What size wire do I need for a 30 amp circuit?

Use 10 AWG copper or 8 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, 10 AWG copper or 8 AWG aluminum carries the circuit.

What size ground wire for a 30 amp circuit?

10 AWG copper or 8 AWG aluminum, taken from the 60 A row of NEC Table 250.122.

Can a higher temperature column let me use a smaller conductor on a 30 A circuit?

No. 12 AWG copper reaches 30 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 20 A; 10 AWG aluminum reaches 30 A in the 75°C column but NEC 240.4(D) caps its overcurrent protection at 25 A; 10 AWG aluminum reaches 30 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 25 A, so the 90°C and 75°C column cannot carry this circuit on a smaller conductor than the 60°C column does.

What size aluminum wire for 30 amps?

8 AWG aluminum at the 60°C column carries 35 A. 240.4(D) caps 10 AWG aluminum at 25 A, so no smaller aluminum conductor works on this circuit.

What Size Wire for 30 Amps? Copper and Aluminum (NEC 2023)

Minimum size by temperature column

Smallest conductor that carries 30 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 30 A circuit.
Conductor	60°C	75°C	90°C
Copper	10 AWG30 A	10 AWG35 A	10 AWG40 A
Aluminum	8 AWG35 A	8 AWG40 A	8 AWG45 A

Equipment grounding conductor

A circuit protected at 30 A takes a 10 AWG copper or 8 AWG aluminum equipment grounding conductor, from the 60 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
10 AWG copper	45 ft	95 ft
8 AWG aluminum	45 ft	90 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 30 A circuit that cap removes a size the ampacity table would otherwise allow.

12 AWG copper carries 30 A at 90°C, but cannot be protected above 20 A.
10 AWG aluminum carries 30 A at 75°C, but cannot be protected above 25 A.
10 AWG aluminum carries 35 A at 90°C, but cannot be protected above 25 A.

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

What a 30 A circuit feeds

Electric dryers on a 240 V branch circuit
RV 30 A pedestals and small water heaters

Thirty amps covers the 240 V appliances below a range: electric dryers on a 14-30 receptacle, RV pedestals on a TT-30, and small storage water heaters. 10 AWG copper or 8 AWG aluminum reaches the rating at 60°C, and the 75°C column does not buy a smaller conductor because 240.4(D)(7) caps 10 AWG copper at 30 A.

A storage water heater with a capacity of 120 gallons or less is a continuous load under 422.13, so its branch circuit must be rated at 125% of the nameplate current. A 24 A heater therefore needs a 30 A circuit and no more. A dryer is not continuous, so a 24 A nameplate fits this circuit directly.

Dryers from older installations often run on a 10/3 cable with a three-wire cord, which relies on the frame-grounding exception in 250.140 and requires an insulated neutral. A new dryer connects with a four-wire cord on a cable that carries a ground, and the receptacles are not interchangeable: the 14-30R has an L-shaped neutral blade that the older 10-30R does not have.

30 A wire size questions

What size wire do I need for a 30 amp circuit?: Use 10 AWG copper or 8 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, 10 AWG copper or 8 AWG aluminum carries the circuit.
What size ground wire for a 30 amp circuit?: 10 AWG copper or 8 AWG aluminum, taken from the 60 A row of NEC Table 250.122.
Can a higher temperature column let me use a smaller conductor on a 30 A circuit?: No. 12 AWG copper reaches 30 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 20 A; 10 AWG aluminum reaches 30 A in the 75°C column but NEC 240.4(D) caps its overcurrent protection at 25 A; 10 AWG aluminum reaches 30 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 25 A, so the 90°C and 75°C column cannot carry this circuit on a smaller conductor than the 60°C column does.
What size aluminum wire for 30 amps?: 8 AWG aluminum at the 60°C column carries 35 A. 240.4(D) caps 10 AWG aluminum at 25 A, so no smaller aluminum conductor works on this circuit.