inverter wire gauge

[jerry f]

I had a general question/concern for d/c wiring to inverters. Say you have a 1200 watt inverter you could simply figure that 1200 watts ( 120v x 10 amps) would need to be sized to carry 100 amps continuous d/c per abyc standards (100amps x 12 volts). However i have no idea how efficient inverters are. If they are 80% efficient and put out 1200 watts then you would have to wire for approx 120 amps d/c. I would assume 1.follow manufactureers wire size if given.2. If not given find out efficiency then compute per abyc. If neither guess on efficiency then compute ????

[Alan Wheeler]

Correct. 80% is about the right efficiency. You need to take into account the length of cable run and x by 2. Big rule, keep cable run as short as possible. And if you can, go to the next size up cable anyway. Most inverters will shut down with only a 2V drop in battery powers supply. if you have even only 1V drop, you only have 1 V of headroom.

[Rick]

Like Wheels points out many inverters "drop out" when detecting 10 Volts at their internals. Most good quality low frequency inverters (they switch at 120 Hz US 100 Hz. Euro) have a 3X power surge capability and, therefore, if you have loads which push that limit for starting or peak loading you should size the cable accordingly to prevent excessive voltage drop to the inverter.

Keep in mind that the voltage sag at the inverter is due to cable voltage drop as well as the voltage drop across the internal battery resistance, which is often substantial and overlooked.

[GordMay]

Another reason for BIGGER Battery Banks
The internal resistance of a battery bank, and the attendant voltage drop, may be reduced by paralleling more batteries into the bank. Doubling the number of batteries reduces the bank's resistance by half
1/R equivalent = 1/R1 + 1/R2 + 1/R3
or
R equivalent = 1/(1/R1 + 1/R2 + 1/R3) etc

FWIW,
Gord May