Minimum size by temperature column
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| Copper | 1 AWG110 A | 3 AWG100 A | 3 AWG115 A |
| Aluminum | 1/0 AWG100 A | 1 AWG100 A | 2 AWG100 A |
Equipment grounding conductor
A circuit protected at 100 A takes a 8 AWG copper or 6 AWG aluminum equipment grounding conductor, from the 100 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 | 120 V | 240 V |
|---|---|---|
| 1 AWG copper | 115 ft | 230 ft |
| 1/0 AWG aluminum | 85 ft | 175 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.
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 100 A that is 83 A, which the 75°C column meets with 4 AWG copper or 2 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 100 A circuit feeds
- Garage, shop, and outbuilding subpanels
- Small service upgrades and feeder runs
One hundred amps is the smallest residential service in common use and a typical detached shop or garage panel. 1 AWG copper or 1/0 aluminum reaches it at 60°C. 3 AWG copper or 1 AWG aluminum reaches it at 75°C, which is what SER cable and conduit terminations usually give you.
Continuous loads on a 100 A circuit top out at 80 A. A shop panel fed from the house is a feeder and does not get the dwelling allowance in 310.15(B)(7), which is limited to service and feeder conductors for the dwelling itself. The equipment grounding conductor for a 100 A feeder is 8 AWG copper or 6 AWG aluminum.
A calculated load between 80 A and 100 A lands on this rating, because 240.6(A) steps from 80 A to 100 A. A feeder that leaves the house for a separate structure also needs its own disconnecting means at that building under 225.31 and 225.32, and the grounding electrode system there is separate from the one at the house.
100 A wire size questions
- What size wire do I need for a 100 amp circuit?
- Use 1 AWG copper or 1/0 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, 3 AWG copper or 1 AWG aluminum carries the circuit.
- What size ground wire for a 100 amp circuit?
- 8 AWG copper or 6 AWG aluminum, taken from the 100 A row of NEC Table 250.122.
- What size aluminum wire for 100 amps?
- 1/0 AWG aluminum at the 60°C column carries 100 A. That equals the circuit rating, so any ambient correction, bundling adjustment, or voltage drop pushes the size up. Terminations on aluminum conductors must be identified for the material.
- Can I use 2 AWG copper for 100 A?
- Table 310.16 lists 2 AWG copper at 95 A at 60°C, 115 A at 75°C, and 130 A at 90°C. It falls short at 60°C, the column this circuit uses. The 75°C and 90°C columns do reach 100 A, and a higher column applies only where every termination is listed for it.