beginner's DC wiring questions

Posted By: Buddy
Date: 1/17/08 10:27a.m.

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Hello Everyone! I'm afraid this will be a long one.... I recently purchased my first boat, an outboard-only 1977 Catalina 27, and I’m in the midst of rehabilitating my poor neglected lady. I’ve just turned my attention to the wiring and (having already had a professional completely replace the AC system), I want to update the ancient DC wiring. I have some very basic beginner’s questions, mostly about wiring gauges, and I’m hoping somebody can point out my blunders before I start rewiring. First off, let me say that I’ve bought and read Don Casey’s “Sailboat Electrics Simplified” and Calder’s “Boat-owner’s Mechanical and Electrical Manual”, taking notes, and I have mapped out my boat’s original wiring—but that’s pretty much the extent of my knowledge. Like all book-read beginners, I’m surely mixed up on some utterly basic points. My new wiring will be modern tinned boat cable. That’s pretty much the only point I’m confident about. GROUND POINT Is there a ground point on an outboard model Cat-27? I’ve traced my wires and do not seem to see one. When I did the hull during a recent haul-out, I didn’t see any strips or signs of through-hull corrosion. Is a ground point necessary? BATTERY TO DC PANEL I do not have a battery switch—the boat only has an outboard motor, and my new late-model smart charger handles both batteries simultaneously, maintaining them. My two deep cell batteries are wired parallel. Thus, I am just connecting my house battery directly to the DC panel. The amp total of my DC panel’s breakers is 60 amps, and the round trip run from my batteries to the panel is 16 feet (including allowances for the circuitous wiring route). One of my books’ wiring charts (labeled “allowable amperage”) suggests 10 gauge wire, but another chart based on the “round-trip length of conductor” suggests 4 gauge wire (due to voltage drop, as I understand). My guess is that 4 gauge is correct. Or is this overkill? Also, I am going to add, per the books, a high amperage fuse just after the positive pole of my battery. Casey suggests that this particular fuse should be “as large as the capacity of the cable allows,” which would be 160 amps in the case of 4 gauge wire (according to one chart). Is 160 amps right? Seems too big to me. Should it be 60 amps (per another chart)? DC PANEL CIRCUITS My new DC panel has 6 breakers (2 X 5 amp; 2 X 10 amp; 2 X 15 amp). Tentatively, I was going to use inline fuses only on the VHF and Depth Sounder, and not on my: water pump; anchor light; compass light; running lights; cabin lights. Is that a mistake? Here are the six panel circuits, and my tentative guesses at wire gauge size. 5 amp breaker: Fresh Water Pump (small pump from tank to galley sink) – Round trip approx 14 ft, and I estimate the amp use as 2 amps. (Notably, I see 6 amps listed as the draw for freshwater pumps in a couple charts, but this pump is just a tiny unit...?) Therefore, 16 gauge wire? 5 amp breaker: Compass light. Only .1 amps and merely 8 feet round trip, so 16 gauge? 10 amp breaker: Anchor Light. The light is only .8 amps, but I’m guessing that the round trip run is about 80 feet, therefore 14 gauge? 10 amp breaker: VHF plus Depth Sounder. A VHF radio spikes to 5 amps on transmit, one chart says. Thus, 5 amps plus .2 amps for sounder is 5.2 amps total. Round trip is only 14 ft. Therefore, 14 gauge? (Each of these instruments will also have an inline fuse, per the specifications of the unit.) 15 amp breaker: Running Lights – My guess is that the bow red/green lights are about 2 amps total, and the round trip to the bow is about 40 feet, therefore 14 gauge? The stern running light I am guessing is 1 amp, with a round trip of 12 feet, so 16 gauge? The mast steaming light is maybe 1 amp, with a round trip of 70 feet, so 14 gauge? 15 amp breaker: Cabin Lights – My problem here is that I cannot access most of the wiring because it runs inside the ceiling. I thought that I’d attach a pull string to each wire, pull it out, and then use the string to pull through the new wire; however, the old wires are sealed inside the ceiling for much of their run and won’t budge, i.e., it feels like they’re fused to the interior somehow. So, I can tidy up the nest of ugly old black-tape splices wherever the wiring is accessible, but it seems the rest of the wire will have to be left intact. Suggestions? Thanks in advance to anyone who can point out my inevitable wrong-mindedness!