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Old 05-05-2005, 07:34 PM   #1
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Fuse/Breaker size

Where can I find a chart to tell me what size fuse or breaker I should use to protect a given size of wire? For example I've run an 8 ga charge wire from the tow vehicle to the batteries in the trailer, but how do I know what size fuse to use on it?

Grant

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Old 05-05-2005, 09:08 PM   #2
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grant

i looked for a chart and this is what i came up with

http://www.the12volt.com/info/recwirsz.asp

however, it does not cover fuses. just off the top of my head i would say a 30 amp fuse would be plenty big for your application. thats what both my chevys run for the charging circuit.

john
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Old 05-05-2005, 09:11 PM   #3
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Hi Grant, I found various sources doing Google searches for "fuse and wire size" among others. I'm not confident interpreting them myself because I think that the wire length and the load expected have much to do with each specific installation. But I'd like to ask a follow up question about the fuse you mention. I assume that the tow vehicle has a fuse on the Hot circuit to my seven way plug and I know that the Univolt has a 30amp fuse for the battery but would you want to protect that wire before it enters the trailer?
Where were you thinking it would go? Does the wire need to be heavier than the corresponding wire in the tow vehicle?

I like messing with electronics but I have lots to learn,

Steve
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Old 05-05-2005, 10:08 PM   #4
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Grant,

That's a tough question without knowing how you will regulate the load between the alternator and the battery. If you have an unregulated source, and a fairly dead battery, it will just continuously trip the breaker.

You calculate the amperage you can push through the #8 wire by calculating the voltage drop (2%) over the length of the line. Did you use 90ºC wire?

Once you know how many amps the line can handle, you have the answer to the fuse/circuit breaker question, but you still need to figure out how to regulate the amp flow. If you already have the regulator, the operation manual should tell you what size circuit breaker (and wire) to use, or how to adjust it for the maximum load based on the wire size.

If you need more info on wire ampacity, you can check NFPA 70, section 551 (Natl Elec Code) or ABYC part E9(Marine)
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Old 05-06-2005, 04:04 PM   #5
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Quote:
Originally Posted by sfixx
But I'd like to ask a follow up question about the fuse you mention. I assume that the tow vehicle has a fuse on the Hot circuit to my seven way plug and I know that the Univolt has a 30amp fuse for the battery but would you want to protect that wire before it enters the trailer?
Where were you thinking it would go? Does the wire need to be heavier than the corresponding wire in the tow vehicle?

I like messing with electronics but I have lots to learn,

Steve
The original charge wire to the trailer was 12 gauge (I think) and was protected by a 30 Amp fuse. I ran an 8 gauge wire (which is about 40 feet long from the fuse block under the hood to the trailer battery) because I had read that this would allow for faster charging of the batteries. I'm wondering whether I should still use a 30 amp fuse, or if I should increase it because the wire is larger.

I think I've seen a chart for this somewhere, but I can't find it now that I need it.

Grant
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Old 05-06-2005, 04:08 PM   #6
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Quote:
Originally Posted by john hd
grant
just off the top of my head i would say a 30 amp fuse would be plenty big for your application. thats what both my chevys run for the charging circuit.

john
Thanks John, that's what was there originally, I was wondering if I should use a larger fuse since the wire was larger.

Grant
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Old 05-06-2005, 05:50 PM   #7
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grant

you can look at it this way, most battery chargers for home use run around 10 to 12 amps. if you have two batteries in your trailer and double it we are still talking 24 amps max.

30 should be plenty even with completely discharged batteries. what you are doing by increasing the wire size is lowering the line loss. that equals more voltage and amperage at the end of the wire.

voltage times amps equals power or watts. the larger wire will allow more wattage to pass and charge the batteries in less time.

john
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