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labatt 02-18-2009 09:30 PM

Battery Cable Sizing
 
We're replacing our Xantrex Freedom 15 inverter/charger with a Freedom 25 in order to increase charging capacity for our new battery bank. We current have 1/0 cables, and the recommendation is to go to 2/0. Our charger goes to a bus bar which also has the alternator hooked to it, and then goes to our batteries (this is a VERY simplified description). The batteries are also connected via 1/0. Do I need to upgrade EVERYTHING to 2/0?

peikenberry 02-18-2009 11:36 PM

I would definitely go with their recommendation. You're going to a higher amperage so you need a bigger cable size to reduce voltage drop and heating of the wire (larger wire, less resistance, lower voltage drop)

hellosailor 02-19-2009 01:05 AM

Labatt-
The purpose of heavier wiring is two things. First, to lower the voltage losses in the system. Second, to safely carry the current that the sytem will be running at.

If you have a short run of cable, i.e. two feet of a ground cable, and it is heavy enough to carry the current without overheating, you may not care about voltage loss since that is based on linear feet of cable. On the other hand, if you had fifty feet of cable running forward to a windlass--you'd NEED the heavier cable as fifty feet of the same "per foot" voltage loss could be crippling.

Do you need to upgrade it all? Probably not, but you need to take a look at each bit and see if it fills both criteria (carrying capacity and voltage loss) as it is. If you are doing a whole system, you may find that buying cable from a 100' spool, or from a spool "end" surplus, is way cheaper than buying it by the foot, and there's enough to replace everything.

But the cable fittings, the lugs, etc. are equally critical. The wrong ones (like the clamp-on ones) will cause voltage loss that you don't need. Crimped fittings need to be crimped properly, not just banged on. So as you spec all the cable runs, also spec each fitting and you may find that the costs of the fittings are as much as the cable is, and for short runs keeping the older smaller cable--with fittings--is a big cost saver without impacting performance.

But either way--the cable size has to be judged by the load and the wire length, you'll have to go over the numbers for each piece to see how critical it is.

jason3317 02-19-2009 08:13 AM

Wire sizing is a function of amperage x length (round trip run).

1/0 is pretty thick

1. What is the distance from the Freedom to the buss bar + buss bar to battery banks?

2. What is the maximum amperage the wire will carry, either the max charger output, or the max DC current load on the batteries from the inverter?

TxLnghrn 02-19-2009 12:25 PM

Labatt,
If you haven't seen it, this site has good information on voltage drop/ wire sizing and a handy calculator. I'm still learning this stuff for a project on my boat, so don't take my advice as worth anymore than you paid for it. :D

American Wire Gauge table and AWG Electrical Current Load Limits

Based on my quick estimations a 5 or 10 foot one-way run should be okay with 1-0 cable (1 and 2% voltage drop). A 20 foot run would be closer to 4% and you may want to upgrade cable if that is the case. I don't know how much the buss bar adds to the voltage drop calculation if at all. For the AC side of the circuit with an ability to surge out 7500 watts, the drop is even less.

Michael

sailingdog 02-19-2009 02:10 PM

Labatt—

It really depends on the length of the runs and the amperages involved. A 10' run may require 2/0 cable, but a short patch cable, like a 1' cable between a buss bar and the main DC panel for instance, may be okay at 1/0. :)

Valiente 02-19-2009 02:20 PM

You are receiving good advice here. Part of my dilemma is that the logical thing for me to do in terms of having 600 lbs. of batteries would be to rebuild the steps into the saloon and put everything right there. The cables would then all be three feet or less.

My solution for windlass and start is to echo-charge and to carry a Honda 2000 for "spot charging" rather than to run heavy cables forward. My ideal is to keep the cabling thick and short.

badsanta 02-19-2009 05:28 PM

While your at it. Make sure to use marine graded tined wire and tined connectors. you only want to do it once. imho

labatt 02-19-2009 06:23 PM

Distance from the charger to the bus bar is about 2 feet, and from the bus bar to the batteries is 2 feet. 100-130Amps will be carried across it. Xantrex recommends 2/0, and I currently have 1/0.

hellosailor 02-19-2009 06:52 PM

Four feet at 100 amps, let's say, for easy numbers.

The charts I have only go up to 0 AWG wire size, not 01/ or 0/2. For 0 AWG cable, the resistance is .0001 ohms per foot, and with a 125A load that would still only be a drop of 0.05 volts.

The drop is roughly--roughly--0.01 volts less each time the cable size goes up one, so I'd guess that's about 0.04 volts for 0/1 cable and 0.03 volts for 0/2 cable. A real difference, yes, but offhand it sure looks like something you could totally ignore.

We're talking about a grand difference of 0.02 volts in a 14.4 volt charging system, or roughly one or two tenths of one percent of your charging voltage being lost.

You can lose more than that with just one poorly crimped cable lug at any point in the system.


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