PV system disconnect size

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Earl Grey

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I am working on a PV system with 5 inverters. Inverter output on units #1 , #2 is 29 amps each. Inverter output on units #3 , #4 , #5 is 9 amps each. Inverters feed a 125 amp sub panel, then feed a sub meter for homeowner use, then feed a outside fused disconnect, then a disconnect at main service area that is fed from the utility / line side of a 400 amp transfer switch. My question is do I need to have the disconnects rated at 125 amps or can I use 100 amps.

29 amps x 125% = 36.25 x 2 = 75.5 (2 - 40 amp 2p breakers in sub panel)

9 amps x 125% = 11.25 x 3 = 33.75 (3-15 amp 2p breakers in sub panel)

or can I use the inverter max. output only

29 x 2 = 58

9 x 3 = 27


85 amps
 
To rephrase my question do I need to take the sum of all my inverter breakers (40+40+15+15+15=125amps) or can I use inverter max output (29+29+9+9+9=85amps) in sizing my 2 disconnects.
 
Earl Grey said:
I am working on a PV system with 5 inverters. Inverter output on units #1 , #2 is 29 amps each. Inverter output on units #3 , #4 , #5 is 9 amps each. Inverters feed a 125 amp sub panel, then feed a sub meter for homeowner use, then feed a outside fused disconnect, then a disconnect at main service area that is fed from the utility / line side of a 400 amp transfer switch. My question is do I need to have the disconnects rated at 125 amps or can I use 100 amps.

29 amps x 125% = 36.25 x 2 = 75.5 (2 - 40 amp 2p breakers in sub panel)

9 amps x 125% = 11.25 x 3 = 33.75 (3-15 amp 2p breakers in sub panel)

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Actually, your subpanel will have to be rated at not less than 83.3% the sum of ocp's supplying power to the subpanel. That is, the subpanel has to comply with the 120% rule of 705.12(D)(2). 83.3% is the inverse of 120%.

Some AHJ's overlook this, but unless you have only one ocpd between inverter and utility, you are technically not connected to the supply side of the service disconnect [705.12(A)].
 
The output of the subpanel is the output of your inverter system, which is a sum of the individual inverters. The fuse needs to be sized for at least 125% of the continuous output current rating of the inverter system, which equals the sum of the max continuous current for all of the inverters. In this case that is: 29 + 29 + 9 + 9 + 9 = 85 A x 125% = 106.3 A. While the next standard size is 110 A, you can use a larger OCPD provided that conductors are sized appropriately. You can't install a 110 A fuse (or larger) in a 100 A disco.

Note that you want the fused disconnect at the source of the fault current, which is the utility grid and not the inverter aggregation panel.
 
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