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Solar wire sizing is a sequence of separate checks, not a direct lookup from array size to AWG: identify the circuit and applicable code, determine its maximum current, establish the required conductor ampacity under actual installation conditions, check overcurrent protection, and assess voltage drop as a separate design question. The locally adopted code and the authority having jurisdiction (AHJ) govern the installation; a code-cycle summary or draft proposal is not a substitute.
Which code edition applies to this installation?
Confirm the code edition adopted by the project’s jurisdiction, including local amendments and the AHJ’s interpretation, before using a calculation or table. The material available for this guide spans three different contexts: EC&M’s worked example addresses the 2014 NEC; EC&M also summarizes 2023 NEC provisions; and an NFPA document published in 2023 contains public-input language for the 2025 NEC cycle. Those references are not interchangeable, and NFPA public-input language is not proof of a final requirement or of local adoption.
For a real project, verify the adopted edition and applicable Article 690 provisions, including the text and tables that apply to the circuit. Treat explanations of another edition as background, not as the project’s governing rule.
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Start by identifying the circuit being sized. The relevant current method can differ for a PV source circuit, PV output circuit, DC-to-DC converter circuit, or inverter output circuit. Do not select a wire gauge before making this distinction.
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PV source circuits
In the NFPA public-input document for the 2025 NEC cycle, one draft method for PV source-circuit maximum current uses the sum of the short-circuit current ratings (Isc) of parallel-connected modules multiplied by 125%. The document also displays other listed methods. This is draft-cycle language, not a final rule to apply automatically; check the final code text adopted for the project.
Converter, inverter, and larger systems
The same public-input document identifies converter and inverter circuit currents by their continuous output ratings in the displayed draft language. EC&M’s 2023 NEC summary also distinguishes systems at or above 100 kW from smaller systems. The NFPA draft describes an engineered approach for systems with inverter generating capacity of 100 kW or greater that involves a documented, stamped professional-engineer design and simulated local irradiance, elevation, and orientation. Confirm the final, locally adopted provisions before relying on either description.
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The practical output of this step is the maximum current determined for the particular circuit using the method required by the applicable code edition—not a generic current inferred from the array’s nameplate capacity.
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How do you determine the conductor ampacity?
Ampacity is the current-carrying capacity required of the conductor under the governing rules and conditions of use. It is not itself an AWG size. After establishing the circuit current, apply the ampacity method required by the adopted code and check whether correction or adjustment for the installation changes the usable ampacity.
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EC&M’s 2023 NEC explanation describes PV circuit conductor ampacity as satisfying the larger of the summarized methods in 690.8(B): one based on at least 125% of current determined under 690.8(A), and an alternative that accounts for correction and adjustment. The NFPA 2025-cycle public-input text also describes multiple methods, including a method based on 125% of calculated maximum currents and a method involving adjustment and correction factors. These descriptions refer to different code-cycle materials. Use the final text and tables for the jurisdiction, rather than combining methods from different editions.
Gather the conditions that affect the calculation
Before selecting a conductor, establish the project conditions relevant to its listed ampacity and any applicable adjustments or corrections. Depending on the installation, these include:
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- Conductor material and insulation type or listing, such as whether the specified conductor is PV wire.
- Temperature ratings of the conductor and connected equipment terminals.
- Ambient temperature and the installation environment, including rooftop conditions where applicable.
- Number of current-carrying conductors and their raceway or cable arrangement.
- The installation method and any applicable adjustment or correction factors.
Insulation temperature ratings and terminal ratings do different jobs in the calculation. EC&M’s 2014 NEC example uses a 90°C PV-wire ampacity for correction calculations but checks the 75°C column for conductor sizing under its stated terminal condition. That is a historical example of why ratings must be checked together, not a direction to use those table values in a current project.
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EC&M’s 2016 article includes the following examples under its stated 2014 NEC context. They illustrate a calculation process; neither result is a universal recommendation for present-day installations.
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| Example | Stated inputs and conditions | Reported result |
|---|---|---|
| PV source-circuit example | Module Isc of 8.90 A, with the article’s stated factors and assumptions. | Calculated required ampacity of 13.91 A; the example identifies 14 AWG rated 20 A at 75°C. |
| Corrected-ampacity example | Size 10 RHH/RHW-2/USE-2 or PV wire rated 90°C; two current-carrying conductors; 90°F ambient; temperature-correction factor of 0.96. | Corrected ampacity of 38.40 A. |
The examples do not establish a suitable size for a different circuit, code edition, conductor, terminal rating, or installation. Use the project’s inputs and governing tables to reach its result.
How is overcurrent protection checked?
Conductor ampacity and overcurrent device selection are related, but they are separate checks. EC&M’s 2023 NEC summary of Article 690.9 describes both circumstances where protection may be unnecessary and cases where it is required. Which rule applies depends on the circuit, available sources, equipment listings, conductor ampacity, and adopted code edition.
Do not choose an overcurrent protective device (OCPD) by applying a single multiplier to the conductor size or circuit current. Check the applicable code text and listed equipment instructions for that circuit, then confirm that the conductor and protection requirements work together.
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Voltage drop is a separate engineering check; passing an ampacity check does not establish that voltage drop meets a project’s design criteria. The sources cited here do not establish a voltage-drop equation, a design target, or a method for comparing the DC and AC portions of a PV system. They therefore do not support quoting a universal percentage or presenting one as a universal NEC requirement.
Before calculating or specifying a target, document the engineering reference and project criteria that support the method. At minimum, define whether the circuit is DC or AC, whether route length means one-way distance or total conductor length, the current and system voltage used, the conductor material and temperature-dependent resistance basis, and whether the chosen target is a project design goal or a code requirement. Apply that method to the actual circuit and route; do not substitute an ampacity result for the voltage-drop analysis.
Quick Recap
What is a defensible sizing workflow?
- Confirm the governing rules. Identify the locally adopted NEC edition, amendments, and AHJ requirements for the project.
- Classify the circuit. Record whether it is a PV source, PV output, converter, or inverter output circuit.
- Determine maximum circuit current. Use the method for that circuit in the governing code; do not treat draft public-input language or a different code-cycle summary as adopted text.
- Establish required ampacity. Apply the applicable ampacity method and account for conditions of use, adjustment, and correction as required.
- Select a listed conductor and verify its usable ampacity. Check the material, insulation or listing, installation method, conductor and terminal temperature ratings, and applicable tables.
- Check overcurrent protection separately. Apply the circuit-specific rules and equipment instructions rather than assuming one multiplier settles the choice.
- Complete a separate voltage-drop analysis. Use a documented engineering basis and explicit project criteria for the circuit and route.
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