Code reference for interconnection to MBO MSP

[Zee]

Whenever I run into a MBO MSP and I land our PV breaker in the SP that it feeds, then I often connect more Amps of PV than 20% of the MSP rating.

What Code reference would you use to support this tie-in method?
---------------
E.g.
#1
(E) 125A rated service with an (E) 2p125A main breaker only.
which feeds the 150A rated SP.
I tie 30A of PV into the SP.
#2
Most recently 2p70A MBO MSP and I tie 20A of PV into the 100A rated sub.

 

[pv_n00b]

This contradicts the code so I don't see a way to support it. NEC 705.12(B) applies to all buses between the inverter POI and the utility POCC. You need to comply at both the subpanel and the MSP.

 

[jwoody]

Your examples can work. There is no bus @ the main panel so you would just use 705.12(B) (2) or (4) for the sub busbars.

E.g. #1: 150A x 120% = 180A - 125A OCPD = 55A total allowable backfeed.

E.g. #2: 100A x 120% = 120A - 70A OCPD = 50A total allowable backfeed.

However if there is a bus @ the main panel, then these amounts would exceed the main bus allowable backfeed. So, If there is a bus @ the MSP (with this scenario of interconnecting @ the sub panels) then you would apply 705.12(B)(3) and just be sure that the sum of all breaker amps on the main bus do not exceed the main bus ampacity. OR, just apply 705.12(B) (2) or (4) to the main bus as well.

 

[Zee]

This contradicts the code so I don't see a way to support it. NEC 705.12(B) applies to all buses between the inverter POI and the utility POCC. You need to comply at both the subpanel and the MSP.

I do comply at the SP.
No bus in MSP to comply with!

It is fundamentaly safe. There is no way to feed too many amps into any bus or feeder conductor.

 

[Zee]

As far as Code reference for the MSP, it seems I would just use the basic 705.12 permission to connect load side?
"The output of an interconnected electric power source shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any distribution equipment on the premises."

I do not see any other Code reference that would be used here.
None of the following and numbered clauses apply at the MSP since not connecting to feeder, no tap, no busbar.

Do i have this right?

BTW - been doing so for decades, no issues, just want to know the exact Code for it.

 

[jaggedben]

What code says you can't do it? Since 2014 the code is very clear that the busbar rules are for panelboards (or, more recently, switchboards) but definitely not a fused disconnect. Oh, you said a MBO, meaning a breaker enclosure. Same thing.

Applying the 120% to a fused disco or MBO is a very six-or more-years-ago confusion at this point.

 

[Zee]

Yes, OK what I thought. It is allowable because it isn't prohibited. Thx

 

[pv_n00b]

BTW - been doing so for decades, no issues, just want to know the exact Code for it.

If you have been doing this for decades and have not found the code reference that allows you to do it then it does not exist. Which probably means there is nothing in the code that says you can't do it either.

 

[pv_n00b]

Applying the 120% to a fused disco or MBO is a very six-or more-years-ago confusion at this point.

It used to be common to apply the 120% rule to feeder conductors between panels too. But as you say, not to an OCPD. Things have evolved a bit.

 

[jwoody]

As far as Code reference for the MSP, it seems I would just use the basic 705.12 permission to connect load side?
"The output of an interconnected electric power source shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any distribution equipment on the premises."

I do not see any other Code reference that would be used here.
None of the following and numbered clauses apply at the MSP since not connecting to feeder, no tap, no busbar.

Do i have this right?

BTW - been doing so for decades, no issues, just want to know the exact Code for it.

Yes you have it correct. As long as nothing (PV) is tapped, then only use 705.12(B) for the busbar ratings.