What size wire for a 150 amp breaker?
Short answer: A 150 A breaker needs at least 1/0 AWG copper or 3/0 AWG aluminum as individual wires in conduit with 75 °C terminations, or 3/0 AWG copper in NM-B (Romex) cable, which is held to the 60 °C column. Long runs need a bigger size to keep voltage drop under 3%. For a 150 A dwelling service or main feeder, NEC 310.12 allows 1 AWG copper or 2/0 AWG aluminum.
Minimum wire size for 150 A by wire type
| Wire type | Minimum size | Ampacity (NEC 310.16) |
|---|---|---|
| Copper NM-B (Romex) | 3/0 AWG | 165 A |
| Copper THHN/THWN-2 in conduit, 75 °C | 1/0 AWG | 150 A |
| Aluminum SER / NM-B (60 °C) | 4/0 AWG | 150 A |
| Aluminum XHHW-2 in conduit, 75 °C | 3/0 AWG | 155 A |
The column matters as much as the breaker. NEC 334.80 limits NM-B cable to its 60 °C ampacity, so the same 150 A breaker needs a larger gauge in Romex than in conduit. Individual THHN or XHHW-2 wires can use the 75 °C column, but only when both ends terminate on lugs rated 75 °C, which is true of nearly all residential breakers and panels sold today but should be checked on the label. NEC 240.4(B) also lets you protect a wire with the next standard breaker above its ampacity (up to 800 A) when the load itself fits the wire, which is how 150 A breakers end up on some sizes listed one step smaller in other charts.
When distance pushes the size up
Smallest size that is both legal for the 150 A breaker and keeps voltage drop at or under 3% at 240 V, with a load of 120 A (80% of the breaker):
| One-way length | Copper, 75 °C | Aluminum, 75 °C |
|---|---|---|
| 50 ft | 1/0 AWG (0.6%) | 3/0 AWG (0.6%) |
| 100 ft | 1/0 AWG (1.2%) | 3/0 AWG (1.3%) |
| 150 ft | 1/0 AWG (1.8%) | 3/0 AWG (1.9%) |
| 200 ft | 1/0 AWG (2.4%) | 3/0 AWG (2.5%) |
| 300 ft | 2/0 AWG (2.9%) | 250 kcmil (2.5%) |
Upsizing for distance does not change the breaker. It does change the ground: NEC 250.122(B) requires the equipment grounding conductor to be increased in proportion when the hot conductors are upsized for voltage drop.
150 A service or main feeder: the 83% rule
For a 120/240 V single-phase dwelling, NEC 310.12 lets service-entrance conductors, and a feeder that carries the whole load of the house, be sized at 83% of the service rating: 124.5 A for a 150 A service. That is why a 150 A service is commonly wired with:
| Material | Size | Ampacity at 75 °C |
|---|---|---|
| Copper | 1 AWG | 130 A |
| Aluminum | 2/0 AWG | 135 A |
The rule does not apply to a subpanel feeder that carries only part of the house's load. A 150 A subpanel uses the full table above.
Ground wire for a 150 A circuit
NEC Table 250.122 sizes the equipment ground from the breaker: 6 AWG copper or 4 AWG aluminum for 150 A. Cable assemblies (NM-B, SER, UF) include a ground already sized for their gauge. A 150 A circuit carries up to 36,000 W at 240 V, or 28,800 W for a continuous load.
Common 150 A circuits
- 150 A subpanel for a large detached shop or an ADU with electric heat.
- 150 A service, common in 1970s–1990s homes.
Feeding a subpanel? It needs four wires (two hots, a neutral and a ground), with the neutral and ground kept separate in the subpanel, and a detached building needs its own grounding electrode (NEC 250.32).
Safety. Installing a 150 A circuit or feeder needs a permit almost everywhere in the US. Equipment nameplates (minimum circuit ampacity and maximum overcurrent protection) override any general table, and local amendments to the NEC can be stricter.