Two necessities for safety (and to comply with local and national electrical codes) are to have excess current protection and a means of disconnection on all voltage sources in any ungrounded conductor. This includes all connections between the PV array, batteries, wind turbine and inverter. Almost all AC sources have some form of protection and means of disconnection in any home or appliance wiring. Even the inverter and generator have their own circuit breaker or certified fuse device.

Common AC circuit breakers and fuses do not function well with DC circuits and should never be used in a DC installation.

Circuit breakers and fuses act like safety valves. An excessive current flow through an appliance or conductor will blow a fuse wired in series to open the electrical path. The same goes for a circuit breaker but it allows a reset after the trip occurs and can also serve as a temporary operating switch.

Every circuit caters to a maximum amount of current based on a worst-case scenario. That amount is calculated from the figures on a manufacturer's data sheet, as listed on the appliance, inverter, wire, PV panel or other component. A 25% safety factor should be added to the rating. The size of cable running through the PV array and inverter must consider the length of the cable, the system voltage and the current in the worst-case scenario as well as the wire capacity. The rating of the insulation at high temperatures must also be considered if the cable is exposed to the sun during the summer.

For example:

1. The system voltage of a PV array with 24V DC will output 30A (max) in all conditions with a one-way wiring of 40 ft (12 m).
2. Preferably, a number 00 (or 2/0) wire should carry this current to give a 1% max loss.
3. A number 00 wire can carry 195A current (max).
4. A 25% safety factor added to the computed maximum PV array current (30A x 1.25 = 37.5A).
5. Compare the worst-case current from step 4 to the maximum wire rating from step 3. If the worst-case current figure does not exceed the wire capacity figure, the current is acceptable.
6. The circuit breaker or fuse size cannot be bigger than the biggest wire capacity rating as determined in the NEC/CEC. Conduit electrical cables or raceways have to be de-rated due to the effects of heating.
7. The environmental temperature or any other condition that affects the temperature of wire insulation can be found in Chapter 2 of the NEC or Chapter 12 of the CEC. You will note the wire selection complexity, placement and type complexity as well.
8. A copy of the NEC/CEC will familiarize you with these terms and issues as you review the appropriate standards. You can also consult your local electrical inspector about these issues throughout the planning stages.

Contact for NEC purchase:

National Fire Protection Association
1 Batterymarch Park,
Quincy, Massachusetts
USA 02169-7471
www.nfpa.org