What size wire do I need for a 20 amp circuit?

Use 12 AWG copper or 10 AWG aluminum at the 60°C column, which sets the default under 110.14(C)(1)(a) for equipment rated 100 A or less, and 334.80 and 340.80 hold NM-B and UF-B cable to that column regardless of the terminals. With 75°C terminations, 12 AWG copper or 10 AWG aluminum carries the circuit.

What size ground wire for a 20 amp circuit?

12 AWG copper or 10 AWG aluminum, taken from the 20 A row of NEC Table 250.122.

Can a higher temperature column let me use a smaller conductor on a 20 A circuit?

No. 14 AWG copper reaches 20 A in the 75°C column but NEC 240.4(D) caps its overcurrent protection at 15 A; 14 AWG copper reaches 20 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 15 A; 12 AWG aluminum reaches 20 A in the 75°C column but NEC 240.4(D) caps its overcurrent protection at 15 A; 12 AWG aluminum reaches 20 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 15 A, so the 75°C and 90°C column cannot carry this circuit on a smaller conductor than the 60°C column does.

What size aluminum wire for 20 amps?

10 AWG aluminum at the 60°C column carries 25 A. 240.4(D) caps 12 AWG aluminum at 15 A, so no smaller aluminum conductor works on this circuit.

What Size Wire for 20 Amps? Copper and Aluminum (NEC 2023)

Minimum size by temperature column

Smallest conductor that carries 20 A. NEC Table 310.16 ampacities with the 240.4(D) small-conductor limit applied. The highlighted column is the one 110.14(C) assumes for a 20 A circuit.
Conductor	60°C	75°C	90°C
Copper	12 AWG20 A	12 AWG25 A	12 AWG30 A
Aluminum	10 AWG25 A	10 AWG30 A	10 AWG35 A

Equipment grounding conductor

A circuit protected at 20 A takes a 12 AWG copper or 10 AWG aluminum equipment grounding conductor, from the 20 A row of NEC Table 250.122. Raise the ground with the ungrounded conductors when you increase them for voltage drop or any reason other than 310.15(B) or (C) derating, because 250.122(B) requires the same proportion. The ground is never required to be larger than the circuit conductors.

Longest run at 3% voltage drop

Longest one-way run before voltage drop reaches 3%
Conductor	120 V	240 V
12 AWG copper	45 ft	90 ft
10 AWG aluminum	40 ft	85 ft

NEC 210.19(A)(1) Informational Note 4 recommends keeping branch-circuit drop at or below 3%. These lengths come from the Table 8 resistance at 75°C for a stranded conductor at unity power factor, rounded to the nearest 5 ft. Reactance is not included, so a large conductor in a steel raceway falls short of the figure. A three-phase run reaches about 15% farther than these single-phase figures.

Small-conductor limit under 240.4(D)

NEC 240.4(D) caps overcurrent protection at 15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper, 15 A for 12 AWG aluminum, and 25 A for 10 AWG aluminum. On a 20 A circuit that cap removes a size the ampacity table would otherwise allow.

14 AWG copper carries 20 A at 75°C, but cannot be protected above 15 A.
14 AWG copper carries 25 A at 90°C, but cannot be protected above 15 A.
12 AWG aluminum carries 20 A at 75°C, but cannot be protected above 15 A.
12 AWG aluminum carries 25 A at 90°C, but cannot be protected above 15 A.

The caps apply after ambient correction and conductor adjustment, so derating the load does not raise them.

What a 20 A circuit feeds

Kitchen, bathroom, laundry, and garage receptacle circuits
Dishwashers, disposals, and 120 V appliance circuits

Twenty amps is the rating for most 120 V receptacle circuits in a dwelling. 210.11(C)(1) requires at least two small-appliance branch circuits for the kitchen, 210.11(C)(2) a laundry circuit, and 210.11(C)(3) a bathroom circuit. 12 AWG copper is the minimum, and 240.4(D)(5) stops protection at 20 A, so the 25 A and 30 A values in the 75°C and 90°C columns never apply to a 20 A breaker. A 15 A receptacle is permitted on this circuit where two or more receptacles are installed, under 210.21(B)(3).

The 125% rule for continuous loads puts the practical limit at 16 A, which is why a 20 A circuit charges an electric vehicle at 16 A and no more. Arc-fault protection under 210.12 covers most dwelling branch circuits at this rating, including the kitchen counters these circuits usually serve.

Twelve gauge wire fits the same device boxes as 14 gauge, so the extra cost is copper and little else. Box fill is where it shows up: 314.16 counts each 12 AWG conductor as 2.25 cubic inches against 2.00 for 14 AWG, so a box that was legal with the old layout in 14 AWG can be too small once the same conductors are 12 AWG.

20 A wire size questions

What size wire do I need for a 20 amp circuit?: Use 12 AWG copper or 10 AWG aluminum at the 60°C column, which sets the default under 110.14(C)(1)(a) for equipment rated 100 A or less, and 334.80 and 340.80 hold NM-B and UF-B cable to that column regardless of the terminals. With 75°C terminations, 12 AWG copper or 10 AWG aluminum carries the circuit.
What size ground wire for a 20 amp circuit?: 12 AWG copper or 10 AWG aluminum, taken from the 20 A row of NEC Table 250.122.
Can a higher temperature column let me use a smaller conductor on a 20 A circuit?: No. 14 AWG copper reaches 20 A in the 75°C column but NEC 240.4(D) caps its overcurrent protection at 15 A; 14 AWG copper reaches 20 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 15 A; 12 AWG aluminum reaches 20 A in the 75°C column but NEC 240.4(D) caps its overcurrent protection at 15 A; 12 AWG aluminum reaches 20 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 15 A, so the 75°C and 90°C column cannot carry this circuit on a smaller conductor than the 60°C column does.
What size aluminum wire for 20 amps?: 10 AWG aluminum at the 60°C column carries 25 A. 240.4(D) caps 12 AWG aluminum at 15 A, so no smaller aluminum conductor works on this circuit.