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How-to Size Fuses in a DIY Camper Van Electrical System

Fuses protect your system from catching on fire in the case of mishaps such as a wire rubbing through it’s insulation and grounding itself to the frame of your camper. The fuse is there to protect the WIRE not the device it’s connected to.  Each device SHOULD have some kind of internal overcurrent protection in addition to the fuse protecting the wire.

Quick note before we get started.  This is just one part of an overarching “How to Install a DIY Camper Van Electrical System” series.  If you’ve just stumbled on this article directly without seeing that, there are likely some things we’ve already covered.  If you want to check out that step by step guide, you can do that here:

Also, we have solar wiring diagrams that are a complete, A to Z solution for teaching you exactly what parts go where, what size wires to use, fuse size recommendations, wire lug sizes, and all kind of other stuff to help save you time and frustration.  You can check that out here:

How to Size Fuses

The goal for sizing fuses is to determine a fuse size that will allow the device to function properly but also protect the wire that is supplying power to the device as well as staying below the short circuit current rating of the device (if available). 

Use the Manufacturers Recommendation

The first and best way to find the recommended fuse size is to use the manufacturers recommendation noted in the user manual of the device that you’re wiring.  For example; the Victron Multiplus 3k 12V recommends a 400A fuse right there on pg 8 of the user manual.

When the manufacturer of a device makes a fuse sizing recommendation, it’s best to follow that recommendation unless there’s another external reason to do otherwise (code conflict, local regulations, poor user manual translation, etc)

Fuse Sizing Calculation

If there is no manufacturer recommendation for fuse sizing; use the following formula to determine a proper fuse size:

Continuous Amperage of Device * 1.2 = Recommended Fuse Size

Note: The 1.2 Multiplication factor is simply 120% of the continuous amperage of the device to allow the device to function as designed without too much overhead to make fuse sizing difficult. This is a rule-of-thumb and not a code/standard.

Tip: You’ll likely have to round the Recommended Fuse Size to the nearest available fuse size. Round down provided that the recommended fuse size does not fall below the continuous amperage of the device. Round up provided that the recommend fuse size does not rise above 150% of the max ampacity of the wire.

IMPORTANT: You must cross-reference this ‘recommended fuse size’ with the maximum ampacity of the wire you are using and verify that the recommended fuse size does not exceed 150% of the max ampacity of the wire you are using.

Source: ABYC E-9.11

Here is a calculator that we’ve made to help:

Fuse Sizing Calculator

This calculator will help you determine an approximate fuse sized based on the circuit amperage when the wire size is already given or known.  This calculator does not superscede manufacturers recommendations.


Note: If the Recommended Fuse Size is larger than the Max Fuse Size, the wire size choice may be too small for the amperage demands of the circuits.

Now that you know how to size the fuses in your DIY Camper Van electrical system, it’s time to put everything you’ve learned to work.

Everything that you are learning here is put to use in our FREE Solar Wiring Diagrams.  If you haven’t yet, check them out as they are a complete solution for a camper van electrical system.  Check them out here:

Remember, this is just one part of a full camper van electrical educational series.  To see all of the individual guides, click here:


Monday 6th of February 2023

Hi Nate, I posted this a couple of times on another part of your site last year, but it sits as "pending", but then came across this section so thought I would try again as maybe I've posted int he wrong place. I've had the conversation with many people who have the same question as me, so would be great to get your understanding on this, thanks. ... Hello from the UK. Great resource both here and YouTube, thanks so much for for your efforts, your videos etc. are a superb resource and clearly a lot of effort goes into them. I have to ask, and apologies if I look stupid, but the one bit I struggle with is your fuse sizing at your +ve busbar (or Lynx Distributor often in your case). For example, feeding your 12V distribution panel is a 6AWG cable (roughly 60AMP cable capacity I believe, we don't use AWG in the UK), but the fuse is a 100AMP Mega Fuse (which if a Victron one would not actually blow until about 250AMP), so how does that protect the cable? Would the cable not just set on fire before the fuse had a chance to blow, so theoretically once the current exceeds the 60AMP cable capacity? I feel I’ve missed something obvious here, but happy to take the embarrassment to understand this better. Thanks again

Edward P

Saturday 31st of December 2022

All your diagrams recommend ANL on the battery bank, but numerous internet people recommend Class T on the battery because of the short circuit rating. Any comments on ANL vs Class T and why you recommend ANL? Thanks

Nate Yarbrough

Sunday 15th of January 2023

Class T is the better choice, but has been pretty much unavailable since 2019. At the moment, at our best estimate, there are only 50-something class T fuses for sale in the entire USA at the moment, none of which are actually available for purchase by individuals who aren't professional installers.


Sunday 13th of November 2022

Hey Nate!

I came to this page from the Battle Born Batteries website:

And there is a picture with table with recommended and maximum fuse sizes for different gauges. And that picture has a water mark from your website.

However recommendations here have different values. So I'm wondering if you know where the picture on their website comes from. And why values there are different there. I'm asking because I would like to understand better why there are different approaches out there. Which ones I should discard as non relative. And where those different values on those pictures come from.

Thanks in advance!

David Forbrigger

Tuesday 5th of July 2022

Dear Sir, We've supplied complete systems for a number of clients including schematics, BOM, and tech support. We've just done a unique little job with 12KW of LiFePO4 storage in 180Ah batteries. We have a 3kw pure sine wave inverter/charger. We have fusing, disconnect and power meters. What we also have is a 300A ANL fuse in the factory housing that has MELTED! We can say definitively that we have never drawn more than 2.2Kw continuous with only 600W motor loads started sequentially. My point, of course being, the fuse block should NOT be "melting". Additionally, as the 300A fuse has remained intact, it seems clear that the current is not at issue. We feed ALL components with 4/0 wire and heavy guage copper lugs which are crimped in our hydraulic crimper. Our wiring is overkill as the total wire run is under 10 feet round trip. We have no issues elsewhere with hot wires or poor connections. Just the fuse block. Is it possible that the run of the mill Amazon units are not built properly? Is there a trustworthy CSA, ETL, UL certified ANL that we can trust to handle the current? We done 3kw systems before and never have we had this issue. It is, of course, not beyond reason that we have apoor crimp hat is generating resistance and thus heat, and of course I will check this when I visit the site. However, all our fittings are built slowly and carefully and we've never had an issue. If our fittings check out, then I'm hoping you may have some ideas or suggestions for us.

Sincerely, David

Nate Yarbrough

Tuesday 5th of July 2022

I have indeed had many issues with 'run of the mill Amazon brand fuses' melting. So much so that I've made a specific announcement and blog post addressing it here: For 3k 12V inverters, I recommend these Blue Sea Fuse Holders: and these Blue Sea Fuses:

Daniel Krivens

Monday 16th of May 2022

Hi Nate, Nevermind my last question, I now see that the 1/0 wire isn't rated for 60C, its 105C giving it a massively higher amp carrying capacity.