Wire ampacity chart (NEC Table 310.16)

Under NEC Table 310.16, 12 AWG copper carries 20 A at 60 C, 25 A at 75 C and 30 A at 90 C, but NEC 240.4(D) still limits it to a 20 A breaker. Most breakers and lugs are rated 75 C, so the 75 C column is the usual limit over 100 A, and the 60 C column at 100 A or less unless the equipment is marked 75 C (110.14(C)).

NEC Table 310.16, allowable ampacity (A)
Size	Copper 60 C	Copper 75 C	Copper 90 C	Aluminum 60 C	Aluminum 75 C	Aluminum 90 C
14 AWG	15	20	25
12 AWG	20	25	30	15	20	25
10 AWG	30	35	40	25	30	35
8 AWG	40	50	55	35	40	45
6 AWG	55	65	75	40	50	55
4 AWG	70	85	95	55	65	75
3 AWG	85	100	115	65	75	85
2 AWG	95	115	130	75	90	100
1 AWG	110	130	145	85	100	115
1/0 AWG	125	150	170	100	120	135
2/0 AWG	145	175	195	115	135	150
3/0 AWG	165	200	225	130	155	175
4/0 AWG	195	230	260	150	180	205
250 kcmil	215	255	290	170	205	230
300 kcmil	240	285	320	195	230	260
350 kcmil	260	310	350	210	250	280
400 kcmil	280	335	380	225	270	305
500 kcmil	320	380	430	260	310	350
600 kcmil	350	420	475	285	340	385
700 kcmil	385	460	520	315	375	425
750 kcmil	400	475	535	320	385	435
800 kcmil	410	490	555	330	395	445
900 kcmil	435	520	585	355	425	480
1000 kcmil	455	545	615	375	445	500

The table assumes not more than three current-carrying conductors in a raceway, cable or directly buried, at an ambient temperature of 30 C. More conductors or more heat call for the adjustments in 310.15; the ampacity calculator applies them.
NEC 240.4(D) caps overcurrent protection at 15 A for 14 AWG copper, 20 A for 12 AWG copper and 30 A for 10 AWG copper, and at 15 A and 25 A for 12 and 10 AWG aluminum.
NM-B and UF-B cable are held to the 60 C column (334.80, 340.80). Dwelling services and main feeders may use the 83 percent rule of 310.12.