No OCPD on service.. How to calculate max allowable load on switchgear busbar?

[BretHeilig]

Hi all,

I'm trying to do a solar PV proposal for a large warehouse in NJ. Code says that the maximum allowable current on a busbar can't exceed the ratings of the OCPDs protecting that busbar. The main switchgear is rated for 1200A, but the service has no OCPD at all (or rather, it probably has one, but it's on the utility side, and nobody knows what its rating is). I'd like to install a PV system which will backfeed through a 750A breaker.. my question is, how do I figure out whether I can do that or not? Do I need to call up the utility and find out what their OCPD is rated for? Or does it fall back to the total allowable loads on the switchgear (which consist of a 700A subpanel breaker, and an 800A subpanel breaker)? Or am I basically home free to land anything up to 1200A? Thanks for any advice,


Bret

 

[Dennis Alwon]

You probably can tell by checking the disco ahead of the meter or by checking the wire size in the distribution panel. That may give you some info if there is no main.

 

[iwire]

You need to find the rating of the bus bars, it will be on a label somewhere.

You also need to look at the conductors supplying the bus bars to determine there ampacity.

 

[BretHeilig]

I assumed the rating of the switchgear (1200A) is also the rating of its busbars, no?

As for searching for a disco upstream of the switchgear, that's just the problem: there isn't one, at least, not one that the property owner can access.

 

[Dennis Alwon]

I assumed the rating of the switchgear (1200A) is also the rating of its busbars, no?

As for searching for a disco upstream of the switchgear, that's just the problem: there isn't one, at least, not one that the property owner can access.

Yes if the switchgear is rated 1200 so is the busbar. I would calculate the cir. mil of the wires (parallel I assume) and see what it's equivalent is in Chapter 3 in T. 310.16

 

[augie47]

Most interesting question. I'm confident someone can, but I'm unable to see how the Code addresses that. There is a good possibility there is no "main" anywhere and it is simply a typical MLO service panel.
Hopefully someone will show how that is addressed. If not, I would request a POCO report on the demand and my busbar was 120% of that.

 

[iwire]

You probably can tell by checking the disco ahead of the meter or by checking the wire size in the distribution panel. That may give you some info if there is no main.

Gus I am not following you

The service is rated as low as it's weakest link.

 

[don_resqcapt19]

If the 700 and 800 amp breakers are in the switchgear, they are the service disconnects. That would make the solar connection a "supply side" connection. Assuming that the maximum output of the solar system is less than the maximum building load, I don't see any code issues connecting it to the bus.

 

[BretHeilig]

Don, yes that seems to be the crux of the issue.. connecting a PV array under this circumstance is no riskier than not connecting the array. Code appears to allow this via 2005 - 230.90 (A) Exception 3. As a followup, I still expect the AHJ to come out and do a double-take about it...does anyone have any practical experience with getting an AHJ to accept a statement of "calculated load" or equivalent? This is a large warehouse.. cataloging every electrical device inside it could take days.

Thanks very much to all for the help!

 

[don_resqcapt19]

I don't see any need for any additional calculations as long as the output of the solar system is less than the full load of the building. The bus will never see more current that the full load of the building. If there is no load at the building all of the solar system output current will be fed to the utility. If the building has load the current on the service entrance conductors will be reduced by the amount of current that the solar system is feeding to the bus. There will never be more current on the bus than that of the full load of the building. For all practical purposes as long as the output of the solar system is less than the rating of the bus there is no issue.

 

[BretHeilig]

But that's just the issue: how do I know what the full load of the building is? The Service Disconnects are rated 700A and 800A, which together can pass 1500A, 25% more than the busbar is rated for. Since the building hadn't burned down before the last time I was there, I assume such an overcurrent situation hasn't happened yet, but that's not guaranteed by the design, by any means.

It's not so much that I think the PV system will create new danger. It's that, in my experience, when you go to install a PV system, you end up owning anything that's wrong with the main switchgear. I'm just wondering if anybody has any experience with being given a hard time over this or similar issues. If nobody has, well seems like good news for me, but watch this space over the next few months!

 

[don_resqcapt19]

Was the service inspected when it was installed? They would have needed a load calculation to show that the total calculated load is less than the rating of the bus bar. If this is a commercial or industrial occupancy, there may be a demand meter and if so you should be able to get the maximum demand from the utility. As long as that shows less than 1200 amps you are good to go.

 

[BretHeilig]

Ahhh, that's a smart way to do it Thanks!