Table of Contents
Circuit Breaker Sizing Basics
Selecting the correct circuit breaker size is critical for electrical safety and code compliance. A breaker that is too small will trip frequently under normal loads, causing inconvenience. A breaker that is too large may not trip during an overload, potentially causing wire overheating and fire. The National Electrical Code (NEC) provides specific guidelines for breaker sizing based on load type and wire gauge.
The fundamental principle is that the breaker must protect the wire. The breaker rating should never exceed the ampacity of the wire it protects. For continuous loads (those expected to run for 3 hours or more), the NEC requires that the load not exceed 80% of the breaker rating, effectively requiring an oversized breaker and wire to handle the sustained thermal load.
Sizing Formula
Where I is the load current, P is power in watts, V is voltage, and PF is the power factor. For resistive loads (heaters, incandescent lights) PF = 1.0. For motor loads, PF is typically 0.8-0.9.
Standard Breaker Sizes
| Breaker (A) | Min Wire (Cu) | Max Cont. Load (120V) |
|---|---|---|
| 15 | 14 AWG | 1,440 W |
| 20 | 12 AWG | 1,920 W |
| 30 | 10 AWG | 2,880 W |
| 40 | 8 AWG | 3,840 W |
| 50 | 6 AWG | 4,800 W |
Frequently Asked Questions
What is the 80% rule?
The NEC 80% rule (Article 210.20) states that continuous loads must not exceed 80% of the breaker rating. A continuous load is defined as one that runs for 3 hours or more. This means a 20A breaker on a continuous load circuit can only safely carry 16A continuously. The 20% margin prevents thermal accumulation in the breaker that could cause nuisance tripping or damage over time.
Can I use a larger breaker to stop tripping?
No. The breaker must match the wire gauge. Putting a 30A breaker on 14 AWG wire is extremely dangerous because the wire will overheat and potentially cause a fire before the breaker trips. If a properly sized breaker keeps tripping, the solution is to reduce the load on the circuit or install a new circuit with appropriately larger wire and breaker.
How do I calculate breaker size for motors?
Motors have high inrush currents during startup, typically 5-8 times their running current. NEC Article 430 allows motor branch circuit breakers to be sized at 250% of the motor's full-load current for standard breakers. This prevents nuisance tripping during startup while still protecting against sustained overloads. Motor overload protection is provided separately by an overload relay.