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Old 20-07-2010, 08:47   #1
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Battery Cable Size on Larger Cats

I am putting together an electrical system for my cat and will have some long battery cable runs. The longest between house and start banks is 24 (7M). The alternators charge the house bank with Xantrex Echos to charge the start batteries so in order to keep voltage drop low I will need to use 4/0 cable. Calder mentions that cables larger than 2/0 are not normally used in small craft but he was writing in an age (5 years ago) in which cats were not common. I am just curious as to what size battery cables members with larger cats are using.

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Old 20-07-2010, 09:02   #2
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See ➥ Wire Size Chart.1 - Read /w "Ohm's Law & You" Cruisers & Sailing Photo Gallery

And ➥ Wire Size Chart.2 - Read /w Wire Size Chart.1 & "Ohm's Law & You" Cruisers & Sailing Photo Gallery

A 4/0 cable, in a 12V circuit, 100 ft long (total Pos + Neg), is good for about 69 Amps @ 3% VD.
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Old 20-07-2010, 09:03   #3
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Not an answer really: Put a minimal starting battery next to each engine. Cable is very heavy and very expensive. Downside is there is no immediate cure for a dead battery, but that should never happen.

Those Xantrex Echo's are a good idea. I've seen some spectacular and unexpected failures when the twin engine charging issue is not handled properly.
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Old 20-07-2010, 10:24   #4
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I agree with daddle... you could get by with a significantly smaller (cheaper) battery if it was within a couple of feet of the engine starter. Your house set is obviously a different critter.
Make or buy a nice battery box to restrain them properly and since the engine compartment is, or should be, vented with fresh air regularly the off gassing is not an issue either. Try to mount them where you can get access to them easily in case of maintenance or replacement being necessary.
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Old 20-07-2010, 10:49   #5
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My bad for not being more clear in my initial post, but I do plan on small start batteries next to the engines. The heavy cables are for the run from the alternators to the house bank. They would also be used to start the engines in case of a problem with the start batteries. Here's a preliminary diagram to give an idea.
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Old 20-07-2010, 11:28   #6
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You might want to consider one of those inexpensive battery boosters. We use them at work all the time and haven't had a problem.
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Old 20-07-2010, 13:02   #7
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You have two engines. One of them will start. You have sails. Maybe you're making this too complicated?

If you're goal is to have the lightest cat possible you do one thing. If not, then the other. It's not that much wire or weight, really. Wait until wife loads all those useless kitchen gadgets (garlic press?) and you load all the spares and tools you'll never use...all that stuff will weigh a hundred times what the wire does, and cost far more and you'll never even think about it.
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Old 20-07-2010, 14:36   #8
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Quote:
Originally Posted by mikereed100 View Post
I am just curious as to what size battery cables members with larger cats are using.
My boat is 48 feet and has a 24VDC house bank. The biggest cables are 1/0. The batteries are in the hulls near the alternators, so smaller cable there. The inverters pack more juice and are centrally located near the generator, so that's where the heavier cable is.
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Old 20-07-2010, 22:06   #9
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Quote:
Originally Posted by mikereed100 View Post
My bad for not being more clear in my initial post, but I do plan on small start batteries next to the engines. The heavy cables are for the run from the alternators to the house bank. They would also be used to start the engines in case of a problem with the start batteries. Here's a preliminary diagram to give an idea.
I have a similar set up. The answer depends on the max current from the alternators, the length of the run from the alternator to the house battery bank, and the %voltage drop you are prepared to accept. Then the tables posted a few posts back will give you the answer as to the cable size.

Mark.
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Old 21-07-2010, 08:24   #10
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Quote:
Originally Posted by JayH View Post
My boat is 48 feet and has a 24VDC house bank. The biggest cables are 1/0. The batteries are in the hulls near the alternators, so smaller cable there. The inverters pack more juice and are centrally located near the generator, so that's where the heavier cable is.
That's cheating! A 24V system would solve the heavy cable problem but just isn't in the budget at this time.

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Originally Posted by mark_morwood View Post
I have a similar set up. The answer depends on the max current from the alternators, the length of the run from the alternator to the house battery bank, and the %voltage drop you are prepared to accept. Then the tables posted a few posts back will give you the answer as to the cable size.

Mark.
So what size cables do you have on your boat? I have already worked out what I should have (4/0 for 100A alterenators and 48' run gives a 2% voltage drop.) I am too lazy to do calculations and don't like tables so I used these:
Blue Sea Systems
Genuinedealz - Technical - Calculators
I have no problem with using 4/0 cables and am happy to do so. They are not that much more weight and I can pick them up at Genuinedealz for less than the cost of lamp cord at WM. It's just that I have never seen them used other than between batteries in a bank and am curious as to whether folks with similar sized cats are using them or are using smaller cables with the attendant voltage drop and higher strain on the alternators. My suspicion is that production builders may use smaller cables and pass the cost of alternator rebuilds on to their valued customers. On the other hand, maybe a 10% drop in the alternator cable is acceptable?

Mike
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Old 21-07-2010, 08:32   #11
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How far is it from your house bank to your start batteries and how much current do your start motors draw?

Having a backup is great but it sounds like the wire gauge is making this impossible. You do have a cat with two engines, so this is a consideration to take in not having any engine start battery backup. The chances of having two dead start batteries are very slim if they are properly isolated.

If you decide you do indeed want the backup then perhaps keep a spare start battery somewhere on board where you could use it to swap out a dead start battery? The spare start battery would probably weigh less and cost less than running 4/0 cable between the start batteries and the house bank.
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Old 21-07-2010, 09:17   #12
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That's cheating! A 24V system would solve the heavy cable problem but just isn't in the budget at this time.
Is the extra cost that much? If you don't do it now, you never will.

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My suspicion is that production builders may use smaller cables and pass the cost of alternator rebuilds on to their valued customers. On the other hand, maybe a 10% drop in the alternator cable is acceptable?
Does the alternator care about the drop? I know the batteries will. If you use a regulator that supports voltage sense at the batteries instead of the alternator it can compensate for the drop.
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Old 21-07-2010, 09:20   #13
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You can always run the alternators voltage sensor wire to the batteries so the alternator can make up for the drop. The downside of doing this is the increased chance of the sensor wire becoming compromised.

..and what Gord says in the next post.
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Old 21-07-2010, 09:23   #14
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... Does the alternator care about the drop ...
Yes.
Voltage drop is wasted (heat) energy, that the alternator will have to develop (by working harder &/or longer) in order to charge the batteries.
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