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Old 09-10-2009, 10:20   #1
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Fuse or Circuit Breaker ?

I'm installing a 12v bilge pump as a sump pump in my house.

I was preparing to buy an inline fuse holder when someone suggested using a circuit breaker so I wouldn't need to look for a fuse should one blow. Seems to make sense, so are there any reasons not to ? For that matter, does it make sense to use circuit breakers rather than fuses on a boat ?

The rated draw of the pump is up to 2.5 amps, and the recommended fuse size is 3 amps, which strikes me as awfully close to the rated draw. Is this typical for a pump? The smallest circuit breaker I find at NAPA is 5 amps, which I would think would be fine. Am I right ?
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Old 09-10-2009, 11:02   #2
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Gee, i'm surprised nobody has answered this yet. According to NEC code the fuse should be rated according to type of wire you use and total loads should not exceed this rating on the circuit. 12 guage wire with THHN insulation is rated for 20 amps and thus would have a 20 amp breaker or fuse. A breaker is about 15 bucks, a fuse about 1 buck. Since you should not be blowing a lot of fuses I would go with the fuse. If you have other loads other than the pump maybe a breaker would be warranted as it is an easy disconnect device.

My 3 cents.
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Old 09-10-2009, 13:24   #3
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Motors need to have protection at the level given by the manufacturer as many will draw more current in a locked rotor condition and can at least get very hot if not catch fire. At least on a boat since usually the only device on that wire is the motor, one fuse/circuit breaker will protect both the wire and the motor, if the protection is the value given for the motor.

I don't know if there is a reason not to use a circuit breaker over a fuse.

John
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Old 09-10-2009, 13:56   #4
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John is right. Do NOT use a breaker and absolutely do NOT use a 20A fuse. Use the fuse size recommended by the manufacturer.

Why?

Because a small motor with it's rotor locked can heat up to the point of ignition (fire). We recently demonstrated this at a West Marine seminar, using a small bilge pump with a fuse just slightly larger than the recommended one.

If you're really worried about fuses blowing, then keep some extras at hand. They're cheap :-)

Bill
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Old 09-10-2009, 22:59   #5
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Thanks all. Sounds like I need to stick with the recommended 3 amp protection. This would rule out circuit breakers unless I can find one smaller than the 5 amp one, which cost 3 or 4 dollars, BTW.

Actually, the more important reason suggested to me supporting a breaker was that if the motor got hot from having to run continuously and blew the fuse,if I'm not around, the fuse won't be replaced. Had it been a breaker, it would automatically reset after cooling, and could continue pumping. Perhaps it doesn't work this way, and perhaps these automatic reset breakers aren't for marine use to begin with.

This is not the typical scenario for a boat, but the amperage recommendation is certainly valid. I'll make sure I keep it at 3 amps, either way.
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Old 15-10-2009, 16:41   #6
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Quote:
Actually, the more important reason suggested to me supporting a breaker was that if the motor got hot from having to run continuously and blew the fuse,if I'm not around, the fuse won't be replaced. Had it been a breaker, it would automatically reset after cooling, and could continue pumping. Perhaps it doesn't work this way, and perhaps these automatic reset breakers aren't for marine use to begin with.
Absolutely correct, marine circuit breakers, as well as household CBs are trip free. When they trip, they cannot be reset until the fault has been cleared.

Be especially careful about the placement of the CB or fuse in the B+ conductor. It needs to be as close to the source of DC power connection as you can achieve. Ask a qualified marine electrician about the 7-40-72 inch rule and take his advice.

Hope this helps.
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