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Wiring

What Size Fuse for a 125 Amp Circuit?

Researched from published standards and manufacturer specifications. Updated .

Quick answer

A 125 amp continuous load is protected at 125 percent, which is 156.3 amps, rounded up to the next standard rating of 175 amps. If the load is not continuous, a 125 amp fuse is sufficient. The conductor must be rated to carry at least 175 amps.

A fuse protects the wire, not the appliance. That single sentence resolves most fuse sizing confusion. The fuse exists so that a fault cannot push more current through a conductor than it can carry without starting a fire, which means the fuse rating must never exceed the conductor's ampacity.

The NEC calls a load continuous when it is expected to run for three hours or more, and requires overcurrent devices to be sized at 125 percent of it. In an RV, a refrigerator, an inverter under sustained load and a converter charging a depleted bank all qualify. Check other loads with the fuse size calculator.

What fuse for a 125 amp load?

Overcurrent protection for a 125 amp load
Load typeSizing factorCalculatedStandard fuse
Continuous, three hours or more125 percent156.3 A175 A
Non-continuous100 percent125 A125 A

Published standard Source: NEC 210.20(A), overcurrent device sized at 125 percent of a continuous load.

What conductor does this fuse require?

A 175 amp fuse needs a conductor rated for at least 175 amps. Over a 10 foot one-way 12 volt run held to 3 percent voltage drop, that means 2/0 AWG copper, which has an ABYC ampacity of 330 amps. Voltage drop usually pushes the conductor larger than ampacity alone would, which is helpful here: it means the wire is comfortably above what the fuse allows through.

Size your own run on the wire gauge calculator.

Which fuse type belongs on this circuit?

Fuse body types are not interchangeable, and a holder made for one will not accept another.

Fuse types by typical current range in RV DC systems
TypeTypical rangeWhere it is used
Blade (ATO, ATC, Mini)1 to 40 ABranch circuits at the 12V distribution panel
MIDI / AMI30 to 200 AMid-sized feeds, solar controller and DC-DC charger circuits
MEGA / AMG100 to 500 ALarge feeds and alternator circuits
ANL35 to 750 AInverter feeds and main battery protection
Class T100 to 400 ALithium banks, where very high fault current demands a high interrupt rating

Convention Source: Typical application ranges. Individual manufacturers publish their own series ratings.. Class T is the type to use on a large lithium bank, because LiFePO4 can deliver enormous short circuit current and ordinary fuses may not interrupt it safely.

Where does the fuse go?

As close to the battery positive terminal as practical, because everything between the battery and the fuse is unprotected. A cable that chafes through before it reaches its fuse is a dead short across a battery that can deliver hundreds of amps. Fuse every positive conductor leaving the bank, including the inverter feed, the solar controller, the DC-DC charger and the distribution feed.

Fuse holders and distribution

Frequently asked questions

What size fuse for a 125 amp circuit?
A 175 amp fuse for a continuous load. The calculation is 125 amps times 1.25, which gives 156.3 amps, rounded up to the next standard rating. For a load that runs less than three hours at a time, 125 amps is sufficient. In both cases the conductor must be rated to carry at least the fuse rating.
Can I fit a larger fuse if mine keeps blowing?
No. A fuse that blows repeatedly is reporting a real problem: an overloaded circuit, an undersized conductor, or a fault. Fitting a larger fuse removes the protection without removing the cause, and the next failure is the wire rather than the fuse. Find the load, measure it, and size the circuit properly for it.
Does the fuse protect my appliance?
Not primarily. The fuse protects the conductor from carrying more current than it can safely handle. Appliances generally have their own internal protection. This is why the fuse rating is tied to the wire size rather than to the device, and why a fuse must never be larger than the conductor's ampacity.
How far from the battery can the fuse be?
As close as you can practically get it. Any length of cable between the battery terminal and the fuse is unprotected, and a short in that section puts the full fault current of the bank across it. Common practice is within a few inches of the terminal, using a terminal fuse block or an inline holder mounted right at the battery.

Wiring safety. Never exceed a conductor's rated ampacity, and always fuse a circuit at or below what the wire can carry, because the fuse protects the wire rather than the appliance. Fuse every positive conductor as close to the battery as practical. Use proper crimped lugs and heat shrink rather than twisted joints, support cable so it cannot chafe through on a frame edge, and keep the battery compartment ventilated. Permanent AC wiring is work for a qualified RV technician or a licensed electrician.