I need to know how much current will be drawn by, say, a 220AH pair of 6V FLA batteries at 50% discharge (or, what the heck, 100% discharge) so I know how to size the supply circuit. It'll be a house battery in a Transit with a 220A alternator. Ford wants you to hook up to their "customer connection point", which consists of 3 terminals, each with a 60A fuse.
The battery will be charged by a pair of 160W solar panels, but if there should come a dark time, I want to be able to use the alternator as a backup.
Thanks for any advice.
When I size that sort of wire, I sort of "circle around" through the various requirements until I arrive at the correct wire size. One is how much current (amps) can my charging sources output, and the other is how much voltage drop do I want (or not want, really) (I don't care whether my battery bank is 50 amp hours or 500 amp hours -- I only need to know about charging sources and loads.)
I don't completely understand a 220 amp alternator going through a 60 amp fuse, but I'm not familiar with the Transit. I can give an example of how I sized things on my E-350; maybe reading through my thought process will help you.
So, I have a start battery in the Ford, and IIRC around a 120 amp alternator (I knew specifically what it was when I was doing the project though!). I have an 18 foot one-way run from the start battery to my house bank. My house bank is three Group 31 AGM's, for 375 amp hours (but none of this is related to the size of the house batteries - the amperage flowing is determined by the charge sources and the loads).
I not only want to be able to charge the house bank, but I also want to be able to "self jump start" if needed. So that adds other parameters. The starter on my engine draws around 150 amps (inrush current doesn't count).
So, I need to be able to fuse up to around 200 amps to accommodate the starter draw. Hence I immediately know I need wire with an ampacity of 200 amps in an engine space. I look on the ampacity table for 105ºC rated wire not bundled (this will be run singly) (I use marine wire by Ancor) and see that anything 1AWG or larger will work. Actually you are allowed some leeway in fusing but since voltage drop will probably make me want to run larger wire anyway, I move on along in my calcs.
Now I want to see about voltage drop when I'm charging my house bank. I go to the calculator and put in my specs (round trip wire length, wire size, amperage, temp, etc.) and see that with 1AWG wire, and presuming my alternator is putting out around 60 amps (I don't typically see more than that heading to the house bank - presumably the engine etc. use the rest). I get a voltage drop percentage of 2.8%. That's okay. I would never accept that for a solar input, as the solar controller is "smart" and can deliver a very precise voltage; but the alternator is just "stupid" bulk charging, basically.
Now I look to see what the voltage drop would be in the other direction, while starting. That would be 6.9%. Not great, but I don't plan on self jump starting often or for long.
But then I don't have any 1AWG wire. I have 2AWG (smaller) or 1/0AWG (larger). So I go with the 1/0.
A friend with the same exact rig elected to go with 2AWG for the same thing. He was able to "fudge" slightly on the fuse size (175 amp blew, 200 amp is fine) (you are allowed to "over fuse" slightly), and he didn't care too much about voltage drop. He has an ample solar system and only uses the start battery connection for occasional bulk charging.
I'm basically in the same position, but I use my start battery charging a little more often, so I wanted less voltage drop, plus I had the wire on hand.
None of the above really had anything to do with the size of my house bank, but rather the amount of current the charging sources could put in, or the loads take out, plus proper fusing (or breakering, as the case may be).
So, questions for you: What is your overall charging plan for the house bank? Is the alternator a primary source or a secondary one? How else do you charge your house bank? What's your usage pattern?
I don't really remember how fussy FLA batteries are about charging voltages. I have AGM and strive to give them just what they want (temperature compensated). If I were using my alternator for a primary charge source I'd be looking into a "battery to battery" charger which could be adjusted.
But even if your house bank is not fussy about voltage, you still will want to be charging at over 13 volts, so won't want too much voltage drop. Plus you will need to be able to fuse both ends of the wire for safety, so need wire that can take the correct fuse size (ampacity).
I'm interested in the 60 amp Transit factor (might want one myself someday).