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

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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: https://www.explorist.life/diy-campervan-solar

Also, we have interactive 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: https://www.explorist.life/solarwiringdiagrams/

TYPES OF FUSES

There are 3 types of 12v fuses I recommend.  They are Spade Fuses, ANL fuses, and Re-settable Breakers.

Spade fuses are the fuses that you’ll commonly find in your car’s fuse panel and are typically for items run out of your distribution block and less than 30 amps (in our case.

Spade Fuses

Spade fuses are what you commonly see in your vehicles fuse block. These are typically used for lower amperage devices and the most common size seems to be 15 amps.

ANL FUSES

ANL fuses come in sizes from 35 – 750 amps.  They are typically less expensive than Re-settable Breakers.  They are more reliable than re-settable breakers as they have no moving parts.  If they blow/trip, they must be replaced.  They require tools to connect/disconnect.

RE-SETTABLE BREAKERS

Re-Settable Breakers come in sizes from 25 amps up to 200 amps.  I prefer these on circuits that I know I may have to regularly ‘disconnect’ for maintenance and or troubleshooting.  Honestly, the only time I don’t use these is when I need a fuse/breaker that is bigger than is available.

HOW TO CHOOSE THE SIZE OF YOUR FUSES

Easiest Way

A lot of components will tell you how big of wire and what kind of fuse to use.  If this is the case, follow it.

Easy Way

  1. Find the continuous amperage of the device you are trying to power and multiply that number by 1.5, then round up to the nearest fuse size you can find.
  2. Cross check that fuse size with the maximum fuse size chart below and verify that your fuse size is below the maximum fuse size listed in the chart.

(more info: ABYC E-911)

Fuse & Wire Sizing Example

For this example, we are going to wire the 12v distribution block to the bus bar then wire a string of lights to the 12v distribution block.  All with appropriately fused with properly sized wires.

Step 1: Find the wire size for the 12v fuse block.

In description of the fuse block, it says the block has a 100A minimum.

Next, determine how far away from the power source you plan on mounting the fuse block.  We are going to mount our fuse block 10 feet away from the bus bar.

Next, we use our wire size chart to determine how big of wire we need.

100 amps over 10ft calls for a 1ga wire.

Now that we know how big of wire we are going to use, we need to determine how big of a fuse we need.

Oh, lookie there, Blue Sea tells us in the instructions how big of a fuse to use.

Now, double check that the 125A fuse requested by the fuse block in within the safe threshold for 1ga wire:

The 125 amp fuse requested by the fuse block is WELL within the maximum fuse size tolerance for our wire.

Next, we are going to determine how big of wire and fuse we need to wire a string of lights.  These 12v lights are the lights we are basing our example on.  We are going to install 2 packs of these 12v lights which will be a total of 8 lights.  The product page says these are 3 watts each.

8 x 3 = 24 total watts

24 watts / 12.6 system voltage = 1.9 amps for the whole string (which we will round up to the nearest whole number)

For the “Distance from fuse block to appliance” we are going to go with the furthest light, which, let’s say, is 15 feet from the fuse block.

2 amps at 15 feet is calling for 12ga wire.

Find the size of fuse by multiplying the amps required by the device (2) by 1.5.

2×1.25 = 2.5 then we will round up to the nearest fuse size (in multiples of 5 amps) which would call for a 5 amp fuse.

Cross Reference that fuse size to verify that it’s below the Maximum fuse size called out by this chart.  As you can see, 5 amps is well within the 52.5 maximum fuse size threshold.

And there you go! There are a lot of steps to this and trust me, you’ll get faster at it with practice. As always leave any questions you have in the comments below and subscribe for updates.

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 Interactive 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: https://www.explorist.life/solarwiringdiagrams/

Remember, this is just one part of a full camper van electrical educational series.  To see all of the individual guides, click here: https://www.explorist.life/diy-campervan-solar

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Mike Cloes

Tuesday 20th of April 2021

If I'm using 200ah of lithium batteries(2x100ah) with 2AWG wire and only a 400w inverter is a 200a fuse ok?

Warren

Monday 19th of April 2021

I don't see any diagrams that don't include inverters. I am trying to work out what size ANL fuse I will need in this case.

I've decided to not use an installed inverter but just use a 500 watt inverter plugged into a 12 volt socket for small and occasional use.

If I allow for an extra 500 watts DC surge for things like heaters and fridges turning on, etc while using the 500 watts inverter, I expect to never draw more than 1000 watts at surge and probably no more than 600 watts continuous. So to get the fuse size I did this: 1000/12 = 83.33 * 1.5 = 125 Amp ANL fuse needed. Is this correct? Or am I thinking about this wrong?

Thanks!

Austin

Tuesday 6th of April 2021

Just wanted to let you know there is a typo. I think you meant 1.5 instead of 1.25 in this formula "2×1.25 = 2.5"

Sam

Thursday 25th of February 2021

If I have a 105ah battery with 2/0 wire, is a 250A ANL fuse what I need? Or do I need to decide on an inverter size too?

Nate Yarbrough

Sunday 28th of February 2021

You need to know inverter size to determine the fuse size between the battery bank & distribution.

Chris S

Monday 22nd of February 2021

I think I'm just about done figuring out the wire sizes and fuse sizes I need for my system thanks to your blog posts, videos and wiring diagrams. Such a great resource, thank you! I do have one question though. On the fuse size chart, why would the fuse sizes be more than the max amps for the wire? Doesn't that mean the wire would fail before the fuse would blow?