How to size a breaker
Find the load current, multiply by 125% if the load runs continuously for three hours or more, then round up to the next standard breaker size. The wire must be rated for at least the breaker size so the breaker protects it.
Example: 32 A Level 2 EV charger
EV charging is continuous, so 32 × 1.25 = 40 A. The next standard size is a 40 A breaker with 8 AWG copper. 8 AWG works as THHN in conduit (50 A at 75°C) and as NM-B cable (40 A at the 60°C rating).
Standard breaker sizes (NEC 240.6)
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400 A and up.
Common circuits
| Appliance | Typical breaker | Copper wire |
|---|---|---|
| Lighting, bedroom outlets | 15 A | 14 AWG |
| Kitchen, bathroom, garage outlets | 20 A | 12 AWG |
| Electric water heater (4,500 W) | 30 A | 10 AWG |
| Electric dryer | 30 A | 10 AWG |
| Electric range | 40–50 A | 8–6 AWG |
| EV charger (48 A) | 60 A | 6 AWG |
Frequently asked questions
What is the 80% rule for breakers?
A standard breaker should carry no more than 80% of its rating continuously. It is the same as sizing the breaker at 125% of the continuous load.
What size breaker for a 5,000 watt heater at 240 V?
5000 ÷ 240 = 20.8 A; × 125% = 26 A, so a 30 A breaker with 10 AWG copper.
Can I put a bigger breaker on the same wire?
No. The breaker protects the wire. Upsizing the breaker without upsizing the wire is a fire hazard and a code violation.