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post #1 of 8 Old 11-01-2010 Thread Starter
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Trying to figure wire gauge

I'm rewiring an old boat. In doing so I'm removing all of the existing wire as much of it is corroded and very little is type III tinned copper. I have the formulas and tables for computing the 'wire size in circular mils' and arriving at the right AWG gauge if I know this one thing: the length of the circuit.

It seems to me that computing the length of the circuit for a circuit that goes from the electrical panel to the load and back to the negative terminal block is easy. When the circuit is more complicated it seems tricky. For example, I'm thinking that I'd like to have one breaker on the electrical panel for the cabin lights on the port side and one for the cabin lights on the starboard side. There will probably be as many as 5 cabin lights on each side as the boat is 48-feet long.

I don't think I want to do 5 home-runs from the electrical panel to each of the individual lights and instead should run one wire from the electrical panel to a terminal strip or even a fuse block that is amidships and then run wires from there to each of the lights. What I'm wondering is how to calculate the wire gauge I need.

I'm thinking that it's a thicker wire from the electrical panel to the fuse block that is capable of handling the load of all 5 lights running at once while each wire for each light can be thinner as 1) it only has to carry the load of one light and 2) the distance is shorter for the lights closer to the terminal strip / fuse block.

For example, 5 x 1.5 amp light bulbs times 20 feet from the electrical panel to the fuse block (40 feet round trip) requires a wire of 7764 circular mils or greater which means 10-gauge. The wire from the fuse block to just one 1.5 amp light 10 feet away (20 feet round trip) requires a wire of 896 circular mils which is quite small but since 16-gauge is the smallest one should use, 16-gauge is right.

So, assuming my math is right on the calculations for the circular mils, is it OK to plan on two different thicknesses of wire: thick wire from the electrical panel to the terminal strip / fuse block and thinner wire from the terminal strip / fuse block to the actual load?

Being new to the forums I sure hope I'm doing this posting thing correctly and apologize if I'm not.

Spoon
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post #2 of 8 Old 11-02-2010
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spoon

your thought process is good, but for the boat - perhaps too theoretical...

you will have the same voltage present everywhere on the lighting circuit (whether 12VDC or 110VAC) and each will have the same current capacity - ie sparks will fly...if shorted to ground. Any load on the circuit has the ability, and will if given a chance, to sink every AMP available until the fuse blows or the wire melts, or the fixture fails.

the idea of one size out and a smaller size back...you have neglected the safety aspect of the return MAY have to carry the full load at some point. as far as the wire is concerned...use the same size both ways. BTW your theory is correct, just not practical in the real world.

16 would be a tad small for 7.5 amps, safety, line length loss, connector loss, etc. 10 would be overkill...besides it is tough to get 10 in the little light fixture.

And one "homerun" or circuit run to the first light, then to the second and so on will do nicely. You may run into issues fusing the same circuit twice. Having hot terminal strips throughout the boat is not a good idea either...every where you break the circuit will be a potential problem, both electrically and corrosion wise. Not to mention that it must be protected from other systems and human contact so that nothing shorts out what is there.

Buy a roll of #14 and #12 stranded for the cabin loads, and a handful of good name brand connectors, some heatshrink and a decent crimp tool..oh, don't forget the fuses or circuit breakers

for higher capacity loads - check one of the better 12 volt bibles/marine wiring books from the library or buy one and work from there.

You are on track to a successful project, but don't caught in the overthink.
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post #3 of 8 Old 11-02-2010
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I don't have a wire gauge table, so will use what I saw online for a table. Seems like if you use 5 10w bulbs (typical Xenon) then a 40 foot RT would be about 12 ga. That may not even count fot a 10% voltage drop which is acceptable on lights. So you could pull 12ga for it.

However, how I would (and have) done it is that I like switches. I would probably pull a 10 like you suggested, to a switch/fuse box that was conventiantly located and make sure it was fused. I would then pull my switches for the boat off of that and use a smaller ga (add it up, but I bet you will get away with 14ish). Make sure every leg from the switch is fused seperate.

Also, if you are going to put fans on the circuts, now is the time to plan for them. Many peopleput them on the same circut as the lights. So it might be worth your while to up to 12 ga with the fans. Just another thought.

My opinions.

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post #4 of 8 Old 11-02-2010
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Wire Size Calculator (LINK)

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post #5 of 8 Old 11-02-2010
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Nice Link!!!

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post #6 of 8 Old 11-02-2010
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I'd recommend going with LED lighting as well, since it will drop your electrical usage considerably.

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post #7 of 8 Old 11-03-2010
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Here's another wire sizing link: The West Advisor: Marine Wire

Our 34 foot boat has 14 ga wire for lighting. It's too small, 'cuz the lights dim if you turn on more than one.

Also remember to wire in parallel not series, 'cuz if one blows it'll be like old Xmas tree lights!

The negative (return) wire has to be the same size as the positive wires going out - it's all one circuit.

Stu Jackson, C34, 1986, M25 engine, Rocna 10 (22#)
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post #8 of 8 Old 11-07-2010 Thread Starter
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Great replies. thank you everyone. Just so you don't think I'm totally stupid, I never intended to say that I figured the return wire would be smaller. The out and the back need to be the same size as, like you say, they are all on the same circuit.
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