| 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, limited to 30 A by NEC 240.4(D)) |
| 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 |
Electric clothes dryers, small water heaters, RV outlets, window and through-wall air conditioners, small sub-panels
| 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 |
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.
| 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 |
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.
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.
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.
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.
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.