ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
UL 508A SPACING
by ale348 - 03/29/24 01:09 AM
Increasing demand factors in residential
by tortuga - 03/28/24 05:57 PM
Portable generator question
by Steve Miller - 03/19/24 08:50 PM
Do we need grounding?
by NORCAL - 03/19/24 05:11 PM
240V only in a home and NEC?
by dsk - 03/19/24 06:33 AM
New in the Gallery:
This is a new one
This is a new one
by timmp, September 24
Few pics I found
Few pics I found
by timmp, August 15
Who's Online Now
1 members (ale348), 302 guests, and 14 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Joined: Jan 2004
Posts: 615
J
Jps1006 Offline OP
Member
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.

Thanks

Stay up to Code with the Latest NEC:


>> 2023 NEC & Related Reference & Exam Prep
2023 NEC & Related Reference & Study Guides

Pass Your Exam the FIRST TIME with the Latest NEC & Exam Prep

>> 2020 NEC & Related Reference & Study Guides
 

Joined: Jan 2001
Posts: 1,044
Tom Offline
Member
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 12:18 PM.

Few things are harder to put up with than the annoyance of a good example.
Joined: Jan 2004
Posts: 1,507
G
Member
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 08:14 AM.

George Little
Joined: Jan 2004
Posts: 615
J
Jps1006 Offline OP
Member
Tom,

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.

Joined: Jan 2004
Posts: 1,507
G
Member
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 03:16 PM.

George Little
Joined: Jun 2004
Posts: 1,273
T
Member
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.


Tesla

Link Copied to Clipboard
Powered by UBB.threads™ PHP Forum Software 7.7.5