Friday, Dec 06, 2013 at 11:01
The backup has to work, otherwise it's not a backup :-)
So you need to consider the temp-compensating/ECU controlled charging system issue. Those systems typically settle down to about 13.0 to 13.2V output, which, after cable losses is not enough to charge a "house" battery. If your tug has one of those you NEED a DC-DC charger. Without it there will be no back-up charging.
A typical DC-DC charger will pull up to about 30 amps, hence my recommendation of 6 gauge (B&S) cabling.
If your tug does not have such a charging system you MAY get enough juice through to consider it as a back-up.
Also, your compressor will be pulling currents in that region of 20 to 30 amps, so again, 6 B&S cabling is appropriate, though 8 would do at a pinch. Voltage is important for your compressor, too. Skinny cable will cause the voltage to drop under load and the compressor will run hot. Probably OK for one tyre, but not so good for 6, so again I lean toward 6 gauge cabling to reduce voltage drop.
In another follow-up you asked about fusing. If you're looking at 30 amps in the circuit, a 40 amp fuse would be appropriate.
Cheers
FollowupID:
803770