Sizing a Disconnect Switch for Output Circuits

[SunFish]

I'm working on a system and am curious about sizing disconnects. I received a quote from a manufacturer for combiner boxes with integrated disconnects and they've quoted a combiner with a 96 A rated disconnect for a combiner with 8 strings of 11 being combined. My module Isc rating is 8.89 A x 8 strings = 71.12 A Isc on my output circuit. So per 690.8 my max cont. current on my output circuit is 71.12 A x 1.25 = 88.9 A. So the combiner disconnect rating is above my continuous current.

My question is, are disconnects required to be sized to a minimum of 125% of the continuous currents similar to wires? If so 125% of my continous currents would be 88.9 x 1.25 = 111.125 A which is over the rating of the disconnect at 96 A. Are disconnects rated for 100% continuous duty? I'm assuming the manufacturer knows what they are doing and would size their disconnects appropriately for an 8 string combiner. What am I missing here?

 

[jtopham26]

The Busbar is after the fuses which limit the current. Busbar current is likely equal to the number of strings times the OCP of the fuses (15). Fuses limit the continuous current of the panels to 9.6A (1.56*9.6=15). After this point I do not believe the extra 1.25 should be required past the fuses although I may be wrong because I don't have the code book in front of me, Please confirm.

 

[ggunn]

I'm working on a system and am curious about sizing disconnects. I received a quote from a manufacturer for combiner boxes with integrated disconnects and they've quoted a combiner with a 96 A rated disconnect for a combiner with 8 strings of 11 being combined. My module Isc rating is 8.89 A x 8 strings = 71.12 A Isc on my output circuit. So per 690.8 my max cont. current on my output circuit is 71.12 A x 1.25 = 88.9 A. So the combiner disconnect rating is above my continuous current.

My question is, are disconnects required to be sized to a minimum of 125% of the continuous currents similar to wires? If so 125% of my continous currents would be 88.9 x 1.25 = 111.125 A which is over the rating of the disconnect at 96 A. Are disconnects rated for 100% continuous duty? I'm assuming the manufacturer knows what they are doing and would size their disconnects appropriately for an 8 string combiner. What am I missing here?

Ask the manufacturer if their disco is rated at 100%. Additionally, some discos are 100% rated if unfused and 80% rated if fused. Square D is that way.

 

[Carultch]

I'm working on a system and am curious about sizing disconnects. I received a quote from a manufacturer for combiner boxes with integrated disconnects and they've quoted a combiner with a 96 A rated disconnect for a combiner with 8 strings of 11 being combined. My module Isc rating is 8.89 A x 8 strings = 71.12 A Isc on my output circuit. So per 690.8 my max cont. current on my output circuit is 71.12 A x 1.25 = 88.9 A. So the combiner disconnect rating is above my continuous current.

My question is, are disconnects required to be sized to a minimum of 125% of the continuous currents similar to wires? If so 125% of my continous currents would be 88.9 x 1.25 = 111.125 A which is over the rating of the disconnect at 96 A. Are disconnects rated for 100% continuous duty? I'm assuming the manufacturer knows what they are doing and would size their disconnects appropriately for an 8 string combiner. What am I missing here?

Most disconnects on combiner boxes I've seen are continuous duty rated, meaning they are sized by 1.25*total Isc, regardless of what total fuse ampacity you use. If a downstream OCPD is part of the system, then you ensure that it is protected by it. For instance, a 200A continuous duty disconnect can be protected by a downstream fuse or breaker up to 250A.

If they are not continuous duty rated, you'd use 1.56*total Isc, even if the fuses sum up to more. Or you'd match it to a downstream OCPD if applicable.

If you are adding your own disconnect, be careful with the fact that it is for a DC application. DC is more difficult to break under load than AC, and could require 2 (if not 3) poles in series to take credit for the full rating. Follow manufacturer's instructions on this DC de-rating whenever available.