Wiring
Wire size calculator
Size the wire by ampacity first, from NEC Table 310.16 in the column your cable type allows (60 °C for NM-B and UF), then check voltage drop over the run and go up a size if it exceeds 3%. A 20 A circuit uses 12 AWG copper; a 30 A circuit, 10 AWG.
10 AWGcopper, NM-B cable (Romex), indoors
- Design current
- 20.0 A
- Breaker
- 20 A
- Smallest size by ampacity
- 12 AWG (20 A, 60 °C column)
- Voltage drop over 50 ft
- 2.42 V = 2.02%
Check: Upsized from 12 AWG to 10 AWG to keep voltage drop under 3%. The breaker stays at 20 A.
How the wire size is chosen
- Design current. The load in amps, or 125% of it for a continuous load (NEC 210.19(A)(1) and 215.2).
- Breaker. The next standard size up from the design current (NEC 240.6(A)).
- Ampacity. The smallest conductor whose ampacity in Table 310.16 covers the design current, in the temperature column your wiring method allows, and that the breaker is allowed to protect: 14 AWG copper is limited to 15 A, 12 AWG to 20 A and 10 AWG to 30 A (NEC 240.4(D)), and a breaker one standard size above the wire's ampacity is permitted otherwise (240.4(B)).
- Voltage drop. If the run drops more than your limit, step up until it doesn't, using resistance from NEC Chapter 9, Table 8.
The column rule most wire charts skip
Table 310.16 lists three ampacities per size: 60 °C, 75 °C and 90 °C. You may not just read the biggest one. NM-B cable (Romex) is limited to the 60 °C column by NEC 334.80, even though its conductors are rated 90 °C. UF cable is limited the same way (340.80). Individual THHN wires in conduit can use the 75 °C column only when both ends terminate on equipment rated 75 °C, which most modern breakers and panels are; circuits of 100 A or less default to 60 °C otherwise (110.14(C)).
That is why a 50 A range circuit in NM-B needs 6 AWG, while the same circuit in THHN through conduit can use 8 AWG. More in the 60 °C rule, explained.
Wire size chart by breaker
Computed from Table 310.16 for runs short enough that voltage drop doesn't apply. "NM-B" means the 60 °C column; "THHN 75 °C" means conduit with 75 °C terminations.
| Breaker | Copper NM-B | Copper THHN 75 °C | Aluminum NM/SE 60 °C | Aluminum 75 °C |
|---|---|---|---|---|
| 15 A | 14 AWG | 14 AWG | 12 AWG | 12 AWG |
| 20 A | 12 AWG | 12 AWG | 10 AWG | 10 AWG |
| 30 A | 10 AWG | 10 AWG | 8 AWG | 8 AWG |
| 40 A | 8 AWG | 8 AWG | 6 AWG | 8 AWG |
| 50 A | 6 AWG | 8 AWG | 4 AWG | 6 AWG |
| 60 A | 4 AWG | 6 AWG | 3 AWG | 4 AWG |
| 70 A | 4 AWG | 4 AWG | 2 AWG | 3 AWG |
| 80 A | 3 AWG | 4 AWG | 1 AWG | 2 AWG |
| 90 A | 2 AWG | 3 AWG | 1/0 AWG | 2 AWG |
| 100 A | 1 AWG | 3 AWG | 1/0 AWG | 1 AWG |
| 125 A | 1/0 AWG | 1 AWG | 3/0 AWG | 2/0 AWG |
| 150 A | 3/0 AWG | 1/0 AWG | 4/0 AWG | 3/0 AWG |
| 200 A | 250 kcmil | 3/0 AWG | 350 kcmil | 250 kcmil |
When distance changes the answer
A 20 A, 120 V circuit running 150 ft to a detached garage passes on ampacity with 12 AWG, but drops more than 3% at full load. The calculator steps it up to 6 AWG (2.5% drop). The breaker stays at 20 A; only the wire gets bigger. Check any run with the voltage drop calculator.
What this calculator does not cover
- More than three current-carrying conductors in one raceway or cable bundle (derate per 310.15(C)(1)).
- Ambient temperatures above 86 °F, such as attics and rooftop conduit (correct per 310.15(B)).
- Motor circuits, which follow Article 430, and service and feeder sizing for dwellings (310.12).
Safety. Turn off the breaker and verify with a tester before touching any conductor. Most US jurisdictions require a permit for new circuits, and some only allow homeowners to wire their own single-family home. When in doubt, hire a licensed electrician.
Common questions
What size wire for a 20 amp circuit?
12 AWG copper. 14 AWG is not allowed on a 20 A breaker (NEC 240.4(D)). On runs over about 100 ft at full load, 10 AWG keeps voltage drop under 3%. 10 vs 12 gauge, compared.
What size wire for a 50 amp circuit?
6 AWG copper in NM-B cable. With THHN in conduit and 75 °C terminations, 8 AWG copper is allowed. Aluminum: 4 AWG in SE/NM-type cable, 6 AWG at 75 °C.
What size wire for a 30 amp circuit?
10 AWG copper, in NM-B or THHN. A 30 A dryer or water heater circuit is the classic 10/2 or 10/3 run.
Can I use aluminum wire?
Modern AA-8000 aluminum is common and code-compliant for feeders and larger circuits (typically 8 AWG and up). Use terminals rated AL or CU/AL, an anti-oxidant where the manufacturer calls for it, and torque to spec. Avoid small-gauge aluminum branch circuits.
Guides that use this calculator
- What Size Wire for a 30 Amp Breaker?
- 10 vs 12 Gauge Wire for a 20 Amp Circuit
- What Size Wire for 50 Amps? Why NM-B Uses the 60 °C Column