Are there exceptions to having inverter OCPD at opposite end of bus?

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Anode

Member
Location
Washington, USA
In this case, we have a 50Amp OCPD (oversized, provided by EC, we only need 20AMP OCPD), and it is already installed at the top of the bus in a 1600AMP SWB?

Any exceptions to the rule? There are next to no chances of the pv system being expanded.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
In this case, we have a 50Amp OCPD (oversized, provided by EC, we only need 20AMP OCPD), and it is already installed at the top of the bus in a 1600AMP SWB?

Any exceptions to the rule? There are next to no chances of the pv system being expanded.

What code cycle?

Under the 2014 code, if all the breakers other than the main don't add up to more than the busbar rating, you're okay.

Also, if the switchboard busbar is rated 50A or more higher than the main OCPD, then you're okay.

If on the 2011 code or earlier then only the second option is available.
 

Anode

Member
Location
Washington, USA
What code cycle?

Under the 2014 code, if all the breakers other than the main don't add up to more than the busbar rating, you're okay.

Also, if the switchboard busbar is rated 50A or more higher than the main OCPD, then you're okay.

If on the 2011 code or earlier then only the second option is available.

2014 Code cycle.

It appears that all our breakers add up to more than the 1600 Amps, so we could not use no. 1 you indicated. What code is that in reference to by the way, I presume something in 705.12?

Capture.jpg

In case that image is too small to see - http://imgur.com/a/lXeCe

I should also say that the position of the 50AMP OCPD is not accurate, it is closer to the top of the bus.

For the second suggestion, the SWB doesn't have a main, but its protected by a 3000AF in the parking garage 20 stories below, in a larger SWB.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
2014 Code cycle.

It appears that all our breakers add up to more than the 1600 Amps, so we could not use no. 1 you indicated. What code is that in reference to by the way, I presume something in 705.12?

View attachment 15761

In case that image is too small to see - http://imgur.com/a/lXeCe

I should also say that the position of the 50AMP OCPD is not accurate, it is closer to the top of the bus.
Read all of 705.12(D)(2)(3)Busbars. There are 4 ways to qualify a busbar. Is there a main breaker on that panel? If the rating of the OCPD protecting the bus is equal to the rating of the bus and the sum of the ratings of all the breakers on the bus excluding the one protecting the bus is equal to or exceeds the rating of the bus, you must move the backfed breaker to the other end of the bus.
 

Anode

Member
Location
Washington, USA
Read all of 705.12(D)(2)(3)Busbars. There are 4 ways to qualify a busbar. Is there a main breaker on that panel? If the rating of the OCPD protecting the bus is equal to the rating of the bus and the sum of the ratings of all the breakers on the bus excluding the one protecting the bus is equal to or exceeds the rating of the bus, you must move the backfed breaker to the other end of the bus.

Thank you for the help. Hypothetically speaking, what if this were a center fed bus SWB? Would it still have to be on the opposite end from where the bus is fed, on either side basically.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Thank you for the help. Hypothetically speaking, what if this were a center fed bus SWB? Would it still have to be on the opposite end from where the bus is fed, on either side basically.

No. If it were a center fed bus (either with a centered main breaker or with a centered feeder connection from an upstream breaker) there would be no way that you could take advantage of the 120% rule, period.

Is there any possibility (based on calculated load, etc.) that you could reduce the size of the current 1600 main or feeder breaker serving the panel? Going to 1500A instead of 1600A would allow you to use the 100% of sources rule.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Could he connect under 705.12(D)(2)(3)(d) with engineering supervision?
Yes and no. Even though the difference is small, with the PV breaker up near the main breaker, the bus below it can draw more current than the bus is rated for. No engineer would OK it.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
For the second suggestion, the SWB doesn't have a main, but its protected by a 3000AF in the parking garage 20 stories below, in a larger SWB.

Are you referring to 3000 amp fuses? Seems like a problem in and of itself to have a 1600A switchboard protected by 3000A OCPD.
 

pv_n00b

Senior Member
Location
CA, USA
In this case, we have a 50Amp OCPD (oversized, provided by EC, we only need 20AMP OCPD), and it is already installed at the top of the bus in a 1600AMP SWB?

Any exceptions to the rule? There are next to no chances of the pv system being expanded.

Never trust the single line to correctly show what is going on in a switchboard. Single lines are drawn for convenience and not physical accuracy because most of the time it does not matter. So you have to get in the switchboard and see what is really going on.

Most likely you will find out this is a center fed bus, it's just easier and cheaper to build them this way so it's really common. That puts you in 705.12(D)(2)(3)(d). Since this is a small back feed on a large bus it would probably pass an engineering study. Unfortunately it's a small PV system having to pay for an engineering study, that is usually problematic cost wise. The problem I always see with the engineering study approach is that it represents a snapshot in time. The switchboard loaded up as it is today might pass but tomorrow someone might install a new load that would then make it fail. Is everyone who comes after the PV install going to redo the engineering study every time they change the loading? Probably not.

Then there is the issue of applying all this again to the switchboard that feeds this switchboard, and any other switchboards it goes through back to the service entrance.

jaggedben; it's a 3,000A breaker frame, most likely with a 1,600A plug.
 

Carultch

Senior Member
Location
Massachusetts

jaggedben; it's a 3,000A breaker frame, most likely with a 1,600A plug.


See if it is possible to dial the 1600A plug down to a rating such that main + PV <= Busbar rating.

This involves checking the load calculation, and the details of the breaker adjustment plug. A 30-day demand recording meter can also suffice, to show what the peak load would be in a practical sense.
 

Anode

Member
Location
Washington, USA
Well, the winning solution was to just move our point of connection to the 400A panelboard fed from the switchgear, which had space for the the 3P 50A OCPD for our inverter output circuit.

With respect to the 400A OCPD protecting that panelboard, if it was not at the end of the bus already, would it technically need to be installed at the end of bus position in the SWB to make it code legal, just as if we were connecting the inverter output circuit OCPD at the SWB?
 

Carultch

Senior Member
Location
Massachusetts
Well, the winning solution was to just move our point of connection to the 400A panelboard fed from the switchgear, which had space for the the 3P 50A OCPD for our inverter output circuit.

With respect to the 400A OCPD protecting that panelboard, if it was not at the end of the bus already, would it technically need to be installed at the end of bus position in the SWB to make it code legal, just as if we were connecting the inverter output circuit OCPD at the SWB?

My interpretation is yes.

I see that if you connect at a subpanel, then its corresponding main panel, and every panelboard/switchboard upstream to the point of common coupling, must also comply with an interconnection rule. Because consider the event that all loads in that subpanel diminish to zero, and its PV operating power is at full capacity. You now have the functional equivalent of a PV inverter directly connected to the main switchboard.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
My interpretation is yes.

I see that if you connect at a subpanel, then its corresponding main panel, and every panelboard/switchboard upstream to the point of common coupling, must also comply with an interconnection rule. Because consider the event that all loads in that subpanel diminish to zero, and its PV operating power is at full capacity. You now have the functional equivalent of a PV inverter directly connected to the main switchboard.
I concur. The language in 705.12(D)(2)(3)(b) starts off with "When two sources, one a utility and the other an inverter are located at opposite ends of a busbar..." It makes no difference if the utility or the inverter is connected directly to the bus or if the connection is through another panel; the 120% rule applies.
 

Ozymandias

Member
Location
Missouri
My interpretation is yes.

I see that if you connect at a subpanel, then its corresponding main panel, and every panelboard/switchboard upstream to the point of common coupling, must also comply with an interconnection rule. Because consider the event that all loads in that subpanel diminish to zero, and its PV operating power is at full capacity. You now have the functional equivalent of a PV inverter directly connected to the main switchboard.

In my pv class project we connected to a subpanel in a garage where the array was and back-fed at the end of the bus. I wondered about any the main panel also following that rule but forgot to follow up on it. Is this code cycle specific or is it a general rule?

EDIT: I just read ggun's answer to my question.
 
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