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Posted: 07/16/22 10:48am Link  |  Quote  |  Print  |  Notify Moderator

time2roll wrote:

50 amp fuse close to the battery or power source. I agree with #8 unless the distance is long go #6 wire.

Fuse to the wire size, the idea is to protect the wire not the device hung on the wire.


"Why use a fuse?

A fuse provides a safe and cost effective way to protect an electrical device or installation from electrical overloads which could damage components or wiring and potentially cause a fire."

Basically you want the fuse to blow out in a controlled manor well before the entire wire becomes a fuse.


"Fuses normally protect wires from burning and setting fire to things"


"When specifying a fuse for an after-market application, the key consideration is that the fuse should be the weakest point (i.e. lowest rated component) so that it always blows before any damage occurs to other parts of the electrical circuit. However, you also do not want the fuse to keep blowing under normal operation (known as a nuisance blow), so the two elements to consider are:

The current rating of the smallest cable in the circuit
The current draw on the circuit under normal expected operating conditions

The fuse rating should lie somewhere between these two values to allow normal operation but blow on overload. For example, if the normal expected current draw is 10A and the cable size is 25A, then a fuse rated at 15A would be appropriate."

50A fuse would be for 6Ga wire, not for 8ga-26ga wire.

Granted, bare UNinsulated wire can sustain higher current than insulated wire, but, it isn't like one is going to install and use UNinsulated wire for this use.

Where to place the fuse?

Generally, as close to the power source (IE 12V battery in the case of the OPs situation) as possible, I have often seen references of 18" or closer to the battery..

On edit..

I will also add that if you have wire which has more capacity than the expected load, you can use a lower amperage fuse.

In other words, if you had 8 ga wire which would typically be fused at 40A but your device (like a 400W inverter) draws 35A at max full load you could fuse to the max load of the inverter which would be 35A.. But cut that too close and you may get nuisance blown fuses..

The idea of using larger wire sizes than needed is not to handle more current, but to lower the voltage drop seen by the inverter.. Just .1V extra drop in the wire can make the difference between the inverter working or shutting down in 12V systems.

* This post was edited 07/16/22 11:09am by Gdetrailer *


Regina, SK, Canada

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Posted: 07/16/22 11:31am Link  |  Quote  |  Print  |  Notify Moderator

freewayrandy wrote:

Thanks for all the info. Pretty lame on electrical here. Inverter primarily for TV. What would be recommended fuse size?

#8 wire with 40 amp fuse (slow blow optional), or a 40 amp breaker. Place the fuse/breaker as close to the battery bank as possible on the positive lead.

We always fuse to the wire size.

Regards, Don
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Posted: 07/16/22 12:37pm Link  |  Quote  |  Print  |  Notify Moderator

Gdetrailer wrote:

50A fuse would be for 6Ga wire, not for 8ga-26ga wire.
NEC allows #8 wire on a 50 amp breaker if the distance is short.

With 105C insulation of marine grade wire the #8 is good for 80 amps if not in a bundle.

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Posted: 07/16/22 01:57pm Link  |  Quote  |  Print  |  Notify Moderator

time2roll wrote:

With 105C insulation of marine grade wire the #8 is good for 80 amps if not in a bundle.

#10 is adequate if the distance is on!y a few feet. 40A fuse, as close as possible, to the battery.

Marine grade wire is pre-tinned and should never corrode. It is worth the extra cost.

Microlite Mike

NW Washington State

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Posted: 07/16/22 05:00pm Link  |  Quote  |  Print  |  Notify Moderator

I find charts for wire size to often be confusing and too broad in their recommendations.

For 12 volt circuits (automotive/RV) I prefer this online calculator.

Parameters needed for calculations are the maximum amperage of the device you're going to power (which for Inverters needs to take into consideration surge capacitiy),and the length of the wire in BOTH directions as it's a DC Circuit. If using a chassis/frame ground at each end then just use the total length of the wire between battery and device PLUS the wire from battery to frame and device to frame.

This calculator also shows voltage drop for each wire size and the max length at stated amperage to achieve the desired max percentage voltage drop.

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Posted: 07/26/22 11:02am Link  |  Quote  |  Print  |  Notify Moderator

8ga minimum 6ga would be better, 40a*12v= 480w ,
If the only thing being powered is the TV and cellphone chargers you can use 10ga, but i prefer going larger as you might someday want full max use from it

I can explain it to you.
But I Can Not understand it for you !


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Posted: 07/26/22 12:18pm Link  |  Quote  |  Print  |  Notify Moderator

Personally I'd use an automotive auto-resetting breaker over a fuse. They are much more waterproof, less of a chance that they'll ruin your outing by nuisance trips, and are just much less of a hassle. At small scales the cost difference is immaterial, for $9 you get 4 fuse holders or 2 breakers, compared to all the other costs in the project it's noise.

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Davison Michigan (East of Flint)

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Posted: 07/26/22 04:55pm Link  |  Quote  |  Print  |  Notify Moderator

larry cad wrote:



I=425/12=35.4 AMPS

10G wire is rated 30amps.

I would go with #8

Add 10 percent for converter loss and overhead So the "rule of thumb" i use is figure AC watts / 10 volts

For that inverter I'd use standard automotive battery cables (4GA) for runs up to around 5-8 feet.. Longer runs bigger wire.. I'd not go shorter.

One Edit.. I said I would.. I also DID. When I had an inverter in that size class.

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Posted: 07/30/22 09:24pm Link  |  Quote  |  Print  |  Notify Moderator

I installed a 1000 watt inverter, I wanted to use 8 gauge wire, but that is hard to find and expensive.
But I bought a cheap set of 12’ 8 gauge booster cables for $13.00 at our local Canadian tire store,
Cut the clamps off, and soldered new round connectors on , and works perfectly.

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