Solar Toolbox – Solar Wiring

Calculating conductor size in a PV system can be based on current capacity or voltage drop, whichever results in a greater size. Often, in PV systems operating at nominal 12 or 24 V, the conductor sizes chosen to limit voltage drop will have ampacity ratings that far exceed the current they are required to carry.

Conductor Selection Based on Voltage Drop:

In PV systems, the maximum voltage drop allowed between the PV array and the battery is 5%, with no more than 3% allowed on any individual portion of the circuit. DC load circuits should be treated similar to AC load circuits with a 5% maximum voltage drop allowed from the battery to the load, with no more than 3% drop on any individual portion of the load circuit. Always calculate the voltage drop using the maximum current that may exist in the conductor. When the maximum current is not known, then use 80% of the rating of the breaker or fuse protecting the conductor.

We have found that there are many on-line calculators for determining conductor size based on voltage drop, but the one we like best is from Southwire and is linked here:

Southwire Voltage Drop Calculator

Sometimes it is just as quick to look at a table. The following table has been provided by Xantrex Technologies. It gives reference information for ampacities up to 60 A.

Conductor Selection Based on Current Carrying Capacity:

Due to the facts that the service voltages of most small, battery based PV systems are between 12 and 48 VDC, and that the conductor voltage drop may have a major impact on the operation of a PV system, the actual voltage, rather than the nominal values should be used.

For Example:

Solar Source Current – The maximum current between the PV array and the battery (or charge controller) is determined by the short circuit current of all the PV power sources (solar panels) x 125 % (Rule 50-008) divided by the temperature correction factor, or:

(Module Isc) x (# of Modules in Parallel) x 1.25 = Current Rating

Current Rating / Table 5A* = Wire Ampacity (based on ambient module temperature, usually 50 deg. C.)

(*Table 5A shows a correction factor at 50 deg. C for Types R90, RW90, T90 to be .80)

Battery Source Current – The maximum current between a battery bank and an inverter will be when the battery voltage is at its lowest. Therefore, in calculating current in this conductor we use the battery open circuit voltage at a discharged state. See the chart below:

 State of Charge
Battery Open Circuit Voltage
12 V Battery
24 V Battery
48 V Battery
  Fully Charged
12.72
25.44
50.88
 75 %
12.60
25.20
50.40
 50 %
12.48
24.96
49.92
 25 %
12.18
24.36
48.72
 Discharged
11.70
23.40
46.80

Remember: When close to the maximum rating for that conductor; always choose the next largest conductor.