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Transformer feed to new panel #161670 04/06/07 10:32 AM
Joined: Jan 2004
Posts: 615
Jps1006 Offline OP
I'm proposing to install a new 15 KVA 480v prim. 120/208 sec. to feed a 3-phase 50-amp panel for a new modular office in a warehouse. I am going to use the 3-pole 480v breaker as disconnecting means as required (NEC 2002 110.58). The panel is in sight about 100' away, as long as no new shelves, machines, etc. aren’t put in the way in the future to obstruct the view.

I thought there was an exception to allow the disconnect not to be within sight as long as there was a means of locking it out. I can’t seem to find one that applies to transformers.

Also, if I protect the primary at 125%, do I not need to supply secondary protection?
Do I understand Table 450.3(B) correctly in that 125% primary protection means no sec. protection, and 250% primary protection means 125% sec. protection?

So, minimum design says 480-volt 25-amp breaker within sight about 100’ away to 15KVA xfmr to MLO panel with branch breakers. What does good design say and why?

Also, grounding only at transformer to building steel right? No bond screw at the panel.


2017 / 2014 NEC & Related Books and Study Guides
Re: Transformer feed to new panel [Re: Jps1006] #161673 04/06/07 11:17 AM
Joined: Jan 2001
Posts: 1,044
Tom Offline
110.58 only applies to only applies to installations in tunnels. Is this office building being built in a tunnel (kind of like some of those underground storage facilities)? If not, then you don't need a disconnect in sight from the transformer. If you did, you would be limited to a maximum of 50 feet to the disconnect (definition in Article 100).

Look at the definition of a lighting and appliance panelboard in 408.34(A). If the panelboard you're installing meets this definition, then you must provide protection for it.

Also, don't forget that you need to protect the conductors from the transformer to the panelboard (article 250).

You can do your grounding & bonding at the transformer or the panelboard, your choice. If done at the transformer, you would not install the bond screw at the panel. Most of the installations I've seen or done, the grounding & bonding is at the transformer.

Last edited by Tom; 04/06/07 11:18 AM.

Few things are harder to put up with than the annoyance of a good example.
Re: Transformer feed to new panel [Re: Tom] #161735 04/07/07 07:04 AM
Joined: Jan 2004
Posts: 1,507
George Little Offline
I don't think you can run a set of conductors more than 25' as a tap from a xformer inside the building unless you provide overcurrent protection at the xformer secondary. 240.21(C)(6). My comment is based on the problem presented by the first post in this thread and on the '02 NEC.

Last edited by George Little; 04/07/07 07:14 AM.

George Little
Re: Transformer feed to new panel [Re: George Little] #161739 04/07/07 08:01 AM
Joined: Jan 2004
Posts: 615
Jps1006 Offline OP

Boy was I way off. I came up with 110.58 by doing a search in my NEC 02 software I bought fron the store on this site. I have to make sure to scroll back a little further to see what it is being applied to. I thought the local disconnect rule was a general rule that applied to all equipment 300VA and larger that was not cord and plug connected.

As to protection of the secondary conductors, isn't that accomplished by the primary protection, both fault and overload? That's, I guess, what I don't get. In Table 450.3(B), under what scenario would the column describing no protection needed be?

George, you may have just answered that question. I don't have my book or CD with me, but I'll be referencing your section for clarification. Thanks.

Re: Transformer feed to new panel [Re: Jps1006] #161765 04/07/07 02:15 PM
Joined: Jan 2004
Posts: 1,507
George Little Offline
Secondary conductors can be protected by the Primary overcurrent device if it is a two wire secondary or a single voltage Delta/Delta xformer. 240.21(C)(1)

Protection of the secondary of the xformer is optional. Good engineering IMHO would have the secondary of the xformer protected. Protection of the conductors taping the secondary or supplying the xformer is not optional. It is quite common to confuse protecting the xformer with protecting the wires connected to it.

Last edited by George Little; 04/07/07 02:16 PM.

George Little
Re: Transformer feed to new panel [Re: George Little] #161802 04/07/07 08:11 PM
Joined: Jun 2004
Posts: 1,273
Tesla Offline
You are feeding a three phase secondary load and will have to have a breaker protecting the 208Y120 panel. I'd go with a back-fed 60 three pole main breaker.

Don't forget to use bonding bushings between this panel and the transformer. You'll want lay-in lugs. Use a four barrel mechanical lug on the Xo or at least one double barrel lug plus singles.

Size your grounding conductor according to 250.66 not 250.122. (#8 minimum)

Consider placing the 15kVA XFMR overhead/above and adjacent to the panel so as to save on space.

Vibration hum can be addressed by isolation pads.



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