Wire & Fuse Sizing

Size the wire to the load and the run; size the fuse to the wire.

How the sizing works

Low-voltage DC circuits are sized by two tests, and the wire must pass both:

• Ampacity: the wire must carry the load current continuously, with a 25% safety margin, without overheating. This calculator uses conservative copper ampacities so the answer holds in conduit, bundles, and summer heat.
• Voltage drop: the wire must deliver the power without losing too much of it along the way. At 12 V, a 3% drop is only 0.36 V, so long runs at low voltage demand surprisingly thick copper. This is the test that usually picks the gauge on a farm.

The fuse then protects the wire, not the load: it is sized above the load current (so it never blows in normal use) and below the wire's ampacity (so the wire can never be asked to carry a current that melts it). If you find yourself wanting a bigger fuse, the honest answer is thicker wire.

The rules this calculator assumes

• Copper wire only. Aluminum has its own tables and its own manners; small DC systems should be copper.
• The run length is one-way. The calculator doubles it internally, because current travels out and back.
• DC-rated protection. Fuses and breakers on battery circuits must carry a DC rating at or above system voltage. An AC breaker can trip and keep arcing on DC.
• The battery's main fuse is its own subject. This tool sizes branch circuits. The main fuse at the battery's positive terminal should be a high-interrupt type (Class T is the usual answer) sized to the trunk wire.
• 3% drop is a good default for equipment circuits; sensitive electronics like 2%, and lighting tolerates 5%.

The reasoning behind every one of these rules, and the dangers they exist to prevent, live on the site's Electrical Safety and Protection page. Read it before you build.

Free under CC BY 4.0. Cite as "Open Agriculture Technology Wire & Fuse Sizing Calculator (openagriculturetechnology.com)".