Minimum size by temperature column
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| Copper | 3/0 AWG165 A | 1/0 AWG150 A | 1/0 AWG170 A |
| Aluminum | 4/0 AWG150 A | 3/0 AWG155 A | 2/0 AWG150 A |
Equipment grounding conductor
A circuit protected at 150 A takes a 6 AWG copper or 4 AWG aluminum equipment grounding conductor, from the 200 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
| Conductor | 240 V single phase | 240 V three phase |
|---|---|---|
| 1/0 AWG copper | 195 ft | 225 ft |
| 3/0 AWG aluminum | 190 ft | 215 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 balanced three-phase run reaches about 15% farther than the single-phase figure.
Dwelling service allowance under 310.15(B)(7)
A 120/240 V single-phase dwelling service or feeder rated 100 A through 400 A may use conductors with an ampacity of 83% of the rating. For 150 A that is 124.50 A, which the 75°C column meets with 1 AWG copper or 2/0 AWG aluminum.
The allowance covers service and feeder conductors only. Branch-circuit conductors, the equipment grounding conductor, and any local amendment are unaffected, and the neutral may be reduced separately under 220.61.
What a 150 A circuit feeds
- Residential service entrances
- Large subpanel feeders
One hundred fifty amps was a common service size before 200 A became standard, and it still appears on older houses and on feeders to outbuildings. 3/0 copper or 4/0 aluminum reaches it at 60°C, and 1/0 copper or 3/0 aluminum reaches it at 75°C, a full size apart.
Because 150 A equipment is less common than 200 A, an upgrade usually goes to 200 A instead of staying here. Continuous loads top out at 120 A. When a service conductor is upsized for voltage drop rather than for ampacity, 250.122(B) raises the equipment grounding conductor with it.
An older 150 A service often has a 60 A or 100 A panel fed from it, and that feeder is sized on its own rating rather than on the service. A service upgrade at this rating usually replaces the mast and the meter socket too, because the existing ones were sized to the old service. The available fault current at the service sets the interrupting rating of the main device under 110.9 and 110.10.
150 A wire size questions
- What size wire do I need for a 150 amp circuit?
- 1/0 AWG copper or 3/0 AWG aluminum at the 75°C column, which 110.14(C)(1)(b) requires for equipment rated over 100 A. On the 60°C column the same circuit would need 3/0 AWG copper or 4/0 AWG aluminum.
- What size ground wire for a 150 amp circuit?
- 6 AWG copper or 4 AWG aluminum, taken from the 200 A row of NEC Table 250.122.
- What size aluminum wire for 150 amps?
- 3/0 AWG aluminum at the 75°C column carries 155 A. Terminations on aluminum conductors must be identified for the material.
- Can I use 1 AWG copper for 150 A?
- Table 310.16 lists 1 AWG copper at 110 A at 60°C, 130 A at 75°C, and 145 A at 90°C. It falls short at 75°C, the column this circuit uses.