Circuit Breaker/Fuse Sizing
Finds the smallest circuit breaker or fuse that may protect a branch circuit or feeder for a given load. Enter the load current and say whether it runs continuously. The page gives the minimum rating the code asks for and the next standard size at or above it. Electricians, designers and inspectors use it as a first step when laying out panel schedules.
Equations
Imin = k·I rating = smallest standard size ≥ Imin
I is the load current in amperes. k is 1.25 for a continuous load and 1.0 for a non-continuous one. The NEC calls a load continuous when its maximum current is expected to last 3 hours or more. A breaker or fuse must then be rated at least 125 % of that load, unless the whole assembly is listed for 100 % duty [1]. The standard sizes are the ratings of Table 240.6(A), from 10 A to 6000 A [1]. Between them the steps grow: 5 A up to 50 A, then 10 A, 25 A, 50 A and larger.
Limits and common mistakes
- For a load with both parts, the code sizes the device for the non-continuous load plus 125 % of the continuous load. Enter that sum as a non-continuous load to get the right size.
- The breaker protects the wire. Check that the conductor’s ampacity, after corrections for temperature and bundling, is at least the load and fits the chosen size.
- Motors, transformers, welders and air conditioners follow their own sizing rules. Use the nameplate or the matching code article, not this page.
- Some EV chargers, water heaters and heaters are treated as continuous by their own code articles, whatever their run time.
- Above 6000 A the page returns an error. Size such a device from the equipment rating.
Worked example
A garage gets a wall-mounted EV charger set to draw 48 A. Charging can last all night, so the load is continuous. What breaker does the circuit need?
- Enter Load current (I) 48 A.
- Choose Load type Continuous (on 3+ hours) and press Solve. The Solution shows:
Required minimum rating: 60 A
Standard breaker/fuse size: 60 A
Check: 48 × 1.25 = 60 A, and 60 A is itself a standard size, so no rounding up is needed. This matches the usual pairing of a 48 A charger with a 60 A breaker. Had the load been 50 A, the minimum would be 62.5 A and the page would pick 70 A, the next standard size.
