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Old 22-08-2011, 12:34   #1
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A Little Confused About the Best Way to Wire My DC

I have a 3 switch beaker panel on my 30ft sailboat. I currently have NO fuses except for the fuse that comes from the alternator to the battery. My question is this. Do i need a fuse box or can i just rely on the breakers to jump if there is a problem? What is the point of a fuse box when cant i just run everything thru a breaker box?? seems like that would have less connections. I know these are kinda stupid questions. Im just kinda confused.
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Old 22-08-2011, 16:52   #2
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Re: A Little Confused About the Best Way to Wire My DC

A circuit breaker and a fuse are basically the same thing (in terms of circuit protection). The biggest difference is the breaker can be reset. You do not need a fuse AND a circuit breaker in the same circuit.
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Old 22-08-2011, 18:02   #3
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Re: A Little Confused About the Best Way to Wire My DC

Circuit breakers are generally used for protecting heavy cabling, well really to avoid a fire if the heavy cabling shorts out. Each time a cable size is reduced the short circuit capacity of that section of cable reduces so a 100amp fuse or circuit breaker at the battery end of a section of 00awg cable will protect that section of cable but a 15awg cable on the other end would fry and burst into flame in the case of a dead short without the fuse or circuit breaker tripping. So a hi rated circuit breaker or fuse at the big cables as close to the battery + terminal as possible, smaller fuses or circuit breakers where the smaller cables branch off like at a fuse box and then equipment protection rated fuses at each appliance. If this isn't convenient then individual cables for each appliance and a suitably rated fuse for each circuit/appliance can be used. Now you know why there are so many wires in a wiring harness :lol:
One easier method is to use the blue Sea fuse boxes close to each appliance cluster. The big cables connect to the links top and bottom and the smaller cables are fused as they run to each appliance from there. Makes fault finding much easier too rather than one central fuse panel and wires running from there.

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Old 22-08-2011, 18:19   #4
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Re: A Little Confused About the Best Way to Wire My DC

My background is aviation (airplanes) and automobiles. In both of those environments, the fuse/circuit breaker is there to protect the wiring (the circuit) and NOT the appliance/device (a common misconception). From my read of Nigel Calder, it is the same in the marine environment. Therefore, the breaker/fuse should always be sized to protect the wire. I have never heard of "equipment protection rated fuses" but, as I am fairly new to the marine environment, I am anxious to learn. :-)
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Old 22-08-2011, 18:36   #5
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Re: A Little Confused About the Best Way to Wire My DC

Correct, breakers and fuses protect the wire which also helps to reduce the chance of a fire. A load that tries to draw more current than it was designed to draw already has a problem.
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Old 22-08-2011, 19:16   #6
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Re: A Little Confused About the Best Way to Wire My DC

Maybe equipment protection was a bad choice of words, how about fire protection from an equipment fault. We all know the $1,000 piece of equipment will go to the after life long before the 20c fuse.

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Old 23-08-2011, 10:58   #7
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Re: A Little Confused About the Best Way to Wire My DC

I suspect we may be saying the same thing. In summary, your circuits should look something like this gross oversimplification (please forgive the word art):

Battery -> short wire -> breaker/fuse -> switch -> long wire -> appliance/device

The breaker/fuse protects the wire from over-current events (short in the circuit or device). The device itself has to protect itself from over-voltage events (i.e. run away alternator), a fuse/breaker is not able to help you here. That is usually the job of fast acting over-voltage detection circuit that usually cuts the main buss from the alternator or (arguably) a transorb on the breaker/fuse. However; over-voltage does not seem to be a concern in the marine environment - I have yet to discover why.
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Old 23-08-2011, 11:57   #8
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Re: A Little Confused About the Best Way to Wire My DC

You need a SEPARATE fuse/circuit breaker for EACH load.

Sounds like you only have the one input fuse for everything, not good...
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Old 23-08-2011, 12:11   #9
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Re: A Little Confused About the Best Way to Wire My DC

Yeah, commonlly on boats one breaker... say 20 amp, will run the house lights. It might not protect for a short in one light. You would need individual fuses for each light. I've never seen that done. Then you may have a breaker for "instruments" (VHF, GPS etc etc) each of those should have it's own fuse near the back of the unit. It's just the way it is. Having more than 3 breakers is handy though. There are some nice little panels out there with 8 breakers and a good digital voltmeter, for not much money.
I bought a very nice 8 breaker one with a switched voltmeter (for two baterry banks) off Ebay for about $130. He custom builds them, with the labels and amperages you specify. That was 2 years ago though.
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Old 23-08-2011, 15:24   #10
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Re: A Little Confused About the Best Way to Wire My DC

Unadar, you are correct, the fuse/breaker is sized to protect the wire. And in an ideal world the wire is also matched to the demand of the load.

More commonly, the wiring on a boat is oversized to prevent voltage drop (as opposed to the wiring in a car where that's usually not a factor) and if the wire can sustain 20 amps, but the dedicated load only draws 5, you would still use a 7.5A fuse/breaker and not a 20A one. That way you are still protecting the wire--but the fuse/breaker will blow faster during any overload, for any reason.
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Old 25-08-2011, 10:49   #11
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Re: A Little Confused About the Best Way to Wire My DC

I have a question?? LIke i said i have a 3 panel breaker box. All of the breakers are 15 amps.. Sure this would be perfect for the 2 cabin lights which are on 1 breaker. But the other 2 breakers are navigation and nav light which combined dont even draw 5 amps. SO the 15 amp breaker sure doesnt seem like its going to do much protection in case of an overdraw. I already bought a blue sea st fuse box that i have been tying everything into and im most likely removing the breaker panel and running everything through the 12 gang fuse block. So am i right in thinking the previous owner was just laxadazickle on his electrical.?
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Old 25-08-2011, 11:10   #12
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Re: A Little Confused About the Best Way to Wire My DC

also since all the breakers were jumped to one another at the positive wouldnt that make them have all the combined capacity, ie 15amps times 3?
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Old 25-08-2011, 11:20   #13
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Re: A Little Confused About the Best Way to Wire My DC

Quote:
My background is aviation (airplanes) and automobiles. In both of those environments, the fuse/circuit breaker is there to protect the wiring (the circuit) and NOT the appliance/device (a common misconception). From my read of Nigel Calder, it is the same in the marine environment. Therefore, the breaker/fuse should always be sized to protect the wire. I have never heard of "equipment protection rated fuses" but, as I am fairly new to the marine environment, I am anxious to learn. :-)
This is ONE use for circuit breakers, they can be used to protect a piece of equipment just like a fuse, obviously it cannot be a shared circuit. My boat has a panel with 40 DC breakers, many of which are sized to protect the equipment, not the wire. ( ie the wire has far greater capacity). I personally beleive that this arrangement is much better then loads of "hidden" inline fuses. In fact I would argue that such fuses have no place on a boat.

In more recent boats, or to save a few $, boat builders have produced smaller panels with circuit breakers reduced to protecting piles of common loads, in this case the primary protection is for the wire.

see E-T-A or carlings site for detailed usage of circuit breakers.

Dave
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Old 25-08-2011, 11:40   #14
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Re: A Little Confused About the Best Way to Wire My DC

I think that worrying about the wire would be a good thing
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Old 25-08-2011, 13:16   #15
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Re: A Little Confused About the Best Way to Wire My DC

so what happens if there is a short before the breaker( in between the battery and the breaker) will the breaker still trip? i just read it wont. so is there like a big fuse that i should have right before the battery?
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