DC-DC charger wire size
A DC-DC charger moves current from the starter battery and alternator to a house battery. The input cable usually carries a little more current than the charger's rated output, because of conversion losses, so check the input current in its manual and size for that. Charging is a critical load: use 3%.
Use 4 AWG (21.2 mm²) copper or larger.
Voltage drop sets the size. 6 AWG would drop 3.95%, over your 3% limit. Ampacity alone would allow 8 AWG.
Check: 4 AWG (21.2 mm²)
| Load current | 40 A |
| Wire in the circuit (out and back) | 2 x 15 ft = 30 ft |
| Resistance of 4 AWG | 0.2485 ohm per 1000 ft; 0.00745 ohm for this circuit |
| Voltage drop | 0.298 V = 2.48% within 3% |
| Voltage at the load | 11.7 V |
| Power lost in the wire | 11.9 W |
| Ampacity of 4 AWG | 85 A (NEC 310.16, 75 C) ok |
Vd = I x 2L x R / 1000 = 40 x 2 x 15 x 0.2485 / 1000 = 0.298 V, and 0.298 / 12 = 2.48%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 7.662 | 63.85 | 4.34 | 10* | drop and current |
| 16 | 1.31 | 4.819 | 40.16 | 7.18 | 13* | drop and current |
| 14 | 2.08 | 3.03 | 25.25 | 8.97 | 20 | drop and current |
| 12 | 3.31 | 1.906 | 15.88 | 10.09 | 25 | drop and current |
| 10 | 5.26 | 1.199 | 9.99 | 10.8 | 35 | drop and current |
| 8 | 8.37 | 0.754 | 6.28 | 11.25 | 50 | too much drop |
| 6 | 13.3 | 0.474 | 3.95 | 11.53 | 65 | too much drop |
| 4 | 21.2 | 0.298 | 2.48 | 11.7 | 85 | smallest that passes |
| 3 | 26.7 | 0.236 | 1.97 | 11.76 | 100 | passes |
| 2 | 33.6 | 0.188 | 1.56 | 11.81 | 115 | passes |
| 1 | 42.4 | 0.149 | 1.24 | 11.85 | 130 | passes |
| 1/0 | 53.5 | 0.118 | 0.98 | 11.88 | 150 | passes |
| 2/0 | 67.4 | 0.094 | 0.78 | 11.91 | 175 | passes |
| 3/0 | 85 | 0.074 | 0.62 | 11.93 | 200 | passes |
| 4/0 | 107 | 0.059 | 0.49 | 11.94 | 230 | passes |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
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DC-DC charger cable size by length at 12V, 3% drop
| One-way length | 20 A | 30 A | 40 A | 50 A | 60 A |
|---|---|---|---|---|---|
| 5 ft (1.5 m) | 12 | 10 | 8 | 8 | 6 |
| 10 ft (3 m) | 8 | 6 | 6 | 4 | 4 |
| 15 ft (4.6 m) | 6 | 6 | 4 | 3 | 3 |
| 20 ft (6.1 m) | 6 | 4 | 3 | 2 | 1 |
| 25 ft (7.6 m) | 4 | 3 | 2 | 1 | 1/0 |
| 30 ft (9.1 m) | 4 | 3 | 1 | 1/0 | 1/0 |
Copper, minimum AWG. * ampacity, not voltage drop, sets this size. "over 4/0" means no single copper conductor up to 4/0 passes: shorten the run, use a higher system voltage or run conductors in parallel. Length is one way; the math uses twice that, out and back.
Worked example
40 A at 12 V, 15 ft one way, 3% drop allowed. The current goes out and comes back,
so the circuit is 2 x 15 = 30 ft of wire, and the allowed drop is 12 x 3 / 100 =
0.36 V. Voltage drop is Vd = I x 2L x R / 1000, with R in ohms per 1000 ft:
- 6 AWG:
40 x 30 x 0.3951 / 1000 = 0.474 V, which is 3.95% of 12 V; over the 3% limit. - 4 AWG:
40 x 30 x 0.2485 / 1000 = 0.298 V, which is 2.48% of 12 V; passes both checks.
Answer: 4 AWG (21.2 mm²), rated 85 A, the smallest size that passes both.
Fusing both ends
A charger run connects two batteries, and either one can feed a short in the cable. A fuse near each battery protects the cable from both sides. Use the fuse sizes the charger's manual gives.
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.
Questions
Should I size for the input or the output side?
Each side separately. The input side runs from the starter battery and carries the input current; the output side runs to the house battery. The two runs often differ in length.