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Old 10-04-2012
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Fusing to the size of the wire....

I know that you're supposed to choose your fuses to the size of your wire, not the device amperage draw. What I'm suddenly having trouble with though, is finding a good chart that shows which fuses for which size wire.

All the tables I can find are voltage drop tables, which aren't helpful I assume. Don't I need to know at what amperage 6 gauge wire spontaneously combusts? My specific project is a 20A 12V engine heater that I would like to fuse. I'll likely be using 6 or 8 gauge wire and the round trip run will probably be around 20-30ft.

My Google-fu is weak, please help me find what I need.

Medsailor.
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Old 10-05-2012
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Re: Fusing to the size of the wire....

You're looking for the 'ampacity' of the various AWG ratings. Try this...

Wire-Gauge Ampacity - Transwiki
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Old 10-05-2012
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Re: Fusing to the size of the wire....

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Originally Posted by Faster View Post
You're looking for the 'ampacity' of the various AWG ratings. Try this...

Wire-Gauge Ampacity - Transwiki
Hmmm..... I'm not exactly sure how to use that table. It looks though like they're talking about house wiring though which would be 120v or 220v which, I believe, makes a big difference because the lower voltage of the 12v is creating more heat.

It also says that the values of the table might not apply for stranded wire, which is what I would be using.

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Re: Fusing to the size of the wire....

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Originally Posted by MedSailor View Post
I know that you're supposed to choose your fuses to the size of your wire, not the device amperage draw.
Not necessarily. It can be either way depending on the situation. Because of the wire sizing requirements of 12 volt circuits, the wire can typically handle way more current than the load requires.

How much current a wire can safely handle depends on the physical size of the wire, the temperature rating of its insulation, whether its run with other wires and how many, and whether its in an engine space.

In your case, if this circuit is soley for the heater, I would size the fuse for the load of the heater which is far less than #8 or #6 wire can safely handle.

Eric
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Old 10-05-2012
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Re: Fusing to the size of the wire....

Here's another ampacity chart that applies to marine under 50 volts from Blue Seas
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Fusing to the size of the wire....-wire-ampacity.jpg  
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Old 10-05-2012
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Re: Fusing to the size of the wire....

MedSailor,

Fusing to the size of the wire is a good overall concept, but it should not be applied slavishly. As Eric intimated, there are times when you would want to use a smaller -- even much smaller -- fuse than the wire size itself would otherwise dictate.

For example, I routinely install SSB radios having a maximum peak draw of 30A using AWG6 cable for runs up to 20' and AWG4 cable for runs over 20'. These cables have an ampacity of 120 and 160 amps, respectively (outside engine spaces). But, as these are cables dedicated to the SSB and it's maximum draw is 30A it would be foolish to use a big fuse or breaker for this purpose. I use 40A ANL and, sometimes, 30A or 40A MRBF (terminal) fuses.

Nothing wrong with using fuses or breakers rated LESS THAN the wire's ampacity, so long as you have definitive knowledge of the anticipated load(s) to be placed on that cable.

Never use fuses or breakers rated HIGHER than the ampacity of the cable they are to protect.

Bill
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Re: Fusing to the size of the wire....

Quote:
Originally Posted by MedSailor View Post
It looks though like they're talking about house wiring though which would be 120v or 220v which, I believe, makes a big difference because the lower voltage of the 12v is creating more heat.
Just chiming in on this (which I actually remember talking with you about before):

The voltage of the bus has nothing to do with ampacity or heat. The voltage drop (and heat generated) is the same for 10A at 120V as it is for 10A at 12V over the same wire.

Differences in specification result from the marine environment, and because you just can't tolerate voltage drop when you only have 12 volts to start with. When you have 120V you can drop a couple of them without your load caring.
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Last edited by asdf38; 10-05-2012 at 03:07 PM.
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