Proper Wire Gauge

[freightdog]

I'm looking to rewire the battery compartment on a Pearson 303 with the correct gauge wire. Currently it is wired with 2/0 AWG wire and is it very difficult to work with. I have a 2GMF Yanmar motor and a starter that is listed as 1KW. If I do my math right, Voltage x Amps= Watts, for a 12V system, I find that the starter draws approx. 85 amps. Thus, to move current a round trip distance of 20 feet, I'm looking at using 2 AWG wire. Does that sound correct that this starter would draw 85 amps? I'd like to use a 4 AWG wire if I can properly get away with it. Any thoughts?

[btrayfors]

Several reasons not to skimp here:

1. Starter motors may draw relatively little amperage when unloaded, but 3-4 times as much when loaded. For example, a Hitachi starter motor rated at 1.2KW will draw 60 amps unloaded and 200 amps loaded.

2. In a marine environment, you want to ensure that your starting cable are large enough to handle whatever amperage is deliverable by the batteries all the way to the starter and back with negligable voltage drop. In your case the total run is 20'...considerable.

3. While AWG2 might work in many instances when conditions are good (batteries fully charged, all connections clean and tight, engine easy starting, etc.), you can't depend on that and have to prepare for less-than-ideal conditions -- when you might really need that engine with no futzing around.

Personally, I wouldn't use anything smaller than 1/0 battery cable for that application, with top-notch connectors, crimps, adhesive heat shrink...the whole 9 yards.

JMO,

Bill

[Ziggy]

The peak current draw of your starter (according to the Yanmar installation manual) is 160A. According to ABYC, you need minimum AWG 2 wire to meet the 160A ampacity requirement in engine space. The resistance of AWG 2 wire is 0.157 ohms/1000', so a 20 foot cable drawing 160 amps will sufffer a voltage drop of 0.5 V. Your AWG 2/0 cable's resistance is 0.049 ohms/1000', for a 0.25V drop. Yes, 0.25V could make a difference with an almost dead battery. The AWG 2/0 is actually specified by Yanmar. Personally, I'd go with the smaller AWG 2 cable and try to make sure my starting battery is always in top shape.

[GordMay]

The 2/0 Cu, specified by Yanmar, is the minimum cable size for a 3% voltage drop on a 160A (12V Nom) starter at 20 feet total cable length.

Don’t make the mistake of “opinion shopping” until you find someone that confirms our natural (but wrong) bias towards cheap & easy. His opinion on “how little you can get away with” will likely prove wrong, in the worst possible manner, at the worst possible time (think Murphy).

See the wire sizing charts at:

http://www.cruisersforum.com/gallery...=500&userid=79
and:
http://www.cruisersforum.com/gallery...=500&userid=79

And read the article (post #1) at:
http://www.cruisersforum.com/forums/...ats-372-2.html

[hanschristian38]

The West Marine catalog has a nice wire size chart in it to find the correct gage based on distance (round trip) and draw.

[FrankZ]

You can also look at WireSizer from Alden Trull Yacht Electrical Design

Allows you to do some what ifs.

[hellosailor]

"The 2/0 Cu, specified by Yanmar, is the minimum cable size for a 3% voltage drop on a 160A (12V Nom) starter at 20 feet total cable length.
"
Gord, in essence you are saying he's wrong but also that he is right. Your spec is for a 160A starter, and he said he's got an 85A starter--HALF OF THAT. Which would make 2AWG excessive.

I'm the last one to advise skimping on wire, but once the numbers have been correctly run...that's that. There are indeed one-kilowatt starters, two kw starters, ten kw starters. They don't all need the same cables.

20 feet round trip with 4AWG cable and an 85A load would produce a 0.42V drop. Shifting up to a 2AWG wire would produce a 0.27V drop, a difference of 0.15V.

That's about a 15% difference in battery charge state, i.e. the 2AWG cable will present the starter motor with a battery that appears to be 15% more powerful than the same battery connected by the 4AWG cables. But either cable can safely accomodate the intermittent load form the starter.

The rest is horse racing: You pays your money, you takes your choices. Neither is "wrong".

[Ziggy]

Originally Posted by hellosailor

Your spec is for a 160A starter, and he said he's got an 85A starter--HALF OF THAT. Which would make 2AWG excessive.

Just to be clear, the maximum loaded current draw on the 1kW starter used on the Yanmar 2GMF is 160A, not 85A. The cable needs to be able to safely handle this current in the engine compartment, thus 2 AWG cable is the minimum needed for safety per ABYC. Yanmar recommendation of 2/0 AWG cable is more conservative, resulting in 0.25A less voltage drop at full load. I agree with Hellosailor that either 2 AWG or 2/0 AWG can safely accomodate the intermittent load form the starter.

As the OP points out, the 2/0 AWG is much more bulky and stiff. Of course it's his call whether the 0.25V voltage drop on a fully loaded starter is a price worth paying for using the smaller cable. Since on a sailing yacht, the engine is only an auxilliary means of propulsion and it's easy to keep a dedicated starting battery fully charged all the time, I personally would (and did) go with the smaller cable. If I had a single screw power boat without another means of propulsion, I would probably go with the more conservative cable size.

[David M]

Why argue with Yanmar, especially if your engine is still under warranty? Your calculations are being done for ideal conditions. A cold wire, a cold environment, no corrosion, nothing loose, a battery that is completely topped up and relatively new and a brand new starter motor that is capable of full cranking power.

Do you think you will always have ideal conditions? Why push your luck in order to save a little money?

Personally, I always over-engineer things on my boat by at least a small margin because stuff does happen. I cant tell you how many times its saved me.

[s/v Jedi]

Why not put the starter battery next to the engine?

I wouldn't care if my boat has sails or not... the engine should start even under the most difficult situations as you need it most when that happens.
We have two (!) 4/0 cables from the house bank to the engine emergency start switch which is mainly for the big alternators but this made me come up with an alternate solution in this case: calculate which size cable you need when you put two in parallel. Two thinner cables are much easier than a single big one. (easy enough because of that 85 or 160A discussion: two AWG 2 cables instead one AWG2/0)

cheers,
Nick.

[Martinini]

I'm chicken, I go with rated size and then buy and install 50% thicker. I don't like surprises when I flip a breaker or switch. When I go to start the engine I want to know it will. But than I bought a boat where some idiot wired the bilge float switch with a lamp cord!

[Strygaldwir]

What never ceases to amaze me is how electrical connections and such seem to move toward their least optimal state on a boat! Vibration, movement, heat, cold, moisture, they all seem to take a toll. Of course there is the notion that things never fail in conveint circumstance. Or, is it we never remember those circumstances. We just remember the time things didn't work or fail when it was most critical.

If you never expect to be in a critical circumstance, if you don't mind inspecting and cleaning on a regular basis, being aggressive about materials is probably not serious an issue. If you sometimes skip a maint cycle, or forget to go over things regularly, I'd over engineer.

[hellosailor]

"What never ceases to amaze me is how electrical connections and such seem to move toward their least optimal state on a boat!"
Amen!
I've gone to using NeverSeize on the actual connections, plus a locking bolt (NyLock) or other means of discouraging loosening. I used to go clean tight and potted with Liquid Lectric or a similar vinyl, but it is easier to redo a connection that just has NeverSeize on it and so far, just as durable and reliable.

[CharlieJ]

For hellosailor: Be cautious with NyLocks; if the stud gets hot enough, the nylon insert can deform and the locking feature of the nut could be compromised. I do agree with using an anti-seize compound (I use NAPA's) on all threaded connections; bus bars, power posts, battery switches, auto charge regulators, inverter/chargers, etc. Anything I work on that has threaded fasteners gets anti-seize. Only takes one galled 3/8" stud on a $70 bus bar to make one a believer.

[hellosailor]

Thanks, Charlie, good point about the NyLocks! I won't feel so crude using the split ring or star washers next time.<G>