Breaker Sizing Guide: How to Choose the Right Circuit Breaker
How Circuit Breakers Work
A circuit breaker is an automatic overcurrent protection device. It trips (opens the circuit) when current exceeds its rating for a sustained period, preventing wire overheating and fire. Breakers have two trip mechanisms:
- Thermal (bimetallic strip): Trips on sustained overcurrent (seconds to minutes). Protects against overloads.
- Magnetic (solenoid): Trips instantly on short circuits (milliseconds). Handles 5–10× rated current.
NEC Breaker Sizing Rules
Rule 1: Breaker ampacity ≤ wire ampacity (NEC 240.4)
| Wire Size | Max Breaker | NEC Reference |
|---|---|---|
| 14 AWG copper | 15A | NEC 240.4(D) |
| 12 AWG copper | 20A | NEC 240.4(D) |
| 10 AWG copper | 30A | NEC 240.4(D) |
| 8 AWG copper | 40A (60°C) / 50A (75°C) | NEC 240.4(G) |
| 6 AWG copper | 55A (60°C) / 65A (75°C) | NEC 240.4(G) |
| 4 AWG copper | 70A (60°C) / 85A (75°C) | NEC 240.4(G) |
Rule 2: 80% Continuous Load Rule (NEC 210.20)
For continuous loads (operating 3+ hours), the breaker must be rated at 125% of the load:
Breaker rating ≥ Load current × 1.25
Example: A 1,800W baseboard heater (continuous) on 120V: I = 1,800/120 = 15A. Breaker = 15 × 1.25 = 18.75A → use 20A breaker with 12 AWG wire.
Common Breaker Sizes and Applications
| Breaker | Wire | Typical Application | Max Load (continuous) |
|---|---|---|---|
| 15A 120V | 14 AWG | Lighting circuits | 1,440W |
| 20A 120V | 12 AWG | General receptacles, kitchen | 1,920W |
| 20A 120V GFCI | 12 AWG | Bathroom, outdoor, garage | 1,920W |
| 30A 120/240V | 10 AWG | Dryer, small AC | 5,760W (240V) |
| 40A 240V | 8 AWG | Electric range, large AC | 7,680W |
| 50A 240V | 6 AWG | Large range, hot tub | 9,600W |
| 60A 240V | 4 AWG | Subpanel feeder | 11,520W |
| 100A 240V | 2 AWG Cu / 1/0 Al | Small service entrance | 19,200W |
| 200A 240V | 2/0 Cu / 4/0 Al | Standard service entrance | 38,400W |
Motor Circuit Breaker Sizing
Motor circuits follow special NEC 430 rules:
- Overload protection: 115% of motor nameplate current (NEC 430.32)
- Branch circuit breaker: 150% of FLC for non-time-delay, 300% for time-delay fuses (NEC 430.52)
- Wire size: 125% of motor FLC (NEC 430.22)
Example: 5 HP, 230V motor (FLC = 28A). Wire: 28 × 1.25 = 35A → 8 AWG. Breaker: 28 × 1.5 = 42A → 45A standard.
Common Breaker Sizing Mistakes
- Using oversized breakers: A 30A breaker on 12 AWG wire is a fire hazard. The wire melts before the breaker trips.
- Ignoring the 80% rule: A 20A breaker can only handle 16A continuous (3+ hours). Space heaters, EV chargers, and baseboard heaters are continuous loads.
- Not matching wire to breaker: Always match: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A. No exceptions for residential branch circuits.
- Double-tapping breakers: Most standard breakers are rated for only one wire. Use tandem/duplex breakers or pigtail for two circuits.
Standards Reference
- NEC Article 240 — Overcurrent Protection
- NEC Article 210 — Branch Circuits
- NEC Article 430 — Motors
- NEC 210.20 — Continuous Loads
Frequently Asked Questions
What size breaker do I need for a 1,500W heater?
At 120V: I = 1,500/120 = 12.5A. If continuous (3+ hours): 12.5 × 1.25 = 15.6A → use 20A breaker with 12 AWG wire. If non-continuous: 15A breaker with 14 AWG wire is acceptable.
Can I put a 30A breaker on 12 AWG wire?
No. NEC 240.4(D) limits 12 AWG copper to 20A maximum overcurrent protection. A 30A breaker on 12 AWG wire is a serious code violation — the wire can overheat and cause a fire before the breaker trips.
What is the 80% rule for breakers?
NEC 210.20 requires that continuous loads (operating 3+ hours) not exceed 80% of the breaker rating. A 20A breaker can handle 16A continuous. This accounts for heat buildup in the wire and breaker during sustained operation.
How do I size a breaker for an air conditioner?
Check the AC nameplate for MOP (Maximum Overcurrent Protection). NEC 440.22 allows special sizing for hermetic compressors — often larger than wire ampacity. The nameplate specifies the exact breaker size. Never exceed the nameplate MOP.
What's the difference between a main breaker and branch breakers?
The main breaker (100A or 200A typical) protects the entire panel and serves as the disconnect. Branch breakers (15–60A) protect individual circuits. The sum of branch breakers can exceed the main breaker — diversity means not all circuits are loaded simultaneously.