Do I size a fuse to the wire or to the load in a camper van build?

Answered by RV Parts Giant · Upland, CA · September 29, 2026

Short answer: The fuse protects the wire. With one device on a circuit, rate the fuse at the device's rating or the cable's, whichever is lower. With several devices on one cable, rate it to the cable. Then check it is DC-rated for your system voltage, slow-blow on inverter feeds, and on lithium, that one fuse can interrupt the bank's short-circuit current.

The fuse is there for the cable

A fuse's job is to stop a cable overheating when something downstream shorts or overloads. So the cable sets the ceiling. Victron's rule is:

One device on the circuit: rate the fuse at the device's rating or the cable's rating, whichever is lower.
Several devices on one cable: rate the fuse to the cable.

A fuse bigger than the cable can carry protects nothing. That is also why you never "fix" a fuse that keeps blowing by fitting a larger one; the cable becomes the fuse.

Size the cable first

The fuse follows the cable, so get the cable right. Use the device manual's figure where there is one; it overrides general tables. Otherwise, Victron's table rates cable by total length, positive plus negative. A 2.5 metre run out and 2.5 back is a 5 metre total. As a check, 16 mm² is good for about 48 amps and 35 mm² about 105 amps over a 5 metre total. Aim for under 2.5 percent voltage drop.

Three ratings people skip

Voltage. The fuse must be DC-rated at or above the system's maximum voltage. Most common DC fuses suit 12 and 24 volts; the MEGA, ANL and MIDI fuses we stock are 32 volt DC parts.
Speed. Inverters and motors have a big inrush, so their feeds want a slow fuse. MEGA and MIDI are slow; ANL is fast.
Interrupt rating. On a lithium bank, at least one fuse in the line must be able to interrupt the battery's maximum short-circuit current. That is usually a Class T at the battery, which we do not stock as a standalone fuse.

Lay it out so each branch has its own fuse

Fuse every conductor at the battery end. Once there are more than a couple of loads, run the main cable to a busbar or fuse block and give each branch its own fuse, sized to that branch's cable, rather than stacking rings on the battery post. The cable feeding the busbar has to be sized for the sum of everything it feeds.

For a quick pick: MIDI fuses in their holder for branch circuits, MEGA fuses (40 to 500 amps) for the inverter and main feeds. Send us your load list and cable runs and we will check the sizing before you order. Everything ships from Upland, California.