The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!


2017 NEC and Related
2017 NEC
Now Available!

Recent Posts
Son of Sparky
by HotLine1
Yesterday at 07:43 PM
Speaking of Plugmold ...
by gfretwell
10/17/16 02:37 PM
Broken battery charger? Check for cobwebs!
by gfretwell
10/17/16 02:30 PM
230 or 345 kV transmission lines?
by annemarie1
10/12/16 01:23 PM
Is this a fix or just a bandaid?
by gfretwell
10/10/16 06:49 PM
New in the Gallery:
12.5A through 0.75mm˛ flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
gfretwell 11
renosteinke 6
HotLine1 6
ghost307 5
sparky 4
Who's Online
0 registered (), 265 Guests and 5 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#199292 - 02/19/11 09:53 AM Transformer wiring
SJT Offline

Registered: 07/24/02
Posts: 242
In regard to the NEC, could a 225KVA transformer be used to only feed a 200 amp/208 V 3 phase panel with a main breaker. If such a transformer was to be used, then what size primary protection would be needed? Would I wire it as if it were a 75KVA transformer and rate the primary and secondary accordingly? It's a strange request from a customer. In the future he may run the proper size primary and secondary conductors, to get the max out of the 225 KVA trans.
Thanks - Good Day

2014 / 2011 NEC & Related Books and Study Guides
#199300 - 02/19/11 01:59 PM Re: Transformer wiring [Re: SJT]
gfretwell Offline


Registered: 07/20/04
Posts: 9023
Loc: Estero,Fl,usa
The way I understand it you use primary protection that protects the transformer and secondary protection to protect the installed secondary conductors.
Greg Fretwell

#199307 - 02/19/11 07:02 PM Re: Transformer wiring [Re: SJT]
George Little Offline

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Need to know how long the Secondary conductors to see if they qualify for the 10' rule mentioned in 240.21(C)(2) or possibly 240.21(C)(3) or 240.21(C)(3). Is it a single voltage secondary qualifying under 240.21(C)(1)? Basically if the xformer is has 225 KVA capacity and you only want to use it at 75KVA there is no problem and the sizing of the primary conductors should be sized to match the secondary load taking in to considration the voltage ratio and maybe an extra 25% would be a good idea to keep the fuses from clearing on start up. There is no need to protect the secondary of the xformer, just the secondary conductors.

Edited by George Little (02/19/11 07:04 PM)
Edit Reason: Edited for added thought:)
George Little

#199322 - 02/20/11 09:03 AM Re: Transformer wiring [Re: George Little]
SJT Offline

Registered: 07/24/02
Posts: 242
I never wired a large transformer to a smaller rating than what it could put out.
Sounds like I'll be OK. I have to check the connection points for the x1, H1, etc. to make sure we use the proper lugs and bolt sizes.

#199667 - 03/04/11 12:20 AM Re: Transformer wiring [Re: SJT]
Tesla Offline

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
On the economics....

A 225 kVA XFMR is huge. Regardless of the true load it will bleed energy to sustain its magnetic circuit.

Correct sizing would mean a XFMR of 75kVA.

So you're bleeding off 225/75 = 3 times as much parasitic losses as would be nominal.

Do run your calculations.

BTW 3 is the lower bound.

Power cost being what it is -- you'd very likely be much better off scrapping/ reselling the gear and downsizing to a new/used transformer.

Talk to Salinger and others in the used electrical equipment market. You'll have your eyes opened.


ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals