WIRE GAUGE SELECTION TABLE
Circuit Amperes		Circuit Watts		Wire gauge (for length in feet)
6V	12V	6V	12V	3'	5'	7'	10'	15'	20'	25'
0 to 2.5	0 to 5	15	30	18	18	18	18	18	18	18
3.0	6	18	36	18	18	18	18	18	18	16
3.5	7	21	42	18	18	18	18	18	18	16
4.0	8	24	48	18	18	18	18	18	16	16
5.0	10	30	60	18	18	18	18	16	16	16
5.5	11	33	66	18	18	18	18	16	16	14
6.0	12	36	72	18	18	18	18	16	16	14
7.5	15	45	90	18	18	18	18	14	14	12
9.0	18	54	108	18	18	16	16	14	14	12
10	20	60	120	18	18	16	16	14	12	10
11	22	66	132	18	18	16	16	12	12	10
12	24	72	144	18	18	16	16	12	12	10
15	30	90	180	18	16	16	14	10	10	10
20	40	120	240	18	16	14	12	10	10	8
25	50	150	300	16	14	12	12	10	10	8
50	100	300	600	12	12	10	10	6	6	4
75	150	450	900	10	10	8	8	4	4	2
100	200	600	1200	10	8	8	6	4	4	2

Find the amperes or watts the circuit is expected to carry on the left and the distance the wiring must run at the top - follow the columns until they intersect - for example, a 12 volt circuit which is 15 feet long and carries 10 amperes should use at least 16 gauge wire.