Calculate the correct wire gauge for any solar circuit. Checks ampacity AND voltage drop per NEC – used by thousands of solar professionals.
Proper wire sizing is critical for safety, code compliance, and system performance. Undersized wires create fire hazards and waste energy. Oversized wires increase material costs unnecessarily. This calculator finds the optimal gauge that satisfies both NEC ampacity requirements and voltage drop limits.
Calculates wire gauge by both ampacity (NEC 310.16) and voltage drop, recommending the larger size to ensure full compliance.
Accounts for elevated conduit temperatures on rooftops per NEC 310.15(B)(3)(c), which can significantly reduce wire ampacity.
Sizes EGC per NEC 250.122 table based on overcurrent protection device rating for complete circuit specification.
Use this calculator whenever you're designing or installing a solar PV system and need to determine the correct conductor gauge. Specifically, you'll need it when:
Electrical inspectors require NEC-compliant wire sizing documentation. This calculator provides the exact ampacity and voltage drop figures you need on your plan set.
Whether residential roof-mount or commercial ground-mount, every circuit from panels to inverter to panel board needs properly sized conductors.
Adding panels or replacing an inverter with a higher-capacity unit? Re-verify that existing wiring can handle the new current levels.
Choose the type of solar circuit: PV Source, PV Output, Inverter Output, or Battery circuit. Each has different NEC sizing rules.
Input your system voltage, current (Isc or Imax), and one-way wire run length in feet.
Select conductor material (copper or aluminum) and insulation temperature rating (60, 75, or 90 degrees C).
Enter ambient temperature and number of current-carrying conductors in conduit for accurate derating.
The calculator shows both ampacity-based and voltage-drop-based sizes, recommending the larger gauge to satisfy both requirements.
The final AWG gauge that satisfies both ampacity and voltage drop requirements. This is the gauge you should specify for your installation.
Minimum gauge to safely carry the derated current per NEC 310.16. Includes temperature correction and conduit fill factors.
Minimum gauge to keep voltage drop below your target (typically 2% for DC circuits, 3% for AC circuits).
EGC size per NEC 250.122 based on the overcurrent protection device rating, ensuring a safe fault-current return path.
Copper conductor ampacity at 75°C (THWN-2) — most commonly used values for solar installations. Temperature derating and conduit fill factors must be applied separately.
| AWG Size | Ampacity (75°C) | Typical Solar Use | Max Isc (with 1.56×) |
|---|---|---|---|
| 14 AWG | 20A | Small string runs | 12.8A |
| 12 AWG | 25A | Residential PV source | 16.0A |
| 10 AWG | 35A | Standard residential PV | 22.4A |
| 8 AWG | 50A | PV output / inverter feed | 32.1A |
| 6 AWG | 65A | Larger inverter circuits | 41.7A |
| 4 AWG | 85A | Commercial string runs | 54.5A |
| 2 AWG | 115A | Large commercial circuits | 73.7A |
| 1/0 AWG | 150A | Main PV feeder | 96.2A |
| 2/0 AWG | 175A | Large feeder runs | 112.2A |
| 4/0 AWG | 230A | Service entrance / large commercial | 147.4A |
The most common mistake. NEC 690.8 requires PV source circuit conductors to be sized at 156% of Isc — not just 125%. This is 1.25 × 1.25 = 1.5625, rounded to 1.56.
Conduit on a rooftop can reach 70°C+ in summer. NEC 310.15(B)(3)(c) adds 33°C to ambient for rooftop conduit exposed to sunlight. This can reduce ampacity by 40% or more.
A wire may pass the ampacity check but still have excessive voltage drop on long runs. Always check both — this calculator does it automatically and recommends the larger gauge.
THWN-2 (90°C) has higher ampacity than THHN (75°C). But if your terminals are only rated for 75°C, you must use the 75°C column regardless of insulation type.
It depends on current (Isc), voltage, wire run length, and conduit temperature. This calculator applies NEC 690.8 multipliers and checks both ampacity and voltage drop to give you the correct AWG size.
NEC 690.8 requires solar PV source circuits to be sized at 125% of Isc for continuous duty, then another 125% for the overcurrent device — totaling 1.56 × Isc.
Copper has better conductivity and smaller gauge sizes. Aluminum is lighter and cheaper but requires larger gauges. Most residential solar uses copper; commercial projects often use aluminum for cost savings on long runs.
NEC recommends ≤2% for DC circuits (panels to inverter) and ≤3% for AC circuits (inverter to panel). Combined total should stay under 5%. Lower voltage drop means more energy reaches the grid.
Higher ambient temperatures reduce wire ampacity. Conduit on rooftops can exceed 70°C in summer. NEC 310.15(B)(3)(c) requires additional derating for conduits exposed to sunlight on rooftops.
The EGC provides a fault-current return path for safety. NEC 250.122 sizes it based on the overcurrent protection device rating — not the circuit current.
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