Fuse Sizing Calculator
Fuse Size by Load Current
Calculate fuse rating based on load current and NEC rules
Max Fuse Size by Wire Gauge
Maximum overcurrent protection for a given wire size
Formula
Continuous: Fuse ≥ 125% × Load | Non-Continuous: Fuse ≥ 100% × Load | Motor: Fuse varies by type (175-300% FLC) | NEC 240.6(A) standard sizes
Frequently Asked Questions
How do I size a fuse or circuit breaker?
For continuous loads: fuse ≥ 125% of load current. For non-continuous loads: fuse ≥ 100% of load current. For motors: varies by fuse type (175-300% of FLC per NEC 430.52). Always use the next standard size up per NEC 240.6(A).
What is NEC 240.4(D)?
NEC 240.4(D) limits the maximum overcurrent protection for small conductors: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A. These are absolute maximums regardless of insulation rating or actual ampacity. Larger wires can use overcurrent protection up to their ampacity.
What is the difference between fuses and circuit breakers?
Fuses are single-use devices that melt when overcurrent flows. They are faster-acting and less expensive. Circuit breakers are resettable switches that trip on overcurrent. They are more convenient but slightly slower. Both provide overcurrent protection per NEC Article 240.
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