taps not over 10 ft.(Art. 240-21(b)1)

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jeff kurtz

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I've got a building(the chapel on a college campus where I work) which was upgraded a few years ago and the entire electrical system was pretty much replaced. The feed is 480V, 3-phase which goes to a 400-amp fused disconnect, then to a 300 kva step-down transformer, to a distribution panel and out to all the power panels. The step-down transformer steps down from 480 to 120/208. Unfortunately, there are some of the old loads which are 240V single phase which are not served by the new system and are fed from a 75 kva single-phase transformer which is not even in the building.
I don't know if this is legal by code, because if you kill the building's main, you do not shut off all the power going through the building, and some of the 240 Volt circuits are in the same conduit as the building's system branch circuits.
In any event, I'm wondering if I can tap a single-phase circuit off of the load side of the 400-amp fused disconnect which feeds the building's step-down transformer, install another smaller appropriately fused disconnect directly on the side of the 400-amp disconnect,(to protect the 75 kva transformer's primary) with wire sized for the smaller transformer, move the single-phase 75 kva transformer inside the building electrical vault next to the newer 300 kva transformer, then feed the 240V loads.
I did put a clamp-on on the building feed wires and each phase is only drawing about 20 amps and the 240V loads don't add up to more than 20 or 30 amps, so overloading is not an issue.
The main on the 240V power panel which is now being fed from the 75 kva transformer is sized at 125 Amps.
My objective is to have a building main which does shut off all power to the buildings.
Is what I'm proposing advisable, legal, and necessary?
Incidentally, I think what happened was an oversite on the part of the EC, not realizing there were 240 V loads because the breaker which feeds the 75 kva transformer is labelled "temp. power", the power panel which feeds the 240V loads is behind the 300 kva transformer and not at all accessible, and there is no mention of it on the remodel prints. I also plan to move the power panel which feeds the 240V loads to a more appropriate location.
 
Re: taps not over 10 ft.(Art. 240-21(b)1)

jeff kurtz said:
Is what I'm proposing advisable, legal, and necessary?
Incidentally, I think what happened was an oversite on the part of the EC, not realizing there were 240 V loads because the breaker which feeds the 75 kva transformer is labelled "temp. power", the power panel which feeds the 240V loads is behind the 300 kva transformer and not at all accessible, and there is no mention of it on the remodel prints. I also plan to move the power panel which feeds the 240V loads to a more appropriate location.
Click to expand...

Ah, don't you just love electrical maintenance?! EC's come and EC's go--they have there orders from who-knows-who and now it's just you to make it right! Been there, done that, got the tee-shirt and now I'm sending it to you! :lol:

I don't know what kind of physical set-up you have but it sounds like all of the electrical panels are in close proximity. Is there a chance the 240V single phase loads are suppose to be moved to slots on your new 240V panel? This would be the best way to keep your loads in one spot and better chance of equal loading on each phase of the feed.

Or you could power your 240 single ph. panel as a sub pan from the new one and keep the additional slots for future expansion. You will need remove the Main Bonding Jumper (MBJ) and run 4 wires to the Main panel if you go this route.

Best of luck!

-Dale

Disclaimer: To all you EC's out there (and I know there are a lot!) please don't take offense to me siding with my fellow maintenance Bro. It's just that I feel his pain. :wink: Hey, some of us have to stick around even after the bills are paid.
 
Okay, I'll settle this. :)

I suggest checking the old 240v loads for being 208v compatible; most are. Those that are not can be fed through buck-boost transformers from the new panel.

How's that? :wink:
 
If you are saying that the EC who did the remodel placed a new 300 KVA transformer in a position that blocked access to an existing (whether or not it is really 'temporary") panel, then that is not an "oversight." It is a clear violation of 240.24(A), and probably a violation of 110.26 as well. So you do have a problem on your hands. You need to fully define the problem before you can be confident in your solution.

One of the first steps to defining the problem is to answer Bob's question:
petersonra said:
does the 75kva xfmr feed off the same service disconnect? that would seem to be what is relevant.
Click to expand...

The new 300 KVA transformer and the old 75 KVA transformer each has a primary side disconnect. Is there a service disconnecting means that is upstream of both? If not, if both transformers are supplied by the utility, then you have a violation of 230.2.

Your proposed actions seem to me to be both allowable under the NEC and a good design. There may be better designs, either in terms of cost or in terms of allowanced for future expansion, but this one does work.
 
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