CRITERIA NEEDED FOR THIS INTERCONNECTION

The example to the right shows what your setup might look like for this type of interconnection. Depicted is a meter/main combo with feedthrough lugs to the interior sub panel. The sub panel will not accept a main breaker. An exterior breaker can still be possible but it will require us to explore some other options for protecting the feeders.

This interconnection has the following requirements that need to be met before it can be considered:

1. Feedthrough conductors are appropriately rated for service (i.e. if it is feeding a 200A service, the feeders are a minimum of 4/0 AWG AL.)

2. There are 2 available stabs to accommodate our PV breaker in the MSP.

   • If you the panel is full but the load center allows tandems you can consider this condition met.

3. There must be no code violations present in and around the MSP. 

   • If there is a plan in place to resolve code violations, then  you can consider this condition met.

4. There must be sufficient available backfeed for the amount of PV we will be landing in this load center.

Now, how are we going to protect these feeders?

NEC 705.12(B)(1): [in regards to protecting connections made to a feeder] The feeder ampacity shall be not less than the sum of the primary source overcurrent device and 125 percent of the power-source output circuit current.

Let's break down the language. 

• The "primary source overcurrent device" is referring to the main breaker in the MSP. 

• The "125 percent of the power-source output circuit current" is referring to the NEC guideline used to calculate total backfeed current from secondary sources like solar.

So what they really mean is:

The sum of the main breaker rating plus the PV backfeed rating will not exceed the rating of the feedthrough conductors.

To subvert this, we could simply derate the main breaker to accomedate this if it is justified by a residential load calculation.