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Breaker and Fuse 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.

Solar Disconnect Schematic

Notes on the above 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 below). 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.
  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.

 

Fuses and Circuit Breakers
 
The purpose of fuses, fused disconnects and breakers is to protect the equipment and wiring 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 explicity 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 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:
 
DC V
Continuous Watts
Go Power
Magnum
Model
Fuse / Breaker Size
Model
Fuse / Breaker Size
12
600
SW600
110
1000
SW1000
110
MMS1012
200
1500
SW1500
200
2000
SW2000
300
MS2012
200
2800
MS2812
300 / DC175
3000
SW3000
400
24
600
SW600
110
1000
SW1000
110
1500
SW1500
200
2000
SW2000
200
2500
3000
SW3000
300
3500
4000
MS4024AE
DC250
48
2500
3600
4000
4400
MS4448AE
DC175
5500
DC V
Continuous Watts
Outback
Xantrex
Model
Fuse / Breaker Size
Model
Fuse / Breaker Size
12
600
1000
1500
2000
FX2012
DC250
2800
VFX2812
DC250
3000
24
600
1000
1500
2000
FX2024
DC175
2500
SW+2524
DC175
3000
3500
VFX3524
DC250
4000
SW+4024
DC250
48
2500
FX2548
DC100
SW+2548
DC100
3600
VFX3648
DC175
4000
SW+4048
DC175
4400
5500
SW+5548
DC175
 
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.
 

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