Switchgear question

[andrew.tkelly]

When a switchgear placard says it has a "Maximum Supply Rating" of 2000A, and a "Maximum Section Rating" of 1200A, which is the value I would apply the 120% Rule to? What is the difference between these two types of values?

Andy

 

[charlie b]

The manufacturer can send you two sections that are to be installed side-by-side and connected to each other internal to the switchgear, each being rated at 1200 amps. But you can't supply more than 2000 amps to the combination. So it depends on how you intend to install the switchgear. Will you have two or more sections, or just one?

By the way, what "120% rule" are you referring to?

 

[andrew.tkelly]

Hi Charlie,

In this particular case, I am putting three separate, small PV systems all on one commercial rooftop. Each system will interconnect into the main panel of the respective office it supplies with power. Each office has a panel filled with branch breakers and is protected by a 150A OCPD located at the top of the panel.

However, these 150amp panels are supplied by a switchgear that is in an adjacent building within the office park. The switchgear seems to be comprised of (3) adjacent sections. Each section has (6) 150 amp switches that feed the aforementioned electrical panels, for a total of 18 electrical panels being supplied by the switchgear. The whole switchgear has a 2000A load break switch. [the switchgear is a Square D Power Style Commercial Multi Metering, 208Y connection]

I'm trying to apply the 120% Rule for backfeeding the busbar throughout the whole system stream.

I'm guessing at the Switchgear sectional level, it would look something like this:

1200 amps (max. section rating) * 120% = 1440 amps
1400 amps - (150 amps * 6 [# of breakers on the section]) = 540 amps
Therefore I can backfeed up to 540 amps on this switchgear section

Is my thinking correct? Please let me know if I can clarify anything. Sometimes it's hard to convey what's in my head into words.

Andy

 

[jaggedben]

If the 1200A buses are protected by a 2000A switch then you're not getting very far by invoking the 120% rule. :happysad:

The 2014 NEC has this 705.12(D)(2)(3)(d):

Connections shall be permitted on multipleampacity
busbars or center-fed panelboards where
designed under engineering supervision that includes fault
studies and busbar load calculations.

 

[andrew.tkelly]

If the 1200A buses are protected by a 2000A switch then you're not getting very far by invoking the 120% rule. :happysad:

The 2014 NEC has this 705.12(D)(2)(3)(d):

Hmmm... this my first time designing a small commercial system where there are multiple busbars of differing ampacities involved. In the past, I've been lucky enough to easily land in a simple MDP. While I've had load calculations performed by hired electricians, I've never had fault studies done --how does one find this kind of service? Also, does "designed under supervised engineering" essentially mean that I'd have to have an electrical PE sign-off on this?

Regards,
Andy

 

[GoldDigger]

...
...
Also, does "designed under supervised engineering" essentially mean that I'd have to have an electrical PE sign-off on this?

Regards,
Andy

Not necessarily, but that would certainly be the best evidence for an inspector that the engineering supervision was exercised.

 

[jaggedben]

... Also, does "designed under supervised engineering" essentially mean that I'd have to have an electrical PE sign-off on this?

That's how I would read it.

Another way around it might be to argue that the switchboard isn't a panelboard, and that thus the 120% rule doesn't apply (because the code says 'busbars in panelboards'). Instead you would argue that the busbar is a feeder, and invoke 705.12(D)(2)(1)(b), pointing out that your existing load breakers protect all downstream loads. That's perhaps a tortured interpretation of the code, but it makes sense from common sense electrical theory point of view.

For what it's worth, I passed one job where I had a miniscule backfeed on one meter of a large meter bank, and I just drew up the plans as if there wasn't a service disconnecting means ahead of the meters. I just didn't think of the meters as panelboard and it didn't even occur to me until afterward what I'd done. No one else noticed.