Load Side Connection in a "Hot Buss" Panel, NEC 2017

[autumnstreamfellow]

I have a question about what I have been calling a "hot bus" Main Service Panel. Utility feed goes straight to the bus bars and you have less than 6 breakers feeding off of the bus. An example panel is rated at 125A and my understanding is that because the existing load breakers are: 100A, 60A & 40A, the panel is already overloaded and must be replaced if we want to do a load side PV connection. However, I have been told by others that you could, in fact connect up to 125A of solar to the open breaker slot and call it a day because the 120% rule does not need to be considered in these type of panels. We are using NEC 2017.

Can someone tell me which is correct? Thank you

 

[electrofelon]

I have a question about what I have been calling a "hot bus" Main Service Panel. Utility feed goes straight to the bus bars and you have less than 6 breakers feeding off of the bus. An example panel is rated at 125A and my understanding is that because the existing load breakers are: 100A, 60A & 40A, the panel is already overloaded and must be replaced if we want to do a load side PV connection. However, I have been told by others that you could, in fact connect up to 125A of solar to the open breaker slot and call it a day because the 120% rule does not need to be considered in these type of panels. We are using NEC 2017.

Can someone tell me which is correct? Thank you

This is typically called a MLO or main lug only panelboard. adding and connecting to a breaker would be a supply side connection, no 120% rule. You can connect up to the rating of the service, which is the lowest of either the service conductors, or the rating of the panel board.

 

[jaggedben]

The panel may be overloaded. See bottom.

Solar is not a load and therefore has no bearing on whether the panel is overloaded. Electrofelon is correct that it is a supply side connection and not subject to the 120% rule.

However, by bringing an inspector out to look at a panel that might be overloaded, you potentially commit your client (or yourself, if you didn't write your contract carefully) to more than was bargained for.

Here's the thing: 230.90(A) Exception 3 allows you to have 200A of breakers on a 125A service but only if a load calculation comes out to 125A or less. So you need to do a load calculation, and find out if your client's existing panel is compliant or not. The easiest way for an existing dwelling is 220.83. Don't include the solar.

 

[romex jockey]

230.90(A)Exception No. 3: Two to six circuit breakers or sets of fuses shall be
permitted as the overcurrent device to provide the overload protection.
The sum of the ratings of the circuit breakers or fuses shall be permitted
to exceed the ampacity of the service conductors, provided the calculated
load does not exceed the ampacity of the service conductors.

Jag,
are there 705 ref's that lead to this?
~RJ~

 

[jaggedben]

Jag,
are there 705 ref's that lead to this?
~RJ~

You mean does 705 reference that section of 230? No, it doesn't. That might be changing in the 2020 NEC, the drafts had some new references. There's the whole changing of the rules in 230 as well.

 

[pv_n00b]

You mean does 705 reference that section of 230? No, it doesn't. That might be changing in the 2020 NEC, the drafts had some new references. There's the whole changing of the rules in 230 as well.

The 2020 NEC 705 will have a new section just for supply side interconnections. The CMP finally decided to provide some guidance.