Breaker / Fuse Sizing Calculator

Enter the load current and tick the box if the load is continuous (running for 3 hours or more). The calculator selects the next standard overcurrent device size per NEC 240.6(A), applying the 125% uplift where required.

Enter a load current above to see the recommended breaker or fuse size.

How It Works

The sizing formula

For a non-continuous load the minimum overcurrent device rating equals the load current. For a continuous load (NEC 210.20(A)), the device must be rated at no less than 125% of the continuous load current:

Required rating = Load (A) × 1.25   (continuous loads only)

The calculator then selects the smallest standard rating from NEC 240.6(A) that is greater than or equal to the required rating.

NEC 240.6(A) standard sizes

The NEC lists the following standard ampere ratings for fuses and inverse-time circuit breakers:

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600

Worked example

A lighting circuit draws 20 A and is classified as a continuous load (office fluorescent lighting that runs all day):

  • Required rating = 20 A × 1.25 = 25 A
  • Smallest standard size ≥ 25 A is 25 A
  • Specify a 25 A breaker or fuse.

If the same circuit were non-continuous (e.g. a window air conditioner used intermittently), 20 A × 1.0 = 20 A, and the smallest standard size ≥ 20 A is also 20 A.

Frequently Asked Questions

Why does NEC require 125% for continuous loads?

A continuous load (one that operates at its full rated current for 3 hours or more) causes sustained heating in the overcurrent device. NEC 210.20(A) and 215.3 require the device to be rated at no less than 125% of the continuous load so the device runs within its thermal design limits and does not trip prematurely under normal operation.

What are the standard breaker and fuse sizes listed in NEC 240.6(A)?

NEC 240.6(A) specifies: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, and 600 amperes for standard ampere ratings of fuses and inverse-time circuit breakers.

What is the difference between a circuit breaker and a fuse?

Both are overcurrent protective devices (OCPDs), but a fuse is a one-time sacrificial element that melts to open the circuit, while a circuit breaker is a resettable electromechanical switch. Sizing rules (and the NEC standard sizes) apply to both. Fuses generally react faster for the same rating.

What counts as a continuous load?

NEC Article 100 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. Examples include commercial lighting, HVAC compressors, electric water heaters, and most industrial process loads. Intermittent loads such as motor starters, convenience receptacles, and kitchen appliances are generally non-continuous.

Can I round down to a smaller standard size?

No. NEC 240.4(B) permits rounding up to the next standard size for conductors rated 800 A or less, but never rounding down. Selecting a breaker smaller than the required minimum ampere rating would leave the circuit inadequately protected and violate the code. Always use the next standard size at or above the calculated minimum.

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