Off-Grid Solar Calculators

Wiring and electrical safety

DC Inline Fuse & Circuit Breaker Sizer

Protect the wire, not the gadget. A 40 A continuous load on 6 AWG needs a fuse of at least 1.25 × 40 A = 50 A, rounded up to the next standard size, and that fuse must not exceed the cable’s ampacity (75 A). On 14, 12 and 10 AWG, NEC 240.4(D) also caps the fuse at 15, 20 and 30 A. If the fuse does not fit, use thicker cable.

Inputs

Units
A

Terms of Service

Results

Please tick the box above to display the Result.

Engineering methodology

Protect the conductor, not the gadget. Minimum overcurrent device = 1.25 × continuous current (NEC-style continuous-load factor). The tool picks the next standard fuse size (NEC 240.6(A)) and compares it with the NEC Table 310.16 copper 90 °C ampacity for the chosen AWG, or with IEC 60364-5-52 Table B.52.4 method C (70 °C PVC) when mm² is shown. On 14, 12 and 10 AWG copper, NEC 240.4(D) caps the fuse at 15, 20 and 30 A, below their table ampacity. Terminals listed at 75 °C can cap the circuit lower under NEC 110.14(C). If that fuse would exceed the cable’s limit, the result is invalid: upsize the cable. A 3% voltage-drop target is not an ampacity limit.

Frequently asked questions

Why size a DC fuse at 125% of load current?

Continuous loads (3 hours or more) are fused at not less than 125% of the current so the fuse or breaker does not nuisance-trip as it heats. The fuse still must not exceed the wire’s ampacity, because the wire is the protected object.

Should the fuse match the appliance rating or the cable?

The cable. An undersized fuse on a fat cable is a nuisance; an oversized fuse on a thin cable is a fire risk. Place the fuse as close to the battery positive as practical.

What if no standard fuse fits between 1.25 × amps and the cable’s limit?

This calculator marks that invalid. Use thicker cable so its limit reaches the next standard fuse, or reduce the continuous current. Do not install a fuse larger than the wire can carry.

More in this cluster

All tools in this cluster