PV Wire Size Guide: 12 vs 10 vs 8 vs 6 AWG for Solar Strings and Home Runs
Oct 06, 2026
PV wire size decides two things on a solar job: whether the conductor is legal for the current it carries, and how much energy the array loses getting to the inverter. This guide walks through both checks for the four copper sizes installers use most, 12, 10, 8 and 6 AWG, with worked numbers you can apply to a real string.
Step 1: Find the circuit current the code makes you design for
NEC 690.8 sets the design current for PV source circuits at 125% of the module short-circuit current (Isc), times the number of strings in parallel. The conductor then has to carry 125% of that value before any temperature or conduit-fill adjustments, or the full value after adjustments, whichever needs the larger wire. In practice that means sizing to roughly 156% of Isc.
Example: a module with Isc of 14 A gives a maximum circuit current of 17.5 A, and a required ampacity of about 21.9 A before adjustments.
Step 2: Check ampacity
Ampacity depends on the insulation temperature rating, the terminal rating and the installation method. UL 4703 PV wire is rated 90 °C, but most inverter and combiner terminals are rated 75 °C, which caps the usable value.
| Size (copper) | NEC 310.16, 75 °C | NEC 310.16, 90 °C | NEC 310.17 free air, 90 °C |
|---|---|---|---|
| 12 AWG | 25 A | 30 A | 40 A |
| 10 AWG | 35 A | 40 A | 55 A |
| 8 AWG | 50 A | 55 A | 80 A |
| 6 AWG | 65 A | 75 A | 105 A |
Rooftop ambient temperatures run well above the 30 °C base in those tables, so apply the correction factors in 310.15 for the actual design temperature, and conduit-fill adjustments where more than three current-carrying conductors share a raceway. Overcurrent limits in 240.4(D) for 12 and 10 AWG also apply. Always confirm against the code edition your AHJ enforces.
Step 3: Check voltage drop
Ampacity tells you the wire won't overheat. Voltage drop tells you how much harvest you lose. Most designers hold DC voltage drop to 1% to 2% from array to inverter. For a two-wire DC circuit:
Voltage drop (V) = 2 × one-way length (ft) × current (A) × resistance (Ω per 1,000 ft) ÷ 1,000
Resistance values below are NEC Chapter 9, Table 8, uncoated stranded copper at 75 °C.
| Size | Ω / 1,000 ft | 50 ft one way | 100 ft one way | 200 ft one way |
|---|---|---|---|---|
| 12 AWG | 1.98 | 2.4 V (0.6%) | 4.8 V (1.2%) | 9.5 V (2.4%) |
| 10 AWG | 1.24 | 1.5 V (0.4%) | 3.0 V (0.7%) | 6.0 V (1.5%) |
| 8 AWG | 0.778 | 0.9 V (0.2%) | 1.9 V (0.5%) | 3.7 V (0.9%) |
| 6 AWG | 0.491 | 0.6 V (0.1%) | 1.2 V (0.3%) | 2.4 V (0.6%) |
Table values assume a 12 A string current on a 400 V string. Scale linearly for your own current and voltage: double the current, double the drop.
Rules of thumb that hold up on most jobs
- 12 AWG: module-to-module jumpers, microinverter and optimizer leads, short runs where ampacity, not drop, is the limit.
- 10 AWG: the default for string home runs to a combiner or string inverter up to about 150 to 200 ft one way on modern high-voltage strings.
- 8 AWG: long home runs, low-voltage strings, and combiner outputs carrying two strings.
- 6 AWG: combiner outputs carrying several strings, long ground-mount trenches, battery and inverter DC leads.
Why the conductor material matters
Every number above assumes solid copper. Copper-clad aluminum and undersized import wire can measure well above Table 8 resistance, which quietly adds voltage drop and heat at the terminals. Our PV wire is US-made, UL 4703 listed, 100% bare copper, so the math on the page matches the wire in the field.
Built to length
Once you have the size and the run length, we build it: 12 AWG MC4 jumpers, 10 AWG MC4 jumpers and home runs, and 8 AWG and 6 AWG leads with crimped copper compression lugs. Browse the full custom cable assemblies line or send a cut list to the engineering desk at 307-206-0534.
This guide is a design aid, not a substitute for the NEC, the module and inverter manufacturer's instructions, or the authority having jurisdiction.
