Key takeaways
- DC wire size is driven by voltage drop, not just ampacity.
- Circular mils needed = (2 × K × amps × one-way ft) ÷ (volts × drop fraction).
- Use K = 12.9 for copper, 21.2 for aluminum; copper runs a smaller gauge.
- Higher system voltage means lower current — so thinner, cheaper cable over the same run.
How to size DC solar wire
On a low-voltage DC system, the cable that just meets the ampacity table is often far too thin. The real limit is voltage drop: at 12 or 24 V, losing even a fraction of a volt in the wire is a big slice of the whole system, starving your loads and charge controller. So you size the conductor to hold the drop under a target — usually 2–3% — and then confirm it also satisfies ampacity and overcurrent rules.
K is the resistance constant — 12.9 for copper, 21.2 for aluminum — and the factor of 2 accounts for current flowing out and back. Since amps sit on top of the fraction, cutting current by raising voltage is the fastest way to shrink the wire.
Worked example: 30 A, 20 ft, 24 V, 3% drop, copper
Circular mils needed = (2 × 12.9 × 30 × 20) ÷ (24 × 0.03) = 15,480 ÷ 0.72 = 21,500 cmil. The first AWG that meets that is #6 copper at 26,240 cmil. Step up to 48 V and the current halves, dropping the requirement enough to use a thinner cable.
AWG, circular mils, and approximate ampacity
| AWG | Circular mils | ~Ampacity (A) |
|---|---|---|
| #14 | 4,107 | 15 |
| #12 | 6,530 | 20 |
| #10 | 10,380 | 30 |
| #8 | 16,510 | 40 |
| #6 | 26,240 | 55 |
| #4 | 41,740 | 70 |
| #2 | 66,360 | 95 |
| #1/0 | 105,600 | 150 |
| #2/0 | 133,100 | 175 |
| #4/0 | 211,600 | 230 |
Low-voltage DC runs are usually voltage-drop-limited, not ampacity-limited, so the gauge you land on is often a step or two larger than the ampacity column alone would imply.
Match the wire to the rest of the system
Wire size depends on the current your array and loads actually draw, so size those first with the solar array sizing calculator and the battery bank sizing calculator. Then choose a higher bus voltage where you can — 24 or 48 V cuts the amps, the drop, and the copper you have to buy for the same job.
Frequently asked questions
What gauge wire do I need for my solar or battery cables?
Find circular mils with cmil = (2 × K × amps × one-way ft) ÷ (volts × drop fraction), then pick the first AWG that meets it. 30 A, 20 ft, 24 V, 3% ≈ 21,500 cmil → #6 copper.
Why does low-voltage DC need bigger wire than household AC?
Voltage drop drives DC sizing. At 12 or 24 V a small voltage loss is a large percentage of the system, so cables run fatter than ampacity alone suggests.
What voltage drop percentage should I target?
Aim for 2–3% on DC runs. The NEC recommends under ~3% on a branch and ~5% overall. Lower drop means a bigger, costlier cable.
Copper or aluminum wire for solar?
Copper (K ≈ 12.9) runs a smaller gauge than aluminum (K ≈ 21.2) for the same drop. Aluminum is cheaper for long runs but needs upsizing and rated terminations.
Does a higher system voltage let me use thinner wire?
Yes. Higher voltage means lower current, and drop scales with current — so 24 or 48 V needs fewer circular mils and a thinner cable than 12 V.
Does wire size set my fuse or breaker rating?
The overcurrent device protects the wire, so the fuse or breaker is rated at or below the conductor's ampacity — never above it.
Ampacity and AWG values follow NEC (NFPA 70) Table 310.16 and PV Article 690. The circular-mil and voltage-drop relationships shown here are exact arithmetic.
Last reviewed June 2026