Solar PV backfed to a Parallel feeder

[CNYSolar]

I have a unique situation which I'll try to explain..
Two parallel sets of 500mcm's feed an 800A switch gear from the utility. We have a 400A panel board with no loads (potentially 240A max of backfed PV). Going to the 800A parallel fed switch gear with one set of 500mcm. I understand 310.10(H)(2) and maintaining length/characteristics of conductors. Other than this what would be the reason you could not tap the 400A panel onto one set of feed conductors near the bus bar. As opposed to rebuilding the inside of the 800A gear to accommodate the new 500mcm tap conductors. Is the problem here the potential to increase the load on the conductors and operating temperature? Because if it is only capable of back feeding wouldn't it only decrease the load on one set of parallel conductors?

Also if you tap both parallel sets of conductors at the same point to the new 400A PV only panel (which has a main breaker of 300A) does this alleviate the problem without creating additional risks?

 

[Carultch]

I have a unique situation which I'll try to explain..
Two parallel sets of 500mcm's feed an 800A switch gear from the utility. We have a 400A panel board with no loads (potentially 240A max of backfed PV). Going to the 800A parallel fed switch gear with one set of 500mcm. I understand 310.10(H)(2) and maintaining length/characteristics of conductors. Other than this what would be the reason you could not tap the 400A panel onto one set of feed conductors near the bus bar. As opposed to rebuilding the inside of the 800A gear to accommodate the new 500mcm tap conductors. Is the problem here the potential to increase the load on the conductors and operating temperature? Because if it is only capable of back feeding wouldn't it only decrease the load on one set of parallel conductors?

Also if you tap both parallel sets of conductors at the same point to the new 400A PV only panel (which has a main breaker of 300A) does this alleviate the problem without creating additional risks?

Do not do anything to intentionally break the symmetry of parallel conductors. This includes tapping onto the middle of them.

I've discussed this in another thread. If you construct Ohm's and Kirchhoff's laws for a system where you do not tap all the conductor sets of the feeder, it is possible to overload that particular conductor set. I'll provide a link when I find it.

If you have two sets of a PV system tapping onto a feeder with 3 sets of parallel conductors, you need to tap onto all 3 sets. This would likely require 8-terminal insulated tap connectors for each phase. 3 for the incoming feeder, 3 for the outgoing feeder, and 2 for the tap.


Another point about tap conductors: if you have 400A of overcurrent device, you need 400A of conductor ampacity. Next size up rule 240.4(B) doesn't apply. Each tap rule in 240.21(B) overrides this.

 

[Carultch]

I have a unique situation which I'll try to explain..
Two parallel sets of 500mcm's feed an 800A switch gear from the utility. We have a 400A panel board with no loads (potentially 240A max of backfed PV). Going to the 800A parallel fed switch gear with one set of 500mcm. I understand 310.10(H)(2) and maintaining length/characteristics of conductors. Other than this what would be the reason you could not tap the 400A panel onto one set of feed conductors near the bus bar. As opposed to rebuilding the inside of the 800A gear to accommodate the new 500mcm tap conductors. Is the problem here the potential to increase the load on the conductors and operating temperature? Because if it is only capable of back feeding wouldn't it only decrease the load on one set of parallel conductors?

Also if you tap both parallel sets of conductors at the same point to the new 400A PV only panel (which has a main breaker of 300A) does this alleviate the problem without creating additional risks?

Here's the example I made up, to demonstrate this concept:
http://forums.mikeholt.com/showthread.php?t=170330&page=3

 

[Carultch]

Consider a 500A service, made up of two parallel sets of 250 kcmil. Suppose we desire to connect 200A of PV, which is 3/0. Ignore the 1.25 safety factor, for simplicity, i.e. assume that this is already included in all applicable ampere figures. The 250 kcmil service conductors run a distance of 500 ft from the transformer to the service disconnect, and the PV is tapped onto set #1, within 1 ft of wire length from the service disconnect. That's a voltage drop of about 10 volts, under full load.

Calculate the current in each resistor (representing one of the parallel sets of service conductors):