# Thread: transformer OCPD

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## transformer OCPD

Hi,

I have a solar installation where I have gotten push back on doing a line side tap so I am looking at the 120% busbar rating rule. I have a 600A rated switchgear with existing 400A fuses. I have 160A at 480V of max operating current for inverters (40A X 4 inverters). I have designed it with a 150KVA, 480V/240V transformer. I came up with 133.01 KVA of actual max backfeed from from the solar (160A x 1.732 x 480V / 1000 = 133.017). So on the 240V side I have 133,017/1.732 = 76,799.65/240V = 319.99A. Can I round down to a 300A breaker? Or do I need to go up to 350A?

For the 120% I have 320A to play with. If I need to go up to 350A breaker I will need to downsize the main fuses to 350A.

Trying to avoid the UL certification report for the line side tap the plan checker asked for.

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Lol, you came up with one thousandth of an amp under the requirement? Seriously though, I think it's just the nominal 480/240, multiply by 2.

The 2014 code goes by inverter output times 125%, not the breaker size, so having a 350A breaker doesn't matter. What you weren't clear on is whether you've already included the 125% in the calcs. That's an important point. You said that was the max operating current, which sounds like the data sheet figure. If that's the case, it's no good. If the datasheet figure is actually 32A then all is okay but you need a 350A breaker and conductors, not 300A.

3. You said"switchgear" in the OP. Some more details may be prudent as this could effect your answer. IIRC, and depending in what code cycle you are on, the 120% Rule is for panelboards.

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So the max operating current is what I was talking about. So the inverter OCPD would be 50A for each. So that would be 200A of OCPD on the 480V side.

It is a switchboard used as service equipment. A GE Type AJ Switchboard to be exact. AJT426R Catalog number.

Now I am looking at different design. 1 inverter max output current 24A and 3 inverters max output current of 40A = 144A at 480 volts. Max output current, not OCPD. OCPD would be 180A.
144A x 480 x 1.732 = 119,715.84/1000 = 119.72KVA. I am using a 150KVA
119,715.84/1.732= 69,120/240 = 288A at 240volts.
I am talking max output current, not OCPD. Can I use 300A OCPD on the 240V side?? Or do I need to go 288A x 1.25 = 360A (or 400A OCPD).

Thanks ahead of time for your help!

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Originally Posted by electro7
...Or do I need to go 288A x 1.25 = 360A (or 400A OCPD).
...
Yes, you do.

(Also you'll probably need a 200A OCPD on the 480V side since there's no standard size between 175 and 200.)

As electrofelon mentioned, you should ask the AHJ if you can ignore the 120% rule because this is a 'switchboard' not a 'panelboard' and the code language applies that rule only to 'panelboards'. Might be a way around the limitation, especially if you could get an EE to approve it.

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Yes, 200A on 480V side.

So your saying it maybe okay to interconnect on the load side of main breaker, and not have to worry about the 120% rule? Nothing in NEC about that? I understand its up to AHJ.

I see there is pre-threaded holes in the bussing on the load side of the main breaker but not at the opposite end of the busbar. Any thoughts on not being at the opposite end of feed?

Jaggedben, whats your opinion on safety if the NEC doesnt say anything about switchboards and the AHJ is okay with it. Do you feel it could be a problematic situation and could cause a hot spot on the bussing at that interconnection point?

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[QUOTE]So your saying it maybe okay to interconnect on the load side of main breaker, and not have to worry about the 120% rule? Nothing in NEC about that? I understand its up to AHJ./QUOTE]

....because it is a switchboard and not a panelboard.

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I don't think my opinion matters; that's why I'd consult an engineer. Or perhaps as the switchboard manufacturer. I don't really know why those code rules are specific to panelboards.

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Shocking, but NEC 705.12(B) and (B)(2)(3) are not consistent.

Up in (B) it says:
(B) Load Side. The output of an interconnected electric
power source shall be permitted to be connected to the load
side of the service disconnecting means of the other source(s)
at any distribution equipment on the premises. Where distribution
equipment, including switchgear, switchboards, or panelboards,
is fed simultaneously by a primary source(s) of
electricity and one or more other power source(s), and where
this distribution equipment is capable of supplying multiple
branch circuits or feeders, or both, the interconnecting provisions
for other power sources shall comply with 705.12(B)(1)
through (B)(5).

It definitely includes switchgear, switchboards, and panelboards here and specifies that (B)(1) through (B)(5) applies to them all. But then you get down to (B)(2)(3) and it says:
(B)(2)(3) Busbars. One of the methods that follows shall be used to
determine the ratings of busbars in panelboards.

Inconsistent I know, but I think it's easy to argue that since (B) says it all applies to switchboards and switchgear even though (B)(2)(3) does not write all three out that (B)(2)(3) applies to switchboards and switchgear. But is there is an AHJ that just reads (B)(2)(3) and not (B) or ignores the connection? Anything is possible. Can someone use this to say they can back feed a switchboard up to the bus rating? They can try but I don't buy it. The same theoretical limitations apply to all three.

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