beginner's DC wiring questions

[Buddy_Y]

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!

I'll answer a couple here and let others go from there. The good news is that you are mostly on the right track.


GROUND POINT ON ENGINE:

An outboard doesn't have a seperate ground point other than the cable running to the battery negative. Your main "ground point" will be on the boat.

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.

I am going to strongly suggest that you install a main battery switch. It is a safety issue. If you have an electircal fire in the panel, you want to be able to turn off the power. It also makes it a lot easier to work on if you don't have to disconnect the 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)?

You might be confusing the ultimate ampacity of the wire with it's design capacity. The ultimate ampacity is that current which heats the wire to the point of fire danger. That's the 160 amp number for 4 AWG wire. You would never try to push this much current through the wire in normal operation, becuase the resistance would cause very large voltage drops. 60 amps is more like the working capacity of 4 AWG wire, although that number does depend on the length of the wire. The fuse is there to protect the wire and prevent it from starting a fire. To avoid annoying un-necessary fuse failure it shoud be as high a rating as needed to prevent the fire, hence the 160 amp number for the fuse. Note that that is independent of the length of the wire.

IN LINE FUSES

You should NOT rely on the inline fuses to protect the wire. They are their to protect the equipment. If there is a short circuit between the panel and the fuse you and/or your boat will be toast as the wire melts and ignites. These kinds of short circuits are not rare on boats where wires move and chafe. Your insurance company would object strongly to unprotect wire runs, and rightly so.

Your wire gauge calculations seem reasonable. But I wonder about the amp used you are getting for the bulbs. You stern light for example. You suggest it is 1 amp. That is only a 12 watt bulb. I am pretty sure that will NOT give you a 2 mile visability. I think you are looking at a 25 watt bulb for that application, but I am going strictly on memory here.

Best of luck! Remember, if you DIY you will take more care than anybody else.

Bill

[GordMay]

See the linked Charts and Article:

Wire Size Chart.1 (Table)
http://www.cruisersforum.com/gallery...=500&userid=79

Wire Size Chart.2 (Notes re Table)
http://www.cruisersforum.com/gallery...=500&userid=79

"Ohm's Law & Boats"
http://www.cruisersforum.com/forums/...boats-372.html


#4AWG has a current carrying capacity of 160 Amps (136A in Engine Room). A 160A fuse, located at the supply end (Battery), will protect #4AWG Copper cable.

Assuming a maximum desired Voltage Drop of 0.375Volts (3% @ 12.5V), #4AWG cable is suitable for 1,250 Amp/Feet - meaning 62.5 Amps over a return distance of 20 feet (Pos + Neg total length).

Your calculated load is 2.0 A Water Pump + 0.1 Binnacle Lite + 0.8A Anchor Lite +5.2A Electronics + 2.0A Nav Lites + 12.5A Cabin Lites (?100 Watts?) = 20.6 Amps @ 12.5V.
I would confirm all actual loads, prior to installation.

Allowing for future additions & slight miscalculations, I’d design for a minimum main circuit load of 30Amp, requiring #10 AWG Copper cable, over the very short distances you cite (perhaps #6 AWG, if circuit length is up to 30Ft).

Larger wire sizes are ALWAYS better, except for very minor cost & weight implications.
#16 AWG Copper (miinimum I’d ever use) is suitable for 75 Amp/Ft,
#14 for 100A/Ft,
#12 for 150 A/Ft,
& #10 for 300 A/Ft.

Each of your described circuits calculates to less than 75 Amp/Feet, so #16AWG Cu would be minimally acceptable; but Larger wire sizes are ALWAYS better, so you might consider using all #14.

As Bill noted, the addition of smaller in-line fuses, after the circuit breaker provide additional protection to the equipment. Smaller overcurrent protection (breakers & fuses) is ALWAY permissible, providing it is not so small as to cause nuisance tripping.

HTH

[Stenn]

Hi Buddy,
....yep, that was a long one....and leaves a LOT to reply to, which is pretty daunting....GreatKetch is right....it'll probably have to be a group-effort to respond to everything.....

First of all, I'd like to refer you to a GREAT guy on eBay, that I bought all my electrical wiring from this past summer....I'm a Quality Engineer in the manufacturing world, so I researched his stuff first....all First-Rate, High quality stuff...amazingly affordable...

....do a search for this eBay "Store".... Greg's Marine Wire Supply

I have an '83 Newport 27-S MKII, that I just got in Sept. of 06 (with in-board Universal 5411 diesel) and I had to re-do some of the wiring as I tried to install modern electronics this past summer, and found some of it (non-original) completely inadequate to the job! What first tipped me off was when my new DSC VHF kept displaying "Low Power" on it's LCD screen every time I transmitted...til I traced the supply and found it being fed with solid-conductor 22 gauge wire!! And the same sort of thing was happening when I tried to fire up my new Lowrance LMS-525c GPS/sonar unit, and it kept rebooting on me.....that was being fed by 22 gauge too! I replaced both feeds with their own 10 gauge supply lines (bought from Greg's), and all is well.

I'll respond to a few points.....

First of all, when talking about a ground-point, and wondering about using your outboard as a ground-point, I guess we could say "all engines are not created equal." Or, maybe I should say, an Outboard is not equal to an inboard's block, when talking about "Ground." The typical "ground" to an engine being talked about in Casey's book is referring to metallic mass of the block, and the connection down through the prop shaft to the water, not just a simple electrical connection like "Positive" and "Negative". ...so the block on an inboard engine represents more of a sufficient mass to be a central ground point that everything, including the batteries, can be tied to, to prevent differences of potential between feeds, the batteries, and devices. An outboard doesn't really perform that mass "function," and you don't leave it dropped down in the water all the time, so the outboard could be thought of as just another "load" running off the batteries, sort of like your VHF or sonar. Casey's book has some safety cautions about the dangers to a swimmer outside your boat, potentially being electrocuted if you have your AC wiring not quite right, because you have that link to the water through a typical inboard. In your case, with an outboard, you don't have that connection. So your battery Negative terminals are all the "Ground" you have. The through-hull ground "strips" you're talking about not seeing on the outside of your hull are not on my boat either....I haven't seen them that often, ...I think Casey even mentions something about builder's not really designing electrical systems as well as they could be doing, particularly for radio transmission and lightning protection.

I agree with GreatKetch about installing a battery switch too. You'll really regret it if you accidentally drain both batteries on an over-nighter or longer, when they're connected in parallel, and have nothing to get your engine going in an emergency. I guess, in that case, you're lucky you have an outboard, because you could probably pull the case off and get it running manually with a pull-rope, but, again, why put yourself in that position? I just replaced my original 25-year old Perko switch this past summer, with a better-made, higher amp "Blue Sea" 9001E switch, for $35.....I thought that was pretty reasonable (and that was from West Marine too...not always one of the most affordable places!) They are designed to have battery cables bolted right to them, and also functions as a handy "jumping off point" to feed up to your breaker box.

About the wiring over the ceiling....that's a pain if you're finding that they glued it down....mine's laying loose above the ceiling, but I found I had to loosen a number of panels before I could pull it, because they were laying across ribs that the panels screwed into, so they were held in place by the panels as they crossed the ribs. I guess you'll have to buy one of the "wire-fish" rolls electricians use to pull ("fish") wire through walls?

My last point is in response to your multiple deliberations over what gauges to use where, on which lines. Why bother with all of that? I was surprised to find how manageable/flexible and routable the marine 10 gauge was that I bought from Greg, so I just bought 100 feet of it (50/50 of black and red) and used it for everything (50/50ft was enough for my needs, I wasn't rewiring the whole boat like you are). Greg does have the stuff that's both positive and negative together, encased in a sleeve, but I didn't use that stuff, just bought a bulk roll of Red and Black.....and No, didn't use Casey's suggestion of Yellow because there was too much pre-existing red and black in the boat, so decided to just keep it all uniform. And speaking of buying in bulk, remember that you can double-up on lines that need more than 10 gauge, 2 runs of 10 gauge = 5 gauge. You might not physically be able to do that all the time, but it's worth keeping in mind...because that big-gauge 6 or 8 gauge stuff gets expensive!

I don't envy you the magnitude of the job you have ahead of you!

"Endeavor to persevere"

Stenn

[GordMay]

Parallel conductors must be de-rated to 80% of their additive ampacity.

So 2 x #10AWG (30Amp*) would NOT equal the ampacity of #5, but actually about 48 Amps (2 x 30A x 0.80).

* ABYC allows higher ampacities than shore-side codes allow.
I recommend using the more conservative NEC (NFPA 78) specifications.

[Stenn]

Thanks for that input. I haven't heard that before.

You say "....must be de-rated by 80%....", but you don't say why. Do you mean "must be" because a safety code dictates it, such as when using two conductors encased together in a protective insulation sleeve, or are you talking about the actual amp-capacity of 2 10-gauge conductors together does not actually equal one 5 gauge? I'm only talking about basic capacity, not somebody's code.

Also, here's another example of how I used the "doubling over" idea.... in cases such as when I was wiring my 1500 watt inverter, the docs said to use an 8 gauge ground wire to the block. But since I already had all that 10 gauge, I just used two lengths of 10 gauge together, and it's fine.

At any rate, I was just suggesting a concept for economizing and simplifying his wire-gauge purchases, doubling over one gauge where necessary (one line somewhere, maybe?, not all over the boat), since it looked like he was actually thinking of using so many different wire-gauges, all based on that load's needs. It almost sounded like he thought he MUST use the small gauge indicated on a chart corresponding to the load he's trying to power. I was basically echoing your point that there's no reason not to use something bigger than the load really needs, so he didn't need to be bothering to figure out how many feet of 16, how many feet of 14, 10, etc. he should buy.....just buying bulk of something like 10 or 12 could be used widely.

Realistically, how much is "Book Perfect" on a boat? Don't we all make do with what we can, or what's available (unless we're rich) ?

[Alan Wheeler]

You say "....must be de-rated by 80%....", but you don't say why.

That's because if you parrelleld two conductors, you have only doubled the diameter. Cable is measured in square area. So two 16mm2 cables parrelleld does not equall one 32mm2 cable. )Sorry I use mm2 not gauge. Please remember your gauge No's. work the opposite way).
However, parrelleling conductors is not a good idea. This is because the current is not equally shared across each conductor. You only need a slight difference in resistance between the two, due to a not so good connection, slight difference in wire length, or what ever, and the cable with the least resistance ends up carrying the greater load. In very high current conditions, like say a starter or winch or battery charger, one cable can end up carrying a great deal of load. The result is that it can heat up and melt because it is not double the current carrying capacity, in fact it ends up not even as double the diameter capacity as it gets hotter.

[Stenn]

OK, I see the possibilities you're talking about.

But as far as slight differences between conductors causing imbalances in current....I was talking about cutting two identical lengths of the same, identical 10 gauge wire, and crimping and soldering them into a shared 4 or 6 gauge end-connector on each end, and only if I absolutely had to because I didn't have and/or couldn't afford anything bigger....not making a standard practice of wiring the whole boat that way....

Of course I'm only talking about my boat, which is only 27', so these lengths are not like wiring some big boy like a 40-something footer....I doubt I'm pushing any theoretical limits.

Stenn

[Alan Wheeler]

Yep you should be fine. Just don't connect up your starter motor with two small wires together thinking you have one big one. You'll have one big one alright, but it won't be the wire :-)

[Cowboy Sailer]

Stenn,
The safe current in a conductor is set by the temperature rise in that conductor. Two wires next to each other somewhat add their heat output to eachother and therefore the current limit due to heat buildup is somewhat less than twice the limit for one.
One point to add to Wheels comment about using doubled conductors to increase the current carrying capacity. (notice I did not say double the current carrying capacity) Since the temperature coefficient of resistance for copper is positive as one wire begans to carry more current its resistance goes up and tends to self-balance the current carried by each so it is really a pretty good technique.
I guess that I should that my education is electrial engineer. I came to be a missionary after retiring from that field.
Good luck on your project. You will get lots of mainly good advice on this forum.

[Stenn]

Thanks fellow sailors.

Alan, I was certainly NOT talking about using this technique on something critical like my starter line......just something intermittent and non-critical like a high power stereo or VHF maybe.

Hmmmm, self-balancing current paths through heat/resistance increase, eh Cowboy? Interesting and logical characteristic.

Again, this is not something I was professing should be done on a regular basis, just a nice-to-know technique in a pinch. But I would stress the need for the shared connector, and both crimped and soldered. I definitely see Alan's point if multiple connectors were used with multiple conductors.

Stenn

[Alan Wheeler]

An important point here. The choosing of a cunductor size should not be based on it's maximum current carrying capacity. (Yeap I know you were not infering that Cowboy, just thought I would make it clear). A conductor under normal operation should never get warm, let alone hot. If there is heat coming from a conductor, it is simply wasted energy, disappating as heat. That is prescious battery energy. We want as much of that battery energy to get to the powered device as possible. So over specing cable is actually the best. Get as much power to the device with the least amount of waste energy.
When it comes to DC, most all systems we will ever come across are usually 12V and 24V. 12V being the more common. Power is the energy available to do work. Work means like turning a motor. More power means the motor will work to its maximum potential. To little energy and the motor will not spin as fast or may even stall compleatly. Or a light may be dim. Power is the result of Voltage times Current. So if you have less voltage, the power also drops. A conductor that is too small will have difficulty delivering the current and thus the power does not get to the device, resulting in it not operating to its fullest potential. (the light went out looking for itself :-)
The only conductors that should ever get warm, is the extreme high current ones powering things like the starter or anchor winch when under heavy load. But these are short term loads usually.

[Alan Wheeler]

Hmmmm, self-balancing current paths through heat/resistance increase, eh Cowboy? Interesting and logical characteris

Yes and no. The point that copper starts to change it's resistance due to heat is way above the heat tolerance of the insulation. So the insulation would have melted on the unbalanced conductors first, causing a few other issues.

[Cowboy Sailer]

Buddy Y,
Good luck on your project. I didn't mean to get into a physics discusion with Gord, Stenn and Wheels and hijack your thread but I just won't let good enough alone.

Stenn, you made a good point about doubling-up overstressed wiring.
Gord, as usual you made knowledgable contributions that should be very useful to the "beginners" of us all.
Alan, the point at which copper starts to have a more or less constant, linear temperature coefficient of resistance is about 15 degrees Kelvin or -258 degrees Celsius. That is mighty cold! The coefficient is about 3.9 X10-3 (.0039) This results in a non-negligible resistance change with temperature, even at low temperatures like a marginally sized conductor (carrying a heavy for its size current) might experience, way below the point where the temperature would damage the insulation.
The change in resistance for a ten degree change in temperature is about 3.9 percent. Thus the current in the hot conducter is 3.9% less than the current in the cooler conductor! The heat produced is porpotional to the current flow squared. In this example the heat produced in the hotter wire would be 7.8% less than the heat produced in the cooler wire That is the self-balancing mechanism.
The difference in resistance caused by slightly differing lenghts of wire or terminal resistance variability is way less than that. They might contribute a 1% difference. That is about what a 3 degree temperature difference would cause. By the way, If the two conductors of a doubled wire system are routed real far apart the current carrying capacity is not derated to 80 percent;it really is twice the capacity of a single wire.
Grins to all, in a little while we cast off the lines.
Jerry

[Therapy]

Originally Posted by Stenn

and both crimped and soldered.

Stenn

There was another long thread a while back where a lot of folks had expert advice.
One piece of was that, for various reasons (and I cannot remember them all) one should NOT solder. Only crimp.
No solder.