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#26924 - 06/26/03 05:25 AM transformer question
sparky806 Offline
Registered: 12/07/02
Posts: 59
Loc: 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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#26925 - 06/26/03 10:56 AM Re: transformer question
Bjarney Offline
Registered: 04/10/02
Posts: 2527
Loc: 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.
#26926 - 06/26/03 03:05 PM Re: transformer question
sparky806 Offline
Registered: 12/07/02
Posts: 59
Loc: Shawnee, KS, USA
thanks. i'm gonna email you right now.

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