DC wire size charts
Every chart here comes from the same math as the calculator: copper resistance at 20 C, the length doubled for the return path, and the NEC 75 C current rating as a floor. Pick the page closest to your job, then change the numbers in its calculator.
Watts are converted: amps = watts / volts.
Source to load. The circuit is 2 x this length (out and back).
Use 8 AWG (8.37 mm²) copper or larger.
Voltage drop sets the size. 10 AWG would drop 3.33%, over your 3% limit. Ampacity alone would allow 14 AWG.
Check: 8 AWG (8.37 mm²)
| Load current | 20 A |
| Wire in the circuit (out and back) | 2 x 20 ft = 40 ft |
| Resistance of 8 AWG | 0.6282 ohm per 1000 ft; 0.02513 ohm for this circuit |
| Voltage drop | 0.503 V = 2.09% within 3% |
| Voltage at the load | 23.5 V |
| Power lost in the wire | 10.1 W |
| Ampacity of 8 AWG | 50 A (NEC 310.16, 75 C) ok |
Vd = I x 2L x R / 1000 = 20 x 2 x 20 x 0.6282 / 1000 = 0.503 V, and 0.503 / 24 = 2.09%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 5.108 | 21.28 | 18.89 | 10* | drop and current |
| 16 | 1.31 | 3.213 | 13.39 | 20.79 | 13* | drop and current |
| 14 | 2.08 | 2.02 | 8.42 | 21.98 | 20 | too much drop |
| 12 | 3.31 | 1.27 | 5.29 | 22.73 | 25 | too much drop |
| 10 | 5.26 | 0.799 | 3.33 | 23.2 | 35 | too much drop |
| 8 | 8.37 | 0.503 | 2.09 | 23.5 | 50 | smallest that passes |
| 6 | 13.3 | 0.316 | 1.32 | 23.68 | 65 | passes |
| 4 | 21.2 | 0.199 | 0.83 | 23.8 | 85 | passes |
| 3 | 26.7 | 0.158 | 0.66 | 23.84 | 100 | passes |
| 2 | 33.6 | 0.125 | 0.52 | 23.87 | 115 | passes |
| 1 | 42.4 | 0.099 | 0.41 | 23.9 | 130 | passes |
| 1/0 | 53.5 | 0.079 | 0.33 | 23.92 | 150 | passes |
| 2/0 | 67.4 | 0.062 | 0.26 | 23.94 | 175 | passes |
| 3/0 | 85 | 0.049 | 0.21 | 23.95 | 200 | passes |
| 4/0 | 107 | 0.039 | 0.16 | 23.96 | 230 | passes |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
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Wire size charts by system voltage
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Reference
Before you buy wire
- Ampacity figures for 14 AWG and larger are from NEC (NFPA 70) Table 310.16, copper, 75 C column, which assumes 30 C ambient and no more than three current-carrying conductors bundled together. Hot spaces, bundles and conduit lower them. 18 and 16 AWG figures are chassis-wiring guidance, not NEC ratings.
- On boats, follow ABYC E-11 for DC wiring. Engine rooms are hot, and conductors there are allowed less current than the same wire elsewhere.
- Use wire rated for the location: heat, oil, water, sunlight and abrasion all matter. Stranded copper is the norm in vehicles and boats, where wire vibrates.
- Fuse every positive run close to the source, sized to protect the wire. NEC 240.4(D) limits the overcurrent device on 14, 12 and 10 AWG to 15, 20 and 30 A.
- This calculator does arithmetic. It does not certify an installation; check your work against the standards and the equipment makers' instructions, or have it checked.