What Size Circuit Breakers Can be Used on a Trailer for Electric Jack and Brake Functions

Updated 06/02/2025 | Published 06/02/2016

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In-Line Circuit Breaker - 20 Amp - Perpendicular Mount Bracket

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Pollak Circuit Breaker - Cycling/Automatic Reset - 30 Amp - Straight Mount Bracket

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Question:

I have two circuit breakers mounted on my trailer frame near my battery box. They are wired in series output lug of breaker #1 wired to input lug of breaker #2 and each breaker has another wire on the output lug. I assume that one breaker is for the electric brakes and the other is for my electric tongue jack I have not traced the wires yet. The problem is that I need to replace the existing breakers because the housing on both is broken and being held together with a zip tie, but I have no way of telling what size they are. I have looked closely and I do not see any amp rating. The only number visible is on the metal contact plate, inside the breaker between the terminals that has U-21 stamped on it. How can I tell what size they are so I can order replacements?

asked by: Jim

Expert Reply:

If those circuit breakers are indeed for the jack and the trailer brakes then they are likely redundant since the 12 volt circuit and brake circuit are already circuit protected on the vehicle side. Electric trailer jacks also usually have an internal circuit breaker and/or in line fuse. If you want to replace them you can.

If you can verify what they go to as the jack and brakes then jack the specs on the jack to determine its amperage draw. Usually electric jacks on trailers are rated to pull up to 30 amps so you can use a 30 amp circuit breaker # PK54530. For the brakes, since most brake controllers will use either a 20 amp or 30 amp circuit breaker (depending on how many brake assemblies there are) you can just go with another 30 amp breaker.

It is also possible that the circuit breakers were there for some kind of winch. The way you describe it sort of sounds like the circuit breaker sets winches use such as # SW87-22873-16 for example. I anticipate that the mystery of the breaker functions will be solved once you can verify where the wires from them lead.

expert reply by: Michael H

Markus W.

6/1/2025

I'm looking to eliminate the lead/acid battery from my travel trailer. I purchased a 2600 watt/h li-ion battery that essentially replicates shore power. It runs everything from the slider to the fridge, etc. The issue that I'm running into is with the electric trailer jack (because it is DC). I'm wondering if I can run the wires to my breaker box, install a 30-amp breaker, and then run it off of the battery (or shore power)? Thanks for your time.

Nick S.

6/2/2025

@MarkusW You could run the jack directly to the battery, it should be 12v already. On the other hand, the distribution box usually has a converter to convert the 110v shore power to usable 12v DC power already in it. Technically you could run the jack to either option and it will work fine for you.