Failed inspection

[ggunn]

I have a client who submitted and got approved a design for a commercial PV system which was to be interconnected through a 100A fused disco via a line side tap to the facility's 100A MDP. During construction they changed the design; they upgraded the panel to a 200A panel and downgraded the main breaker to 100A so that they could interconnect through a 100A backfed breaker in the MDP and get under the 120% rule for the panel. It looks like a safer design to me, since under the changed design there are no conductors unprotected by OCPD and the resultant current in the feeder to the MDP is unchanged.

The inspector failed the installation, saying that when they upgraded the panel they should have also upgraded the feeder to 200A conductors even though the main breaker is still 100A and the only thing that changed is the rating of the busbars. Is there code that stipulates this?

 

[Carultch]

I have a client who submitted and got approved a design for a commercial PV system which was to be interconnected through a 100A fused disco via a line side tap to the facility's 100A MDP. During construction they changed the design; they upgraded the panel to a 200A panel and downgraded the main breaker to 100A so that they could interconnect through a 100A backfed breaker in the MDP and get under the 120% rule for the panel. It looks like a safer design to me, since under the changed design there are no conductors unprotected by OCPD and the resultant current in the feeder to the MDP is unchanged.

The inspector failed the installation, saying that when they upgraded the panel they should have also upgraded the feeder to 200A conductors even though the main breaker is still 100A and the only thing that changed is the rating of the busbars. Is there code that stipulates this?

I see no reason why you should have to do this. The service conductors need to meet or exceed the sizing requirements per the service disconnect's OCPD rating.

If the main breaker was 175A, they would only need 175A of service conductor.
If the main breaker was 150A, they would only need 150A of service conductor.
If the main breaker was 125A, they would only need 125A of service conductor.

So I do not see how a 100A main service breaker would be any different. The 120% rule applies to busbar/breaker assemblies, rather than service or feeder conductors.

The main reason for the 120% rule, is to keep the breakers in the panelboard from overheating and nuisance tripping one another. From a pure Kirchhoff current law view, there is never a chance that an opposite fed panel can experience an ampacity in excess of the breaker on either side feeding it. The current will diminish to zero in the middle of the busbar, when fed from opposite ends. There also is no chance in this example, for the current on the service conductors to exceed the MCB's rating and not be protected by the MCB.

There is the rule that the sum total ampacity of the interconnection overcurrent devices cannot exceed the rating of the service, but we aren't violating that rule. An example of that would be 125A of PV interconnected on a 200A MDP with a 75A main breaker and 75A of service conductors.

 

[Smart $]

... Is there code that stipulates this?

No. The only pertinent section is 230.90.

Did the inspector provide a section?

 

[ggunn]

Thanks; that is what I thought as well. Now there is the matter of letting the inspector save face.

 

[Carultch]

No. The only pertinent section is 230.90.

Did the inspector provide a section?

Is the burden of proof on the inspector, to provide a code reference when citing a violation?
Is there a code reference that backs up this burden of proof assumption of mine?

 

[ggunn]

Is the burden of proof on the inspector, to provide a code reference when citing a violation?
Is there a code reference that backs up this burden of proof assumption of mine?

It's more a matter of getting the inspector to change his position and letting him do so gracefully. He is the only solar inspector in the AHJ's territory and it pays to stay on his good side.