Max PV System Capacity to POC (Point Of Connection)

[bendesa]

Hi all,

This seems to be a nice foru, with a lot of available knowledge. I do have a question about the following.

When connecting a high capacity (1MW and up) PV System to the POC (Point Of Connection) how do you know if it is possible to the
available MDP (Main Distribution Panels)?

I've read about the 120% rule but it's not completely clear to me. I do have an example here of an distribution panel. What could be the maximum
PV System capacity to connect?



I hope somebody can help me out on this

Many thx in advance

Kind regards

Ben

 

[ggunn]

If you re connecting through a backfed breaker in an MDP with OCPD you must satisfy 705.12(B)(3). If you are connecting on the supply side of the MDP OCPD or if the MDP is MLO, you must comply with 705.11. Most larger systems interconnect under 705.11 because with a supply side interconnection you are only limited in system size by the utility transformer rating and the ampacity of the service conductors, whichever is less.

 

[electrofelon]

If you re connecting through a backfed breaker in an MDP with OCPD you must satisfy 705.12(B)(3). If you are connecting on the supply side of the MDP OCPD or if the MDP is MLO, you must comply with 705.11. Most larger systems interconnect under 705.11 because with a supply side interconnection you are only limited in system size by the utility transformer rating and the ampacity of the service conductors, whichever is less.

What has been your experience with load side interconnections on switchboards vs panelboards? The 120% rule seems to only apply to busbars in panelboards in 2017. Is this any different in 2020?

 

[ggunn]

What has been your experience with load side interconnections on switchboards vs panelboards? The 120% rule seems to only apply to busbars in panelboards in 2017. Is this any different in 2020?

Can you demystify this for me? I have never treated them differently except that the other end of the busbar rule can't be applied if the busbar is fed from both ends.

 

[electrofelon]

Can you demystify this for me? I have never treated them differently except that the other end of the busbar rule can't be applied if the busbar is fed from both ends.

2017 states the following:

(3) Busbars. One of the methods that follows shall be used to determine the ratings of busbars in panelboards.

I am assuming that was an oversight by the code writers but it does say what it says, and a switchboard is a very different animal than a panelboard. The OP has a switchboard. Was this changed or clarified in 2020?

 

[jaggedben]

'Panelboards' was removed in the 2020. I believe, buy I'm not certain, that the reason was to include switchboards, although the non-specifying language also raises some odd but probably not very consequential questions as discussed in this thread.

 

[GeorgeB]

When connecting a high capacity (1MW and up) PV System to the POC (Point Of Connection) how do you know if it is possible to the
available MDP (Main Distribution Panels)?

I've read about the 120% rule but it's not completely clear to me. I do have an example here of an distribution panel. What could be the maximum
PV System capacity to connect?

Your 400V system suggests this is not in the USA. Are you (is this installation) subject to the NEC? If not, what is the regulating authority?

 

[wwhitney]

Was this changed or clarified in 2020?

It is easy to see the 2020 version by looking here:

https://up.codes/viewer/alabama/nfpa-70-2020/chapter/7/special-conditions#705.12_(B)_(3)

Cheers, Wayne

 

[ggunn]

2017 states the following:

(3) Busbars. One of the methods that follows shall be used to determine the ratings of busbars in panelboards.

I am assuming that was an oversight by the code writers but it does say what it says, and a switchboard is a very different animal than a panelboard. The OP has a switchboard. Was this changed or clarified in 2020?

Thanks, but I am not interested intaking advantage of oversights, typos, etc. in the NEC. I think I will just continue considering a busbar a busbar.

 

[wwhitney]

Thanks, but I am not interested intaking advantage of oversights, typos, etc. in the NEC.

Isn't TX on the 2020 NEC? In which case you have leeway as an engineer to use (2020) 705.12(B)(5).

Cheers, Wayne

 

[ggunn]

Isn't TX on the 2020 NEC? In which case you have leeway as an engineer to use (2020) 705.12(B)(5).

Cheers, Wayne

I will and I have. I just won't try to defend my design to an inspector by saying "this is a switchboard not a panelboard".

 

[pv_n00b]

'Panelboards' was removed in the 2020. I believe, buy I'm not certain, that the reason was to include switchboards, although the non-specifying language also raises some odd but probably not very consequential questions as discussed in this thread.

It's an interesting path for this change. In 2017 and before NEC the 120% rule was limited to panelboards and not intended to apply to switchgear or switchboards. Switchboard design is entirely different and a switchboard's ability to dissipate heat is much greater than a panelboard. But that left people hanging, how does one deal with switchboard interconnections? Most people did it under engineering supervision or just used the 120% rule even though it did not apply.
In the 2020 NEC this was made more explicit by removing the restriction to panel boards and adding an allowance for design under engineering supervision. So 705.12(B)(3)(2) removed the restriction to panel boards and 705.12(B)(3)(5) allows interconnections to switchboards, switchgear, and panel boards that do not comply with the 120% rule if made under engineering supervision. Or if you don't have an engineer around the 120% rule can be applied. So it's clearer now than before.
We do almost all our load-side interconnections to switchboards under engineering supervision as they would not meet the 120% rule. We still use the 120% rule for panel boards.

 

[electrofelon]

It's an interesting path for this change. In 2017 and before NEC the 120% rule was limited to panelboards and not intended to apply to switchgear or switchboards. Switchboard design is entirely different and a switchboard's ability to dissipate heat is much greater than a panelboard. But that left people hanging, how does one deal with switchboard interconnections? Most people did it under engineering supervision or just used the 120% rule even though it did not apply.
In the 2020 NEC this was made more explicit by removing the restriction to panel boards and adding an allowance for design under engineering supervision. So 705.12(B)(3)(2) removed the restriction to panel boards and 705.12(B)(3)(5) allows interconnections to switchboards, switchgear, and panel boards that do not comply with the 120% rule if made under engineering supervision. Or if you don't have an engineer around the 120% rule can be applied. So it's clearer now than before.
We do almost all our load-side interconnections to switchboards under engineering supervision as they would not meet the 120% rule. We still use the 120% rule for panel boards.

Yeah I agree panelboards and switchboards are totally different, so I don't see it at all as a "Technicality" or odd that the rules might be different. That said its hard to imagine there wasn't a mistake in the 2017. (e), the part that allows engineering supervision for multiple ampacity bus bars (which certainly is directed at switchboards as I have never heard of such a thing in a panel board) is still under the "busbars in panelboards" section. So you are saying that they did indeed intend to leave out switchboards from the 120% rule but just messed up the structure of those sections?

 

[jaggedben]

...That said its hard to imagine there wasn't a mistake in the 2017. (e), the part that allows engineering supervision for multiple ampacity bus bars (which certainly is directed at switchboards as I have never heard of such a thing in a panel board) is still under the "busbars in panelboards" section. So you are saying that they did indeed intend to leave out switchboards from the 120% rule but just messed up the structure of those sections?

Basic editing errors in the NEC? I'm shocked. Shocked, I tell you.

 

[pv_n00b]

Yeah I agree panelboards and switchboards are totally different, so I don't see it at all as a "Technicality" or odd that the rules might be different. That said its hard to imagine there wasn't a mistake in the 2017. (e), the part that allows engineering supervision for multiple ampacity bus bars (which certainly is directed at switchboards as I have never heard of such a thing in a panel board) is still under the "busbars in panelboards" section. So you are saying that they did indeed intend to leave out switchboards from the 120% rule but just messed up the structure of those sections?

I don't know about the intention, not being at the table in the CMP4 discussions, but switchboards have been left out for a while and it was not unknown to CMP4. I'm actually surprised at the number of people, including AHJs, who just never noticed that the 120% rule was limited to panelboards. Most people just blank that part out and assume it applies to everything. I don't know about the multiple ampacity busbars either. Not used in panelboards that I know of but the section does not say multiple ampacity busbars in switchboards so who knows what CMP4 was getting at? Overall, 705.12(B)(2)(3) is poorly written so the rewrite in 2020 was really needed.

 

[LarryFine]

Basic editing errors in the NEC? I'm shocked. Shocked, I tell you.

 

[jaggedben]

...I'm actually surprised at the number of people, including AHJs, who just never noticed that the 120% rule was limited to panelboards. Most people just blank that part out and assume it applies to everything. ...

I think one contributing factor is that in the 2011 NEC it was indeed a blanket requirement for all 'busbars or conductors', and that was the era many AHJs first had to bone up on 705. Many just never noticed any of the changes and still just think '120% rule.'

 

[electrofelon]

I think one contributing factor is that in the 2011 NEC it was indeed a blanket requirement for all 'busbars or conductors', and that was the era many AHJs first had to bone up on 705. Many just never noticed any of the changes and still just think '120% rule.'

so we should be able to look up the comments and reasoning of the addition of the word "panelboard" then. Who gunna do it?

 

[jaggedben]

so we should be able to look up the comments and reasoning of the addition of the word "panelboard" then. Who gunna do it?

I don't think you'll find a specific comment on that because it was part of a comprehensive re-write of 705. IIRC, the reports say something about how the proposal was the result of discussions among a large group of stakeholders, or some such. So you'd probably have to track down some of those people and quiz them. I'm sure Bill Brooks remembers something, although it was 10 years ago now.

 

[Carultch]

I will and I have. I just won't try to defend my design to an inspector by saying "this is a switchboard not a panelboard".

Agreed. Until we have a specific rule that specifies the math governing interconnections on switchboards, I'd treat switchboards just like panelboards, whether the NEC sneaks the word panelboard in the article or leaves it for applying to busbar equipment in general. Load centers / panelboards/ switchboards / MCC's / switchgear, are all the same beast to me, with the essential difference being scale and application.