710.15 (B)

mbrooke

Batteries Not Included
Location
United States
Occupation
*
I'm a bit confused on this one. I'm reading this as the conductors need to be sized based on the sum of all the gensets and not the feeder OCPD at the parelleing gear. Normally this would not be an issue, but this is over kill N+ applications where many more generators are present than load requires to account for failures or repair.
 

steve66

Senior Member
Do you have a stand alone system? Per the handbook, if its interconnected with a AC system, 705 applies instead.
 

steve66

Senior Member
Ok, but it looks like you have ATS's so only one generator at a time can be connected any particular load.

IMO (and this seems logical, but I don't have any references to support this) the paragraph you referenced is intended to apply when multiple small sources in parallel serve a connected load that is larger than the total of the sources. I'm thinking of something like several low wattage PV panels that supply a dwelling. They generally produce enough power, but maybe the resident can't run the washer, dryer, dishwasher, and charge the electric vehicle all at the same time.

So IMO this paragraph is basically meant to be an "exception" to the requirement of of sizing the service or feeder conductors for the maximum load. Since the total combined source capacity would be smaller than the connected load, the feeder conductors could be smaller.

So I don't think this was meant to apply to your situation because you only connect one generator to any particular load at a time, and I assume your conductors are sized for the connected load.

I'd think this paragraph should be revised and clarified in future editions of the NEC.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Ok, but it looks like you have ATS's so only one generator at a time can be connected any particular load.

IMO (and this seems logical, but I don't have any references to support this) the paragraph you referenced is intended to apply when multiple small sources in parallel serve a connected load that is larger than the total of the sources. I'm thinking of something like several low wattage PV panels that supply a dwelling. They generally produce enough power, but maybe the resident can't run the washer, dryer, dishwasher, and charge the electric vehicle all at the same time.

So IMO this paragraph is basically meant to be an "exception" to the requirement of of sizing the service or feeder conductors for the maximum load. Since the total combined source capacity would be smaller than the connected load, the feeder conductors could be smaller.

So I don't think this was meant to apply to your situation because you only connect one generator to any particular load at a time, and I assume your conductors are sized for the connected load.

I'd think this paragraph should be revised and clarified in future editions of the NEC.

Right, but the way its written I'd be forced to size based on all the gen nameplates in sum. Technically. Though you are correct if I needed to make an argument.
 
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