Wiring

PV system wiring is substantially different from conventional AC wiring as they generally use lower voltage and often have larger wire sizes compared to AC systems. NEC article 690 - Solar Photo Voltaic Systems - has complete details on the code requirements for wiring and other electrical aspects. Some key information is also described in the Standards and Codes section on this site. Wire size selection is based on two important criterions - Ampacity and Voltage drop. Ampacity refers to the current carrying ability of a wire. The larger a wire is, the greater its capacity to carry current. Using a wire with an Ampacity less than the current flow will cause the wire to overheat leading to losses and even fire. Table 310-16 in the NEC code book lists the standard Ampacity of copper wires. Voltage drop is the loss of voltage due to a wire's resistance and length. It is important to take into consideration the voltage drop and minimize the energy loss in the wiring in designing the wiring for your PV system. Using a larger wire size, decreasing the current flow or decreasing the length of the wire are all solutions to reduce voltage drop.

A good design practice is to keep the wiring voltage loss at 2%.

The Ampacity ratings for insulation types RHW-2 and THWN-2 with copper conductors at 30 degrees C is shown below in table W-1. The Wire resistance of the same copper conductors is shown below in table W-2.

Wire Resistance ratings for the above and effect of temperature on resistance
Table W.2

When sizing wire in a PV system, start by calculating the maximum current load which the maximum current flowing through the circuits. NEC code requires you to then multiply this value by 1.25 so that the conductor never carries more than 80% of its rated capacity for safety. NEC 690-8 also requires designers to have an addition 1.25 factor of safety for wires connecting the PV panels to the battery bank.

When designing the wiring between the different components, ensure the voltage drop is less than 2% of the rated voltage.