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Old 17-03-2013, 16:39   #1
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j29 re-wire

about to set off on my spring project of re-wiring my J-29. it's a relatively simple 12 volt DC setup, with an outboard engine, so no alternator. attached is my plan. looking for any feedback to make sure i don't have to christen my fire-extinguishers. i've read don casey's book cover-to-cover, and have delved into the relevant chapters of calder's. have also browsed the web trying to dig deep into my college physics memory of circuits and ohm's law. my specific questions are with regard to choice of wire gauge. i can figure out the individual circuit gauges using the 3% tables, but i want to verify the size of the battery wires... do i just add up the amp-draws of the entire curcuit? my plan is to go with 4 awg to and from batteries to busbars, 6 awg for nav lights and bilge pump, 10 awg for cabin lights and mast light, and 16 awg for instruments (which draw a miniscule current). does that sound reasonable???
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Old 17-03-2013, 17:24   #2
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6 sounds way too heavy for nav and bilge pump. Even regular lights don't draw much, LEDs draw almost nothing
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Old 17-03-2013, 17:26   #3
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Re: j29 re-wire

I agree, up the size of the battery connections and lower to 10awg for the bilge and nav lights.

And yes, circuits are sized for their max current draw accounting for the length of the run.
Let your heart tell you where to go, but let your brain tell you how to get there.
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Old 18-03-2013, 04:46   #4

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Re: j29 re-wire

Greetings and welcome aboard the CF, amtdys.
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Old 19-03-2013, 01:15   #5
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Re: j29 re-wire

thanks for the advice! hope to get started this weekend.
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Old 27-03-2013, 11:41   #6
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Re: j29 re-wire

Check the actual draw listed on the devices you choose. It's easy to say generically what they require but it's best to check the equipment spec. If you are reusing installed equipment you may want to pick up a multi-meter with a current clamp and check what the equipment is currently drawing. I picked up a meter with a DC clamp (make sure it's rated for DC many at the Home Depot and Lowes are not) for about $60.00 this fall and find it invaluable around boats and campers. Mind you this price won't get you great accuracy like some of the Flukes I have at work but for this type of thing they are wonderful. There are ABYC wire charts online and blueseas also has a wire calculator on their site. Circuit Wizard - Blue Sea Systems
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Old 27-03-2013, 17:46   #7
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Re: j29 re-wire

Two observations:

Your branch wire sizes look way too big. Even your 4 gauge main feeder looks big. I would expect 8 to 10 gauge for the main and 14 gauge for most branches. And I doubt seriously if your bilge pump pulls 15 amps. Most pull 6-8 amps.

Your fuse sizes look like they are sized to protect the load. The fuse should be sized to protect the wire. Increasing the size will limit spurious trips.

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Old 27-03-2013, 17:59   #8
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I am currently rewiring my boat, using much smaller wire in general than you describe. I have led lights and am using 16 for them. I'd check those numbers.
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Old 28-03-2013, 06:10   #9
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Re: j29 re-wire

Yeah, seconding the comments to reassess some of those load currents before investing in wire.

other comments:
- you only require one 30 A fuse in the battery run. Omit the fuse to the right of the battery switch
- I would upsize the wires between the battery and the bussbars, and select a battery fuse type that you could later upsize... to permit future expansion.
- why not install a small DC breaker panel instead of fuses and switches?
- add a circuit for your VHF radio. How 'bout a stereo
- I like to add a cigarette-lighter-type 12v socket or two on a 10A breaker for stuff like a 12v cooler, fans, spotlight, trouble-light, computer charger, small (75w) plug-in type AC inverter, 12v to USB for phone charging, etc etc.

If you could update your drawing with rough estimates of the wire runs, it would be easier to double-check your wire gauge choices.
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