Step 1: Size the Wire

The majority of wire gauges for 12V, 24V, and 48V DC circuits can be determined by simply using the chart here. The more current, the larger the wire. For long wire runs, the voltage drop across the wire will begin to play a role in the equipment’s performance. In some cases it is necessary to upsize the wire, not because of it’s current carrying capacity, but in order to reduce the voltage drop from point A to B. For almost any circuit between batteries, busbars, inverters, and main panels, we suggest it be treated as “critical” for best operation and highest efficiency. For DC loads such as lighting, refrigeration, pumps, and more, it is typically safe to size the wires for “non-critical” unless the manufacturer recommends otherwise. If your DC circuit needs to carry more current than is shown in the chart, it’s best to contact a professional.

Step 2: Size the Fuse

With very few exceptions, fuses should be sized to protect wires, not appliances. Therefore, the maximum fuse size allowable for any wire in a DC circuit can be determined using the chart here. Be sure to pay attention to whether the wire is a single wire where plenty of air flow is present (not tightly bound to other wires or encased in a fully sealed conduit) or bundled together with others (using zip ties or conduit). Bundled wires should not be relied on to carry as much current as they otherwise could, because they are surrounded by other warm conductors instead of air. Be sure to also consider whether your wires are in an engine room. Inside of an engine room on a boat, or engine bay of a vehicle, wires are already in a hot environment, so it takes less additional heat to push them over the edge that would cause insulation to melt. Therefore, less current should be conducted through wires in an engine bay in order to avoid overheating. If you have made it through step 1 (sizing your wire) and come to step 2 (fusing your wire) only to realize your maximum allowable fuse size is within 20% of the constant load you expect on the wire, it’s best to upsize the wire and start again.