Determining fuse sizes - Fiberglass RV


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Old 01-11-2010, 11:18 PM   #1
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I am wiring up the 12 volt system now. So how do you determine the fuse size needed for each circuit? I will be using the glass-type fuses.

The Shurflo pump specified a 10 volt slow-blow fuse, so I've got that in place.

Now I've got a bunch of lights and a 12v socket left to fuse.

Thanks for advice
Fran
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Old 01-12-2010, 07:12 AM   #2
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The fuse rating in amps, not volts, is in the curcuit to protect the wiring, not the appliance.

The fuse should be rated no larger than the current capacity of the smallest wire in the circuit.

Ideally, ALL the wires in a circuit would be the same size, but this is not always the case as circuit extensions are commonly added by the user who sometimes uses what he has on hand.

If you have a camper that has wiring added by a previous owner, you may have to play detective to confirn that the added wiring is of sufficient size.

If you presume that the wire size is sufficient, then the fuse should be sized a little over the anticipated load. For instance, a 12 or 15 amp fuse for a 10 amp load.

Some appliances pull a lot more current on start up than during normal running. In this case a slow blow fuse is used so that the fuse dosn't blow when the unit is turned on. This is usually not a problem but something you should be aware of.

In most cases, what blows a fuse is that there has been a short in the circuit. When this happens there is a HUGE increase in the current draw that will blow most any fuse. Therefore, any size is better than none.
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Old 01-12-2010, 08:25 AM   #3
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This should help.
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Old 01-12-2010, 11:23 AM   #4
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Kevin, I have done some residential wiring and 14 gauge solid wire is 15 amps, 12 gauge is 20 amps. So where is your list from? Is multi-strand different or are vehicles different from residential?
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Old 01-12-2010, 11:31 AM   #5
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The fuse rating in amps, not volts,..........
Agree, however fuses are also rated for maximum AC or DC voltage. So using automotive fuses usually rated at 32V in 120V application is not good. Blowing a low voltage fuse in a high voltage application will not necessarily open the circuit.

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Old 01-12-2010, 12:37 PM   #6
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Found the info here
http://www.oznium.com/forum/topic15761
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Old 01-12-2010, 01:08 PM   #7
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I have never seen a 32 volt fuse, all my fuses are 240 volt.

Bill K

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Agree, however fuses are also rated for maximum AC or DC voltage. So using automotive fuses usually rated at 32V in 120V application is not good. Blowing a low voltage fuse in a high voltage application will not necessarily open the circuit.

George.
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Old 01-12-2010, 02:46 PM   #8
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I have never seen a 32 volt fuse, all my fuses are 240 volt.
Bill K
As far I know the maximum voltage rating standard for most automotive fuses such as ATC, ATM and MAX is 32 volts.
http://stevenengineering.com/Tech_Support/.../06AUTOFUSE.pdf
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Old 01-12-2010, 03:50 PM   #9
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Guess I am showing my age, I was talking about glass tube fuses. Sorry about that.

Bill K

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As far I know the maximum voltage rating standard for most automotive fuses such as ATC, ATM and MAX is 32 volts.
http://stevenengineering.com/Tech_Support/.../06AUTOFUSE.pdf
George.
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Old 01-12-2010, 06:14 PM   #10
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Talking

Here is a good discussion of fuses:

http://en.wikipedia.org/wiki/Fuse_(electrical)

Don't forget that when you are dealing with electrical circuits you are trying to do a task which professional electricians spend years learning how to do correctly.

Every task looks simple until you get into the nitty-gritty of all the ramifications of every action.
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Old 01-12-2010, 06:32 PM   #11
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DC Wiring: The type of fuse that George showed are the one's in my converter box and they lite up if the fuse is BLOWN and I think that is far superior to the old glass fuse that is somewhat hard to tell without taking out and sometimes testing with a ohm meter.

On the glass fuse: Some are SLOW Blow meaning they will accept a higher START UP load without blowing, The regular ones will blow immediately if the load is exceeded during startup.

Me, I would install the heaviest wire (10 or 12 Gauge) and then fuse for the load. With the heavy wire you will cut the voltage loss and never have to worry about you wire melting the insulation with it fused at a lower amperage.

15Amp seams to be the most common. I guess you could have one of those 12DC Coffee pots and that may take a larger fuse and If you installed (10 or 12 Gauge) wiring you could fuse up to accommodate it and still be safe by not exceeding the wire capability.
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Old 01-13-2010, 12:40 AM   #12
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Thank you to all who responded. Sounds like we're here to learn a thing or two from eachother.

My fuse terminal block uses the glass fuses, so that's what I'm working with.

So say I use 12g wire for a circuit, but the light I am using has skinny wires (18g) in the fixture itself. Is this difference relevant to anything?

Also, how do you join 12g wire to 18g wire? Those crimping type connectors are sized for only one gauge.


Fran
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Old 01-13-2010, 07:16 AM   #13
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Wire Nut.
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Old 01-13-2010, 10:11 AM   #14
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You really need some electrical assistance. By asking this question it tells me that you should purchase a really good "How To Do It" book and solicit someone with the experience so you will not hard boil your egg and/or kill yourself.
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