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Old 01-14-2022, 12:11 PM   #1
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How to calculate the size of fuse for a small battery bank

I'm increasing my RV's available 12v power from a 75ah lead acid battery on the tongue to a 412ah battery bank under the bed. All the schematics I see online show an inline fuse on the positive cable between the batteries and the charger but I'm unsure if I need one and if I do what size I'll need.

I'll have 8' of 4awg neg/pos cabling between the charger/converter and the battery box, a Victron smart shunt on the negative cable, and the battery box will contain two 206ah LiFEPO4 batteries connected in parallel. There is not a power inverter anywhere in the system - this is only designed to operate the 12v items in the RV and not any of the 115vac items. Such as the refrigerator, water pump, awning, slide, lighting, and possibly an occasional cycling of the furnace. But not the TV, microwave, A/C, or any wall outlets.


How do I determine what size fuse to install? I know the power center has a fuse for the charging system, but I don't know if that is sufficient or if I should incorporate a separate fuse inside the new battery box. Oh, and I matched the new charger that is lithium rated to the old one that I'm removing - both charge at a max output of 55 amps.
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Old 01-14-2022, 12:23 PM   #2
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The fuse at the battery box is designed to protect the cable between the batteries and the converter/inverter. Based on the cable you are using you would normally use a large fuse, however if you are only running 12V items and charging at 55A max I might start at 60A and see how it goes. If decide to use the inverter for 120VAC you will need to increase it.
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Old 01-14-2022, 12:25 PM   #3
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Thanks Flybob. I think I'll mount a fuse fixture so that I can increase the size down the road if need be. Appreciate your input!
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Old 01-14-2022, 02:42 PM   #4
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Quote:
Originally Posted by Flybob View Post
The fuse at the battery box is designed to protect the cable between the batteries and the converter/inverter. Based on the cable you are using you would normally use a large fuse, however if you are only running 12V items and charging at 55A max I might start at 60A and see how it goes. If decide to use the inverter for 120VAC you will need to increase it.
don't want to rob the thread, but I was to run all my circuits from an inverter (2000 watt), what would I need, for 4-6vt batteries. Thanks
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Old 01-14-2022, 02:49 PM   #5
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What you need to do is determine the wire gauge you are using then look up a chart to determine how many amps it will handle, any size up to the wire capacity is ok as that is all you are trying to protect.

If that is not enough for your planned load you need larger cable.
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Old 01-14-2022, 03:36 PM   #6
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what you need to do is determine the wire gauge you are using then look up a chart to determine how many amps it will handle, any size up to the wire capacity is ok as that is all you are trying to protect.

If that is not enough for your planned load you need larger cable.
/\ /\ /\ this any other advise how to size the fuse is wrong
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Old 01-16-2022, 09:26 PM   #7
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Wire gauge and fuse sizing...

Go here.. https://www.wirebarn.com/Wire-Calculator-_ep_41.html

I plugged in 55A, 13.8V, 8 feet and 10% voltage drop. It says that 14 AWG is ok for your application.

I used 10% since it's a charging circuit and not running anything. For a 2% drop max (V), then min gauge is 6 AWG.

Now you know your wiring is adequate, you're oK with a 55A or 50A fuse.

Use this calculator or similar chart for all your DIY's to be safe.
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Old 01-16-2022, 09:38 PM   #8
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Go here.. https://www.wirebarn.com/Wire-Calculator-_ep_41.html

I plugged in 55A, 13.8V, 8 feet and 10% voltage drop. It says that 14 AWG is ok for your application.

I used 10% since it's a charging circuit and not running anything. For a 2% drop max (V), then min gauge is 6 AWG.

Now you know your wiring is adequate, you're oK with a 55A or 50A fuse.

Use this calculator or similar chart for all your DIY's to be safe.
Am I correct this is your very FIRST time calculating the correct wire size for a given fuse size?
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Old 01-16-2022, 09:44 PM   #9
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Thanks for that. I actually bumped it up from 4awg to 2 awg so even better.
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