Off-Grid Solar Calculators

Wiring and electrical safety

DC Inline Fuse & Circuit Breaker Sizer

Protect the wire, not the gadget. A 40 A load on 6 AWG needs a fuse at least 1.25 × 40 A, then the next standard size, and that fuse must still be at or below the cable’s ampacity. If it is not, use thicker cable.

Inputs

A

Results

1.25 × continuous
50.0A
Next standard fuse
50.0A
Cable ampacity (conservative)
75A

Recommended gear

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 and compares it with a conservative chassis ampacity for the chosen AWG. If that fuse would exceed ampacity, the result is invalid: upsize the cable.

Frequently asked questions

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

Continuous loads are typically fused at not less than 125% of current so the device does not heat-nuisance-trip. The fuse still must be at or below the wire’s ampacity so 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 ampacity?

This calculator marks that invalid. Use a thicker AWG so ampacity rises above the next standard fuse, or reduce continuous current. Do not install a fuse larger than the wire can carry.

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