Calculating wire size and voltage drop

Just watch the dynamic physical loads of huge wires on battery terminals. Bus bars and flexible (say robotic or welding fine wire cable) battery connection wires in a star configuration avoid this. I'm just starting to realize the kind of stresses the terminals on even a well secured battery can see on an RV.

Jim

I'm leaning towards the 2 ga at this point. I could probably get by with 4 ga. but other sources I'm reading recommend even bigger wire.

IMO, anything up to 2/0 is easy to work with. I have everything from #10 to 4/0 in my rig. Only the 4/0 was difficult. Westend, if that 90A of inverter goes through that wire (90"/102"), size your wiring for that. You can go a little small if that 90A is short in duration. And what mean by small is size to current handling. With inverters you don't want the low voltage alarm sounding off so keep the voltage drop above that.

When I was adding a second battery that was going to be 10' from the first I did some calculations for voltage drop based on a 20' combined DC run from the charge controller to the battery.

2ga - drop of .53% 14.49 volts to 2nd battery
4ga - drop of .85% 14.38 volts to 2nd battery
6ga - drop of 1.36% 14.3 volts to 2nd battery
8ga - drop of 2.15% 14.2 volts to the 2nd battery

Usually systems are wired to avoid a drop greater than 3% loss. Current capacity is a function of the cross section of the wire or its area and not its diameter. This is why the change is not linear with the diameter of the wire.

Whether you use 2/4/6 gauge wire is not going to make a noticeable difference in performance. Thicker wire is much harder to run and more difficult to properly attach the connectors. I do recommend solder slugs and connectors made for them over using a crimp connection for anything heavier than 8ga. Delcity.com is a good source for these. The solder connector is mechanically stronger and keeps out moisture far better than a mechanical crimp. For ultimate long term performance marine grade wire will greatly reduce corrosion problems with a crimped connector and Jamestown Distributors and West Marine are good sources.

mena661 wrote:
Which wire is going to the inverter, 102" or 90"??

Neither---The (+), 90" and (-), 102" cables from the batteries are terminated on distribution busses. A 4AWG cable of 12" connects the bus to the inverter. I get different results from different calculators. I would sure like 4AWG to work since I have a partial roll of it but want to get this right the first time and am willing to get the necessary size of wire.

Which wire is going to the inverter, 102" or 90"??

Voltage drop calculator

If you don't like that one there are many more..

Hi westend,

1 conductors per phase utilizing a #2 Copper conductor will limit the voltage drop to 0.92% or less when supplying 30.0 amps for 10 feet on a 12 volt system.
For Engineering Information Only:
95.0 Amps Rated ampacity of selected conductor
0.1808 Ohms Resistance (Ohms per 1000 feet)
0.045 Ohms Reactance (Ohms per 1000 feet)
0.12 volts maximum allowable voltage drop at 1%
0.109. Actual voltage drop loss at 0.92% for the circuit
0.9 Power Factor
**Note to User:All ampacity values are taken from the Section of 310-15 of the NEC. The conductor characteristics are taken from Table 9 of the NEC. The calculations used to determine the recommended conductor sizes for branch circuits are based on 60°C ampacity ratings for circuits rated 100 amps or less or marked for use with #14 AWG - #1 AWG. Circuits rated over 100 amps or marked for conductors larger than #1 AWG are determined using 75°C ampacity ratings. Calculations to determine service and feeder conductor sizes are based on overcurrent device ratings rather than actual expected loads which are conservative and may yield oversized conductors. No calculations take into account temperature correction factors or conductor de-rating.
This voltage drop calculator is applicable only to NEC applications. It does not optimize conductor sizes for several different loads at various points in a circuit. The total combined load and length of the circuit must be used. Consult with an engineer if your application requires more complex engineering calculations.