Breaker Sizing Formula
For continuous loads: design amps = load amps x 1.25. For non-continuous: design amps = load amps. Select the next standard breaker size equal to or greater than the design amps. Wire gauge must match or exceed the breaker's ampacity rating.
Frequently Asked Questions
Breaker size (amps) = load amps x 1.25 for continuous loads (operating for 3+ hours continuously) per NEC 210.19(A)(1). For non-continuous loads, the breaker can equal the load amps. Always select the next standard breaker size up from your calculated value.
Standard residential breaker sizes are: 15A, 20A, 30A, 40A, 50A, 60A, 70A, 80A, 100A, 125A, 150A, 175A, 200A. The most common for household branch circuits are 15A (lighting, outlets) and 20A (kitchen, bathroom, garage circuits).
The NEC (National Electrical Code) section 210.19(A)(1) requires that continuous loads (those operating 3 or more hours) not exceed 80% of the breaker's rated ampacity. This means a 20A breaker should not have more than 16A of continuous load. Non-continuous loads can use 100% of the breaker rating.
For a 240V appliance, amps = watts / 240V. A 5,000W electric dryer: 5000/240 = 20.8A x 1.25 = 26A → use a 30A breaker. An 8,000W electric range: 8000/240 = 33.3A x 1.25 = 41.7A → use a 50A breaker (skipping 40A since it doesn't meet the 1.25x requirement).
Only if the wiring supports the higher ampacity. A 15A breaker is matched to #14 AWG wire. Replacing it with a 20A breaker without replacing the wire with #12 AWG creates a fire hazard — the wire can overheat before the breaker trips. Always match breaker size to wire ampacity.
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