Battery cable size calculator

At 12 V a 3 percent drop is only 0.36 V, so battery cable is sized by voltage drop long before ampacity matters. Enter the one-way distance; the calculator counts the run out and back.

Result

Voltage drop, 4 AWG: 0.62 V, 5.13 percent
At the load: 11.38 V
Smallest size for this load at or under 3.00 percent: 1 AWG

Shop 1 AWG wire

Drop by size
Size	Drop (V)	Drop (%)	Max load (A), 75 C
18 AWG	15.90	132.50	cord size
16 AWG	9.98	83.17	cord size
14 AWG	6.28	52.33	15
12 AWG	3.96	33.00	20
10 AWG	2.48	20.67	30
8 AWG	1.56	12.97	50
6 AWG	0.98	8.18	65
4 AWG	0.62	5.13	85
3 AWG	0.49	4.08	100
2 AWG	0.39	3.23	115
1 AWG, meets target	0.31	2.57	130
1/0 AWG	0.24	2.03	150
2/0 AWG	0.19	1.61	175
3/0 AWG	0.15	1.28	200
4/0 AWG	0.12	1.01	230
250 kcmil	0.10	0.86	255
300 kcmil	0.09	0.72	285
350 kcmil	0.07	0.61	310
400 kcmil	0.06	0.53	335
500 kcmil	0.05	0.43	380
600 kcmil	0.04	0.36	420
700 kcmil	0.04	0.31	460
750 kcmil	0.03	0.29	475
800 kcmil	0.03	0.27	490
900 kcmil	0.03	0.24	520
1000 kcmil	0.03	0.22	545

Uses stranded conductor resistance at 75 C from NEC Chapter 9, Table 8. Single phase and DC: 2 x I x R x L. Three phase: 1.732 x I x R x L. AC reactance, which adds a little on large conductors in steel conduit, is not included. Max load comes from the ampacity column you pick, with 14, 12 and 10 AWG copper held to 15, 20 and 30 A (12 and 10 AWG aluminum to 15 and 25 A). NM-B and UF-B are held to the 60 C column. A continuous load needs 125 percent, and more than three conductors or high heat lower it.

Use it to size a run, then check the size against your local code or with a licensed electrician.

The minimum size shown also meets the building wire ampacity of NEC Table 310.16. Battery cable to SAE J1127 is often rated higher; check the cable rating and the equipment maker's instructions.