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

10 AWG copper or 8 AWG aluminum, per NEC Table 310.16 at the 75°C column. The equipment grounding conductor is 10 AWG copper per NEC Table 250.122.

What is the maximum continuous load on a 30 amp breaker?

24 amps. NEC 210.19(A)(1) and 210.20(A) require the circuit to be rated at 125% of a continuous load — one that runs for three hours or more — which works out to 80% of the breaker rating available for continuous use. Non-continuous loads may use the full 30 A.

What size ground wire for a 30 amp breaker?

10 AWG copper or 8 AWG aluminum, from NEC Table 250.122. The table lists maximum overcurrent device ratings, so you round up to the next listed row. If the circuit conductors are upsized for voltage drop, NEC 250.122(B) requires the ground to be upsized proportionally too.

Can I use 10 AWG NM-B cable on a 30 amp breaker?

Yes. NEC 334.80 requires NM-B ampacity to be taken from the 60°C column of Table 310.16, where 10 AWG copper is rated 30 A. That is why NM-B sometimes needs a larger conductor than THHN in conduit for the same breaker.

30 Amp Breaker Wire Size

Copper Wire

10 AWG

Aluminum Wire

8 AWG

Ground (Copper)

10 AWG

Double-pole (or single-pole at 120 V) · 240 V or 120 V · Max continuous load 24 A · NEC Tables 310.16 and 250.122

30 Amp Circuit — Specification Summary

Item	Value
Breaker type	Double-pole (or single-pole at 120 V), 240 V or 120 V
Recommended copper conductor	10 AWG
Recommended aluminum conductor	8 AWG
Minimum per NEC Table 310.16, 75°C	10 AWG copper (35 A)
Minimum NM-B cable, 60°C column	10 AWG copper (30 A)
Equipment grounding conductor	10 AWG copper / 8 AWG aluminum (NEC Table 250.122)
Maximum continuous load (125% rule)	24 A
Maximum non-continuous load	30 A

Recommended conductor sizes follow the 75°C column of NEC Table 310.16, the column that matches the termination rating of virtually all modern breakers and lugs (NEC 110.14(C)).

Common Uses for 30 A Circuits

Electric clothes dryers, small water heaters, RV outlets, window and through-wall air conditioners, small sub-panels

What a 30 Amp Breaker Powers

Load	Notes
Electric clothes dryer	4-wire 240 V, NEC 250.140
Water heater (small / point of use)	240 V, continuous load
RV receptacle (TT-30)	120 V, 30 A, NEC 551.71
Window air conditioner (large)	240 V
Small sub-panel	Feeder to a shed or shop

Always size a circuit from the equipment nameplate or a load calculation, not from a general table. Air conditioners and heat pumps in particular carry a minimum circuit ampacity (MCA) and maximum overcurrent protective device (MOCP) rating that override the general rules.

NEC requirements for this circuit

NEC 240.4(D)(7) caps 10 AWG copper at 30 A. A 30 A circuit is the largest that still falls under the small-conductor rule, so above 30 A Table 310.16 governs directly.

GFCI required for 30 A receptacle outlets in dwelling garages, outdoors, basements and similar locations under NEC 210.8, which now covers receptacles up to 250 V. Dryers hard-wired or on a 30 A outlet in a laundry room are covered by 210.8(A)(10) in the 2020 and later NEC.

Voltage Drop by Distance — 10 AWG Copper at 30 A

One-way run	Drop	% of 240V	Verdict
25 ft	1.86 V	0.8%	Within the 3% branch-circuit target
50 ft	3.72 V	1.5%	Within the 3% branch-circuit target
75 ft	5.58 V	2.3%	Within the 3% branch-circuit target
100 ft	7.44 V	3.1%	Over 3% — upsize one gauge
150 ft	11.16 V	4.6%	Over 3% — upsize one gauge
200 ft	14.88 V	6.2%	Well over 3% — upsize two gauges or more

Single-phase, 240V, full 30 A load, 1.24 Ω/1000 ft DC resistance (NEC Chapter 9, Table 8) at 75°C terminations. NEC 210.19(A) Informational Note recommends keeping branch-circuit drop at or below 3%, and 5% total across feeder plus branch circuit. See the voltage drop formula for the arithmetic.

The 80% rule: a continuous load is one expected to run for three hours or more — EV chargers, electric heat, and most commercial lighting. NEC 210.20(A) requires the overcurrent device to be rated at not less than 125% of that continuous load, which leaves 24 A usable on a 30 A breaker. A breaker that trips after an hour or two under a steady load is usually a continuous-load sizing mistake, not a faulty breaker.

Other Breaker Sizes

15A 20A 30A 40A 50A 60A 100A 200A

Wire size for 30 amps — full page → 10 AWG full specifications → NEC Table 250.122 — ground wire size → NEC Table 310.16 — conductor ampacity → Wire size calculator →