How to Size a Circuit Breaker — NEC 2023 Guide Step-by-Step Sizing
Sizing a circuit breaker requires understanding two NEC principles: the 80% rule for continuous loads (NEC 210.20) and matching wire ampacity to breaker rating (NEC 240.4). For continuous loads (3+ hours), the breaker must be rated at 125% of the load — meaning a 40A continuous load needs a 50A breaker. For non-continuous loads, the breaker simply equals or exceeds the load current.
The 80% Rule Explained
NEC 210.20(A) states that where a circuit supplies continuous loads, the overcurrent device rating must be not less than 125% of the continuous load plus 100% of the non-continuous load. In practical terms, this means a standard breaker can only carry 80% of its rating continuously. A 20A breaker can carry 16A continuously. A 50A breaker can carry 40A continuously. This derating accounts for heat buildup in the breaker during extended operation. Note: 100%-rated breakers (like Square D QO or Eaton CH-type) can carry their full rating continuously but cost more.
Step-by-Step Sizing Process
Step 1: Determine the load in amps. Check the appliance nameplate or calculate: Amps = Watts ÷ Volts. Step 2: Classify the load — continuous (3+ hours: EV chargers, water heaters, fixed lighting) or non-continuous (intermittent: receptacles, motors, cooking). Step 3: For continuous loads, multiply by 1.25. For mixed loads: (continuous × 1.25) + non-continuous. Step 4: Round up to the next standard breaker size: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100A. Step 5: Select wire that matches the breaker per NEC 240.4 and 310.16.
Common Breaker Sizing Examples
EV Charger (48A continuous): 48A × 1.25 = 60A breaker with 6 AWG wire. EV Charger (32A continuous): 32A × 1.25 = 40A breaker with 8 AWG wire. Electric Water Heater (4500W/240V = 18.75A continuous): 18.75 × 1.25 = 23.4A → 25A or 30A breaker with 10 AWG wire. Kitchen 20A receptacle circuit: Non-continuous, 20A breaker with 12 AWG wire. Lighting circuit (continuous): For 12A of fixed lighting: 12 × 1.25 = 15A breaker with 14 AWG wire.
Wire Must Match the Breaker
Per NEC 240.4, the wire ampacity must equal or exceed the breaker rating. Standard pairings: 15A breaker → 14 AWG. 20A → 12 AWG. 30A → 10 AWG. 40A → 8 AWG. 50A → 6 AWG. 60A → 4 AWG. These are based on NEC Table 310.16 at 60°C for NM-B cable. For THHN in conduit (75°C column), slightly smaller wire may be acceptable but the above pairings are standard practice and always safe.
| Load | Amps | Continuous? | Breaker | Wire (Cu) |
|---|---|---|---|---|
| LED Lighting | 10-12A | Yes | 15A | 14 AWG |
| Kitchen Outlets | 16-20A | No | 20A | 12 AWG |
| Electric Dryer | 24A | No | 30A | 10 AWG |
| EV Charger 32A | 32A | Yes | 40A | 8 AWG |
| Electric Range | 40A | No | 50A | 6 AWG |
| EV Charger 48A | 48A | Yes | 60A | 6 AWG |
| Sub-Panel | 100A | Mixed | 100A | 1 AWG |
When the Next Size Up Is Permitted
NEC 240.4(B) allows the next standard overcurrent device size above the conductor ampacity for conductors 800A and below — but only for non-continuous loads. Example: 10 AWG wire (rated 30A at 60°C) can be protected by a 30A breaker. But for continuous loads on 10 AWG, the max continuous current is 24A (30A × 0.80). This nuance trips up many DIYers — always account for the 80% rule first, then size the wire to the breaker.
Disclaimer: For educational reference only. Consult a licensed professional.