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transformer question #26924
06/26/03 07:25 AM
06/26/03 07:25 AM
sparky806  Offline OP
Joined: Dec 2002
Posts: 59
Shawnee, KS, USA
does anybody here have expereance with back feeding transformers? here's the deal...i have a customer with a 240 volt grounded b 3 phase service. he has an x-ray machine that requires 480 volt. i am going to install a 480 t0 240 volt transformer. i know if i back feed it, i will get the needed 480 volts, but i'm not sure on conductor sizing. is it the same as a normal install? is there anything else i should be aware of? Thanks

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Re: transformer question #26925
06/26/03 12:56 PM
06/26/03 12:56 PM
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
'806 — It's duck soup. Get a 480∆-240∆ dry-type transformer...15kVA and up are fairly common.

If you don't mind a 2MB pdf, see page 21 at

Suggest getting a transformer with both FCAN and FCBN taps—they work in the opposite direction for a reverse-fed arrangement. For sensitive electronics, electrostatically shielded units are suggested. If heat is an issue…pick an 80°C-rise transformer on page 27.

For reverse-fed operation DO NOT connect the 240V-side X4 terminal to anything.

Overcurrent protection of 480V windings on a ∆-∆ transformer can be by the usual 125% rating on [only] the 240V source. No system grounding is required by NEC for the 480V side.

If you like, contact me off-board for figuring the kVA-rating, breaker/fuse and conductor sizes needed.

Re: transformer question #26926
06/26/03 05:05 PM
06/26/03 05:05 PM
sparky806  Offline OP
Joined: Dec 2002
Posts: 59
Shawnee, KS, USA
thanks. i'm gonna email you right now.


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