240.21(C)(4) ? Transformer Secondary Taps

[charlie b]

(NOTE: I see that there are two current threads discussing issues similar to mine. But I think my question is different enough to give it its own thread.)

I am involved in the design of a new laboratory facility (presently a dirt field). Two medium voltage switches will serve two transformers (12.47kv to 480/277V, 3000 KVA each). They in turn will feed a ?Main-Tie-Main? switchboard that is located perhaps 150 feet away. The MV switches, the transformers, and the switchboard will all be within the same building, at grade level. We are early enough in the design process that I can call for conduit runs under the slab.

Question: If I run the secondary conductors from the transformers to the switchgear below the slab, can I treat these conductors as being ?outside secondary conductors??

I would like to take advantage of 240.21(C)(4), and not install an overcurrent device at the transformers. But these conductors will be "within the building" from their point of connection to the transformers until they reach the underground duct bank below the transformers. Does that fact cause a problem with my plan?

I know that 230.6 states that conductors can be considered ?outside the building? in the type of installation I am contemplating. But that is not a ?definition,? being outside Article 100, and I don?t know if it would apply to Article 240. Also, in 230 it is presumed that the conductors begin their run from a point outside the building, and never become "within the building" until they are stubbed up from below.

FWIW, the 2004 California Electrical Code applies, and it is based on the 2002 NEC. But I don?t see anything significantly different between this and the 2005 NEC.

 

[iwire]

Charlie I believe you will be fine and will find a reference to 230.6 in the Article for outside feeders and branch circuits.

 

[rbalex]

In the 2004 California Electrical Code it's Section 225.32. I wouldn't have any problem with what you wanted to do either. In fact, its fairly common with many smaller continuous process facilities.

You may want to review Section 110.9 closely if the tie is operated closed or there is a closed transition transfer for maintenance purposes or "return to normal.?

 

[steve66]

So the transformers are indoors?

If that's right then I know it seems like a minor point, but the source of the conductors is inside the building. There is a phrase in there that says its OK if the termination of the conductors is inside, but it is silent on the source of the conductors.

I really don't think the code is worded to allow that, but I'm not sure it's really a safety issue. Especially if the transformers are in a vault or something similar. You might try asking your AHJ what he thinks.

I think you either need transformers with non-flammable fluid, or a transformer vault.

Steve

 

[charlie b]

Many thanks for the comments. But the issue has become moot. It has been decided that we will use busway from the transformer room to the main electrical room. So I will be adding a secondary OCPD.

 

[UND75]

I have a similar issue, and will wind up resolving it in the same way (i.e., busway).:smile:

 

[RB1]

Charlie:

For conversational purposes, the transformer secondary conductors are not permitted to originate in the building. Only load terminations are permitted inside the building. This language was added in the 2002 Code cycle.

 

[charlie b]

I'm not sure I understand what you are saying. But in case this might clarify the situation, the transformers are in a room entirely by themselves, inside the building, and down the hall from the main electrical room. The primary switches are also in the building, and also in a room by themselves.