing system. This guide provides the necessary formulas and steps to master these calculations.

Why Voltage Drop in PV Circuits Is a Critical Concern

Voltage drop refers to the reduction in electrical potential (voltage) as electricity flows through a conductor. While no conductor is perfect, this loss becomes a significant issue in solar installations. Every volt lost in the wiring is wasted energy that never reaches the inverter, batteries, or appliances. This translates directly into lost kilowatt-hours and reduced financial returns for the system owner. Over the 25+ year lifespan of a PV system, even a seemingly small percentage of power loss can amount to a substantial loss of generated energy. The National Electrical Code (NEC) recommends keeping voltage drop below 3% for general branch circuits to maintain efficiency, a guideline widely adopted for PV systems.

Beyond efficiency, excessive voltage drop can impact equipment operation. PV inverters are designed to operate within a specific DC input voltage range. If the voltage from the array falls below this threshold due to line losses, the inverter can cease production or fail to start up, particularly during periods of lower irradiance. In extreme cases, undersized conductors can overheat due to increased resistance, posing a potential fire hazard.

Understanding Key Terminology: PV Source vs. PV Output Circuits

The NEC, in Article 690, makes a clear distinction between different parts of a solar installation’s DC side. Understanding these is crucial for accurate calculations.

• PV Source Circuit: This circuit connects a group of modules wired in series (a string) to a common connection point, typically a combiner box. Calculations for these circuits are based on the string’s maximum power current (Imp) and maximum power voltage (Vmp).
• PV Output Circuit: Also known as a “PV array circuit,” this runs from the combiner box, where multiple source circuits are connected in parallel, to the DC input of the inverter. The current in this circuit is the sum of the currents from all the combined strings, making conductor sizing particularly important here.

The distinction matters because the current, voltage, and wire lengths are different for each. A system may involve a series vs parallel circuit configuration to achieve the desired voltage and current levels for the inverter, and each wiring segment must be calculated accordingly.

The Voltage Drop Formula for Electricians

The calculation for voltage drop is a practical application of Ohm’s Law (V = I × R). For PV circuits, a more detailed formula is used to account for conductor length and resistance. The formulas differ slightly for single-phase (common in DC circuits and split-phase AC) and three-phase AC systems.

For DC and Single-Phase AC Circuits:

VD = (2 × L × R × I) / 1000

For Three-Phase AC Circuits:

VD = (1.732 × L × R × I) / 1000

Where:

• VD = Voltage Drop (in volts)
• L = One-way length of the conductor run (in feet)
• R = Conductor Resistance in ohms per 1,000 feet (Ω/kFT). This value is found in NEC Chapter 9, Table 8 for DC circuits.
• I = Current of the circuit in amperes (use Imp for PV source circuits).
• 2 = This factor accounts for the total length of the circuit (to the load and back).
• 1.732 = The square root of 3, used for three-phase calculations.

A Step-by-Step Guide to Wire Size Computation for a PV Circuit

Proper wire size computation ensures that your chosen conductor meets both ampacity requirements and voltage drop targets. Here is a practical, step-by-step process for a DC circuit.

1. Determine Circuit Parameters: Identify the key values for your circuit. You’ll need the maximum power current (Imp), maximum power voltage (Vmp) from the module datasheet, and the one-way distance (L) from the panels to the combiner or inverter.
2. Define the Target Voltage Drop: A common industry best practice is to limit voltage drop to 2% or less for the DC side of the system (both PV source and output circuits combined). To learn more about acceptable limits, you can review details on acceptable voltage drop according to the NEC. Calculate your maximum allowed voltage drop in volts (e.g., 2% of 480V Vmp is 9.6V).
3. Choose a Trial Conductor Size (AWG): Select an initial wire gauge (AWG) that meets the ampacity requirements of the circuit. Remember that NEC 690 requires conductors and overcurrent devices to be sized to at least 125% of the maximum current from the PV source. For more general information on this process, see this article on how to calculate voltage drop.
4. Find Conductor Resistance (R): Using the chosen wire size, look up its resistance value (R) in NEC Chapter 9, Table 8. This table provides DC resistance in ohms per 1,000 feet (Ω/kFT) for both copper and aluminum conductors at a standard temperature. The table is based on a conductor’s circular mil area.
5. Apply the Voltage Drop Formula: Insert your values (L, R, and I) into the formula: VD = (2 × L × R × I) / 1000. This will give you the calculated voltage drop in volts.
6. Compare and Finalize: Compare your calculated voltage drop to the target you set in Step 2. If the calculated drop is higher than your target, you must return to Step 3 and select a larger conductor (a lower AWG number), then recalculate. Repeat until the voltage drop is within your acceptable limit. For an example calculation, check out our guide on how to calculate voltage drop with the NEC.

Factors Influencing Voltage Drop and Conductor Sizing

Several factors beyond basic calculations must be considered for accurate and safe conductor sizing.

• Conductor Material: Copper has lower resistance than aluminum of the same size, making it a more efficient but typically more expensive choice.
• Conductor Size (AWG): This is the most direct way to control voltage drop. A larger wire (lower AWG number) has less resistance and will result in a lower voltage drop.
• Circuit Length: The longer the wire run, the greater the total resistance and the higher the voltage drop. Doubling the length will double the voltage drop if all other factors remain constant.
• Ampacity and Temperature: A wire’s ability to carry current safely (its ampacity) is reduced in high-temperature environments, such as on a hot rooftop. An ampacity chart and NEC tables for temperature correction factors must be used to ensure the chosen wire is safe. This often requires balancing ampacity needs with voltage drop goals, as explored in this detailed voltage drop calculation guide.

The Role of the NEC Code Book in PV Installations

For any professional electrician, the nec code book is the ultimate source of truth. Article 690, “Solar Photovoltaic (PV) Systems,” provides comprehensive rules covering everything from circuit sizing and overcurrent protection to disconnecting means and grounding. Adhering to the latest version of the NEC ensures that an installation is not only efficient but also safe and compliant with legal requirements. Specifically, NEC Chapter 9, Table 8 is indispensable for finding the conductor resistance values needed for accurate voltage drop calculations. Familiarity with these sections is non-negotiable for anyone working in the solar industry.

These calculations are fundamental for any journeyman electrician or master working on solar projects. For those looking to deepen their expertise, our online electrical courses provide comprehensive training on the NEC and practical applications. Maximize system performance. Master voltage drop calculations today.

Primary Sources

All technical standards and calculation methods are based on guidelines from the National Fire Protection Association (NFPA), publishers of the National Electrical Code (NEC). For the most current standards, refer to the official NFPA website.

Frequently Asked Questions (FAQ)

What is an acceptable percentage for voltage drop in PV circuits?

While the NEC does not set a mandatory limit for voltage drop as a safety rule, it recommends a maximum of 3% for branch circuits to ensure efficiency. For PV systems, industry best practice is often more stringent, aiming for a combined 2% or less across the DC circuits (PV source and output circuits).

How does the voltage drop formula differ for single-phase vs. three-phase systems?

The core formula is similar, but the multiplier changes. For DC and single-phase AC circuits, the formula uses a factor of 2 to account for the round-trip distance of the current. For balanced three-phase AC circuits, it uses the square root of 3 (approximately 1.732) because of the phase relationship between the conductors.

Can I use an online voltage drop calculator instead of manual computation?

Yes, a voltage drop calculator can be a quick and useful tool for estimations. However, a professional electrician should understand the manual wire size computation process to verify the results and understand the underlying factors, such as temperature correction and ampacity adjustments, which a basic calculator may not account for.

Why is NEC Chapter 9, Table 8 so important for wire size computation?

NEC Chapter 9, Table 8 provides the standardized DC resistance values for different conductor sizes (AWG) and materials (copper/aluminum). This table is the foundational data source needed to use the voltage drop formula accurately, ensuring your calculations are compliant with industry standards.

Image Alt Text Suggestion: Diagram showing voltage drop in a PV source circuit from solar panel to combiner box.