i was wondering if any of you guys have ever back fed a transformer , or in other words used a step down as a step up x former.
i have never done this before and if any of you guys have , any problems from this....
i will be going from 208 3 phase to 480...
thanks for any input...

The transformer would have to be listed as a step up transformer.

[This message has been edited by russ m (edited 02-15-2004).]

Technically, it works fine if you do it right. I'm an engineer with an electric utility and we do it on a bigger scale quite often. Don't know about code, etc.

If you already have a 480 - 208/120 transformer you plan on back feeding you should talk to the manufacturer and see if they have documentation that says that is OK.

Buck Boosts are usually labeled 240/120 - 24/12 but have documentation that shows their use for other voltages.

If you do back feed a 480-208/120 transformer you will not tie in XO, leave it open.

I have done this for temp 480 power before.

as far doing it right....can u go into detail
a little more...if the x former is rated at 3 kva , i know i should not exceed the rating of this depending upon the load being used...the primary side would require less of a load than 3kva , right , wrong ?
thanks for the replies..

It should work OK, but be aware of a couple of things. Iwire's floating XO is a good suggestion. Full load at 480V is only 3.6 amperes. It's likely there is no 480V neutral, like you'd find with a 480Y/277V circuit. Even for temporary use, I would protect the 480V winding with 4½-amp 600V fuses—a standard dual-element size.

I have done this for a power source to be used in telecom overvoltage testing. I needed to take 208 or 240 in and produce an AC power supply adjustable from 50 to 1000 VAC. The output is then connected through current limiting resistors (.17 to 20 amps, momentary) to the tip and ring leads of various telecom equipment to determine their immunity to "power cross" conditions.

In my application the 240 comes in, is fused, goes to a powerstat 246U 0-280V 4.2 KVA autotransformer. 0-280 VAC output of autotransformer goes to two sola/hevi-duty HS5F3AS 3KVA 240 volt-wired secondaries in parallel. The Sola/hevi-duty transformers' primaries are configured for 480V each and put in series to get the 1000VAC output I need. Definitely for lab use only! This was built mostly with parts that were on hand (had some nice parts lying about). I will be rebuilding this one with bigger and better parts over the next two months. Unfortunately, still using 240 on the input - don't want to get the facilities department involved with a 480 v installation. Also want some portability - "portability" LOL, the new transformers I'm looking at are going to total about 500 pounds, plus about 100 pounds of 225 watt resistors. Most of the stuff in the (well grounded metal) box I've got is rated to 600 VAC but being used at 1000 VAC. As I said, lab use only.

Don't think this sort of thing is advisable for a permanent installation...

I did one last summer. 240 grounded B to 480 delta. worked great. I got all my advice for it right here. Bjarney was extremly helpful with conductor sizing, and breaker size.

One thing that comes to mind here is this Transformer might possibly be an Open Delta "Tee" configuration - since it's <15KVA with a 208/120 3Ø 4W Secondary.

Thinking the output on the side with the "H1", "H2" and "H3" terminals might be a little funky from this input reversal scenario. Sound reasonable?

BTW: Would like to get the cost difference and back-order time difference between purchasing a typical Transformer - wound + meant to be a Step-Down type / vs. one which has been wound + is meant to be a Step-Up type.

I've done it a few times - but then again that was when I was younger and knew alot less. Thought the same "Transformers work exactly the same connected either way", back in the mid 1980's.
After learning a little bit 'O EEE-Lekkt-rikkle Injun-earring, the thoughts on things like this has changed quite drastically!

Scott35

I HAVE ABOUT 30 TO INSTALL NOW.POWER CO HAS NEW SERVICE UG.AND THE NEW VLTG IS 208 FROM OLD 240 SYS.ANYWHERE FROM 2OOAMP TO 400AMP 3 PH.PLUS THE ONES I HAVE DONE IN THE PAST.

BEST THING TO DO IS GET YOUR LOCAL SUPPLY HOUSE GET YOU 1 FROM MFG. NO PROB.