Solar Toolbox – Breakers and Fuses

The purpose of fuses, fused disconnects and breakers is to protect the wiring (and the equipment attached to it) of your system from a fault condition that could damage them. Breakers and fused disconnects also provide the ability to electrically isolate equipment during servicing. The interruption of a DC versus an AC current arc is very different. DC is much more difficult to interrupt because the current is continuous. Unless explicitly stated, fuse and circuit breaker ratings are AC and not DC.

Solar Array Breaker/Fuse Sizing:

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

Current Rating / .80 = Breaker/Fuse Ampere Rating

The load on a photovoltaic source circuit must be considered continuous, therefore the breaker/fuse size is calculated as the current rating divided by .80 which generates a breaker/fuse size that will run at 80% of its rated capacity. The current rating for the photovoltaic source circuit is calculated by applying the 1.25 rule to the module Isc.

There is a maximum fuse size stated in the specifications for any UL listed photovoltaic module. This maximum size takes into account both the 125% factor, and the maximum circuit loading factor of .80. Although this size is taken from the NEC, it should meet the requirements of the CEC as well.

Inverter Disconnect Breaker/Fuse Sizing:

The selection of which inverter fused disconnect or breaker needed is based on the inverter maximum current that the inverter is able to draw under normal conditions, and also be capable of interrupting the short curcuit current that may occur in the event of a short to the negative battery terminal.

Transformer based inverters usually have a very high maximum surge rating over electronic based inverters. We therefore always recommend that you follow the instructions provided by the inverter manufacturer when sizing your fuses or breakers for the inverter. Technically, you can divide the maximum surge rating of the inverter by the inverter operational voltage, then multiply by 1.25 to find out what the approximate size breaker/fuse you should use might be, but it is best to ask the manufacturer.

For your convenience, we have provided some common inverter battery disconnect listings by Manufacturer here:

The above ratings are based on inverter manuals using the appropriately sized wire. For more information about wire size and maximum length, see the wire toolbox.

Disconnecting Means

Photovoltaic modules are energized whenever they are exposed to light, and a battery is always energized – therefore, it is important that you are able to disconnect all equipment from all ungrounded conductors of all sources (CEC Rule 50-012(1)). The illustration below shows the placement of disconnecting means in a PV system.

Notes on the illustration:

1. The charge regulator in a PV system is typically supplied by two energy sources and therefore needs a disconnect means for each source in order to electrically isolate it during servicing.
2. The size of the disconnects for the PV source circuits are based on the maximum current of the PV modules (see above). Note: simply disabling a PV array by covering the modules with an opaque covering does not isolate the equipment and is not permitted.
3. The size of the disconnects for the charge regulator are based on the rated current of the PV array on the input side, and the rated current of the charge regulator on the output side.
4. The size of the disconnect for the inverter is based on the maximum current that the inverter is expected to draw – and be capable of interrupting the short circuit current from a short to the negative battery terminal.

In addition to appropriate capacity selection for all disconnect devices, they must be strategically placed such that any piece of equipment can be rendered dead for maintenance. Also, the above diagram does not illustrate the required AC disconnects, especially if the inverter has an AC input feed line.