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Start with the circuit and the governing code
Before calculating anything, identify which conductors you are sizing: a PV source circuit, PV output circuit, DC-to-DC converter circuit, or inverter output circuit. The applicable current method depends on the circuit category and the code edition in force for the project.
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Confirm the adopted code edition, local amendments, and AHJ requirements first. The available explanations cover different code cycles: an NFPA public-input document published in 2023 for the 2025 NEC cycle, an EC&M example based on the 2014 NEC, and an EC&M explanation of the 2023 NEC. They are not interchangeable. The NFPA public-input document is draft-cycle material, not final code text or evidence of what a locality has adopted.
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Determine maximum circuit current
Do not choose a wire gauge from a module’s nameplate current alone. Use the current calculation for the circuit type under the adopted code, and check the final code text rather than relying on a summary of a different edition.
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PV source circuits
In the NFPA public-input text for the 2025 NEC cycle, one stated method for PV source circuit current is to add the short-circuit current ratings (Isc) of the parallel-connected modules and multiply the sum by 125%. The document also shows other listed methods. Treat this as draft language, not a universal or final requirement.
Converter and inverter circuits
The same NFPA draft text identifies converter and inverter circuit currents by their continuous output ratings in the shown language. EC&M’s 2023 NEC summary also describes source-circuit current calculations and distinguishes provisions for systems at or above 100 kW. It says the larger-system engineered method is tied to a documented, stamped professional-engineer design using simulated local irradiance, elevation, and orientation. Confirm the adopted code’s exact requirements before applying any of these methods to a project.
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Calculate conductor ampacity for actual conditions
Maximum circuit current is an input to conductor sizing; it is not the conductor’s ampacity requirement by itself, and ampacity is not an AWG answer. Under the 2025-cycle NFPA public-input material, 690.8(B) describes conductor ampacity methods that include one based on 125% of calculated maximum currents and another involving adjustment and correction factors. EC&M’s 2023 NEC summary likewise describes checking the larger of its summarized 690.8(B)(1) and (B)(2) methods. These are summaries of different code-cycle materials, so use the locally adopted text and applicable tables for the actual calculation.
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- Conductor material and insulation type or listing, such as PV wire where the equipment and installation call for it.
- Temperature ratings of both the conductor insulation and the equipment terminals.
- Ambient temperature and whether the conductors are on a rooftop or in another temperature-sensitive setting.
- Number of current-carrying conductors and the raceway or cable configuration.
- Applicable correction and adjustment factors, along with equipment instructions and installation requirements.
A conductor may have one temperature rating for correction calculations but still need to be checked against a lower terminal rating when determining usable ampacity. EC&M’s 2014 NEC example illustrates this distinction by using 90°C PV wire ampacity for correction calculations and then checking the 75°C column because of the stated terminal conditions. The table references and code context are historical; verify current tables and requirements rather than carrying those values into a new design.
What historical worked examples do—and do not—show
EC&M’s 2016 article gives examples under the 2014 NEC. They demonstrate how assumptions and conditions affect a calculation; neither establishes a generally suitable wire size for a modern PV installation.
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| Example | Reported result | Scope |
|---|---|---|
| PV source circuit using module Isc of 8.90 A | 13.91 A calculated required ampacity; the article identifies 14 AWG rated 20 A at 75°C | The article’s specific 2014 NEC example and stated assumptions; not a general recommendation. |
| Size 10 RHH/RHW-2/USE-2 or PV wire, rated 90°C, with two current-carrying conductors and 90°F ambient | A 0.96 temperature-correction factor and 38.40 A corrected ampacity | The article’s specific historical example, not a current universal sizing result. |
These numbers are useful for seeing why a calculation must account for circuit and installation details. They are not substitutes for checking the applicable code edition, conductor listing, terminal ratings, and conditions at the site.
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Conductor ampacity and overcurrent protective device (OCPD) selection are related, but they are distinct design checks. EC&M’s 2023 NEC summary discusses Article 690.9, including circumstances in which protection may be unnecessary and cases in which it is required. The answer depends on the circuit, available sources, equipment listings, conductor ampacity, and adopted code. Do not reduce OCPD selection to a single multiplier without checking the applicable code text and equipment instructions.
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Assess voltage drop as a separate design question
Ampacity compliance does not establish acceptable voltage drop. The cited sources address current, ampacity, temperature correction and adjustment, and overcurrent protection; they do not establish a voltage-drop equation or recommended target. They also do not support treating a particular percentage as a universal NEC requirement.
To calculate voltage drop for a specific solar system, first select a documented engineering method and a project-specific design criterion. Define the inputs the method requires, including:
- Whether the segment is DC or AC and where it sits in the system.
- The route length and whether the method uses one-way distance or total conductor length.
- The current used in the calculation and the system voltage.
- Conductor material and resistance at the relevant operating temperature.
- Whether the selected voltage-drop target is a project design goal, an equipment constraint, or a code requirement—and the source for that classification.
Without a sourced equation and a selected target, a numerical voltage-drop answer would be unsupported. Obtain the applicable engineering reference, equipment requirements, and project criteria before choosing a conductor based on this check. Evaluate DC and AC sections using the method and assumptions appropriate to each; do not infer that one section’s result establishes the other’s acceptability.
Quick Recap
A practical sizing sequence
- Identify the circuit. Record whether it is a PV source, PV output, converter, or inverter output circuit.
- Confirm the governing rules. Verify the locally adopted NEC edition, amendments, and AHJ interpretation.
- Calculate maximum current. Apply the correct method for that circuit in the adopted code; do not substitute draft-cycle language or a different edition’s summary.
- Determine required ampacity. Check the applicable ampacity methods and account for correction and adjustment factors under the adopted requirements.
- Select a compatible conductor. Verify material, insulation or listing, installation method, temperature conditions, current-carrying conductor count, and terminal ratings.
- Verify OCPD requirements. Apply the circuit-specific code provisions and equipment instructions independently of the conductor ampacity check.
- Assess voltage drop. Use a documented method, defined assumptions, and a project-supported target for each relevant DC or AC segment.
- Review the complete design. Confirm that the selected conductor and protection are compatible with the equipment, actual installation, and AHJ requirements.
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