Conductors between Sub-panel and Main sizing per 690.64(B)(2) or 705.12(D)(2)

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Local AHJ using John Wiles recommendations are requiring using the SUM of both the subpanel breakers and the feeder breaker when sizing the conductor.

The system is two strings of (17) Micro Inverters landing in a subpanel with two 20amp breakers, the feeder circuit in the main is a 40amp, using John Wiles guide the plan reviewer id taking the sum of (20+20+40)x 1.20 = 88 amps requiring #4 wire between the Main and Sub. This is before voltage drop, number of conductors, and temp corrections are calculated.
This is all based on NEC 2011, CEC 2013 705.12(D)(2)

705.12(D)(2) Buss or Conductor Rating: The Sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.

In practice we use this code to determine the allowable PV backfed, Bus rating x 1.20 ? MBR. ( 225 Bus x 1.20 with 175 amp main break would be 95 amps backfed allowed. (125x1.20= 270 ? 175 = 95)

I don?t see how this would require using #4 wire for a 40amp circuit.
 

GoldDigger

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Local AHJ using John Wiles recommendations are requiring using the SUM of both the subpanel breakers and the feeder breaker when sizing the conductor.

The system is two strings of (17) Micro Inverters landing in a subpanel with two 20amp breakers, the feeder circuit in the main is a 40amp, using John Wiles guide the plan reviewer id taking the sum of (20+20+40)x 1.20 = 88 amps requiring #4 wire between the Main and Sub. This is before voltage drop, number of conductors, and temp corrections are calculated.
This is all based on NEC 2011, CEC 2013 705.12(D)(2)

705.12(D)(2) Buss or Conductor Rating: The Sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.

In practice we use this code to determine the allowable PV backfed, Bus rating x 1.20 ? MBR. ( 225 Bus x 1.20 with 175 amp main break would be 95 amps backfed allowed. (125x1.20= 270 ? 175 = 95)

I don?t see how this would require using #4 wire for a 40amp circuit.
I think the plan reviewer has gotten it backwards.

1. Take 20 + 20 + 40 =80. (Just for reference, 80 x 1.2 is 96, not 88!)
2. Make sure that 80 is not more than 1.2 times the ampacity of the feeder wire.
3. 80/1.2 = 67, so 80 is not more than 67 times 1.2.
4. You need wire rated for 67A. #4 Cu should be fine. You cannot drop down to a rating of only 40 regardless of how sensible it may seem. That is changing in new code cycle however.
5. Any additional calculations for continuous load on the wire, and other ampacity adjustments and corrections do not necessarily apply to this calculation as far as I can see. All that matters is the "rating" of the wire.
 
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Where I am getting confused is why is the sum added together to calculate wire size, 200 amp main, 40 amp feeder, and two 20 amp circuits.


200 amp OCP is for all circuits and feeders.

40 amp OCP for Feeder servicing the sub panel.

20 amp OCP for each string of Micro Inverters.


the feeder circuit between the main and sub should only be required to support the 40 amps of backfed, the system has no load so why is there the need to calculate sum of the 40 amp and 2 20 amp breakers.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
Where I am getting confused is why is the sum added together to calculate wire size, 200 amp main, 40 amp feeder, and two 20 amp circuits.


200 amp OCP is for all circuits and feeders.

40 amp OCP for Feeder servicing the sub panel.

20 amp OCP for each string of Micro Inverters.


the feeder circuit between the main and sub should only be required to support the 40 amps of backfed, the system has no load so why is there the need to calculate sum of the 40 amp and 2 20 amp breakers.

You may not find the following persuasive, but it is the logic used when that section of the code was written:

Well, at one end of the wire we have a source of 40A.
At the other end of the wire, we have an independent source of 40A.
Now if someone opens up a J-box part way along the feeder and installs an new subpanel with an 80A main breaker and 80A worth of loads, it could draw more current than the feeder could handle.

The idea is that anyone who adds a new MLO panel, for example, thinking that the feeder is protected at the 40A level now and does not pay any attention to the amount of load he adds is going to get into trouble.
In fact, even if you allow for the installation of such a new set of loads, the current in each length of feeder wire will not be more than 40A on either side of the attachment point. So to give you some credit for that they allow you to multiply by 120%.

As I said, the situation is handled much more rationally in [2014].
 
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