NEC 310.16 Wire Sizing
What Size Wire for a 100 Amp Breaker?
The copper and aluminum conductor, ground wire, and code basis for a 100 amp circuit, per NEC Table 310.16. Common on a small whole-home service or main power feeder.
Wire Size by Breaker Amperage (NEC 310.16, 75°C)
Copper and aluminum conductor and copper ground by breaker rating. Sized to the 75°C column with the 240.4(D) small-conductor limits applied.
| Breaker | Copper | Aluminum | EGC (Cu) |
|---|---|---|---|
| 20A | #12 | #10 | #12 |
| 30A | #10 | #8 | #10 |
| 40A | #8 | #8 | #10 |
| 50A | #8 | #6 | #10 |
| 60A | #6 | #4 | #10 |
| 100A | #3 | #1 | #8 |
| 200A | #3/0 | 250 kcmil | #6 |
Copper THHN/THWN-2 at the 75°C column (75°C-listed terminals). For NM-B cable (Romex), use the 60°C column per NEC 334.80 and 110.14(C)(1)(a). Service and feeder ratings can use the NEC 310.12 dwelling exception. Verify terminal temperature rating and voltage drop on long runs.
How a 100 Amp Circuit Is Sized
The breaker protects the conductor, so the conductor ampacity must be at least the breaker rating (NEC 240.4). For a 100 amp breaker, the smallest 75°C copper conductor that meets or exceeds 100 amps is #3, and the aluminum equivalent is #1. For #14, #12, and #10, the 240.4(D) small-conductor rule caps the breaker at 15, 20, and 30 amps regardless of the table ampacity.
The dwelling exception (NEC 310.12). A single-phase dwelling service or main power feeder can be sized at 83% of the rating, because a home rarely draws its full service continuously. That is why a 100 amp home service commonly runs #4 copper or #2 aluminum instead of the #3 copper a 100 amp branch circuit would require. Confirm your load with a service calculation before you rely on the exception.
Size a Specific Circuit
Enter your load, run length, insulation, and conditions for exact NEC 310.16 sizing with temperature derating, bundling, terminal temperature limits, and voltage drop.
Frequently Asked Questions
What size wire do I need for a 100 amp breaker?
A 100 amp single-phase dwelling service needs #4 copper or #2 aluminum under the NEC 310.12 dwelling allowance (conductors rated for 83% of the service rating). Its grounding electrode conductor is #8 copper, sized from the service conductors per NEC Table 250.66. A 100 amp feeder that is not a dwelling service is sized at the 75°C column of Table 310.16 (#3 copper) and grounded by a #8 copper equipment grounding conductor per Table 250.122.
Can I use aluminum wire for a 100 amp breaker?
Yes. Aluminum is allowed and common on larger circuits and services where it saves on cost. For a 100 amp breaker, use #1 aluminum at the 75°C column versus #3 copper. Apply antioxidant to the terminations, torque to the listed spec, and remember aluminum drops more voltage, so check the run length on long pulls.
What size ground wire for a 100 amp breaker?
It depends on whether this is a service or a feeder. For a 100 amp dwelling service, the grounding electrode conductor (GEC) is #8 copper or #6 aluminum, sized from the service-entrance conductors per NEC Table 250.66 (not by the breaker). If this 100 amp panel is instead a subpanel fed by a feeder, the equipment grounding conductor run with the feeder is #8 copper per NEC Table 250.122. A service uses a GEC to the grounding electrodes; a feeder or branch circuit uses a 250.122 EGC.
Why can a 100 amp service use smaller wire than a 100 amp branch circuit?
NEC 310.12 lets a single-phase dwelling service or main power feeder use conductors rated for 83% of the service rating, because a whole house rarely draws its full rating continuously. That is why a 100 amp home service commonly uses #4 copper or #2 aluminum instead of the #3 copper a 100 amp branch circuit would need. The exception applies only to dwelling services and main feeders, not general branch circuits.
Do I size the wire to the breaker or to the load?
Size the conductor to the load first, then protect it with a breaker at or above the conductor rating (NEC 240.4). The breaker protects the wire, so the wire ampacity must be at least the breaker rating, with the small-conductor limits of 240.4(D) for #14, #12, and #10. On long runs, voltage drop can force a larger conductor than ampacity alone requires.
Wire Size for Other Breakers
20 Amp Wire Size
#12 copper or #10 aluminum, #12 ground.
30 Amp Wire Size
#10 copper or #8 aluminum, #10 ground.
40 Amp Wire Size
#8 copper or #8 aluminum, #10 ground.
50 Amp Wire Size
#8 copper or #6 aluminum, #10 ground.
60 Amp Wire Size
#6 copper or #4 aluminum, #10 ground.
200 Amp Wire Size
#3/0 copper or 250 kcmil aluminum, #6 ground.
Wire Size Calculator
Full NEC 310.16 sizing with derating, bundling, and terminal temperature limits.
Voltage Drop Chart
Max copper and aluminum run distances at 3% drop, for long runs.
Long run or a continuous load? Check it.
Ampacity is only half the job. A 100 amp circuit can pass the table and still fail on voltage drop over distance. Run the numbers before you pull the wire.