I recently installed a buck-boost transformer in one of our buildings with 208Y/120v service to support a 1hp submersible well-pump rated for 230v single-phase.

The transformer is a GE 9T51B0158 which is labeled for 208v primary x 240v secondary, up to 500va.

Per the label diagram, I connected 208v nominal (210v measured at the transformer) to X1 & X2, tied H2 to H3, and then took a meter reading at H1 & H4 before connecting to the well-pump control. I measured 259v at H1 & H4, unloaded. Is there some reason to expect this kind of over-voltage? Would you expect the secondary to drop down closer to 240v under some load? Or is it more likely that I have either misunderstood the wiring diagram or acquired a dud unit?

Any thoughts are appreciated.



  • With a Wye I don't usually see big swings, 500va is a bit small in my opinion for a 1 hp pump, I think you will see the voltage drop quite a bit with a load.
    – Ed Beal
    Jun 25, 2018 at 19:11
  • @EdBeal - Thanks for your thoughts. My understanding is that VA on Buck Boost transformers is measured on the voltage differential, so 208-to-240v would be 32v, or a max of 15.63 amps @ 500VA. I can't see the nameplate on the pump motor, but I don't expect it would show a running current over 10 amps...
    – dewhite04
    Jun 25, 2018 at 19:59
  • I just realized you were boosting from 208 to 240 you have the calculation. Right but the motor is going to draw close to 24 amps at startup.
    – Ed Beal
    Jun 25, 2018 at 20:53
  • 1
    Well the VA is the value volts x amps. Since yo with motor loads the draw is normally 3x FLA (full load amperage) or more on start up. The NEC has table 430.248 for motor amperages. A 1 hp 230v motor is rated a 8 amps a 208v motor 8.8 that's why I said the transformer was a bit small was not thinking of boost.
    – Ed Beal
    Jun 25, 2018 at 21:03
  • This isn't adding up. First off, that is not a buck boost transformer. That is a standard power or control transformer with a 208V primary and 120/240 secondary. Actual buck/boost transformers have very low voltage secondaries meant to buck or boost the line voltage by tens of volts. And ~260V is exactly what I would expect if someone wired a transformer with a 208V primary to a 240V supply. The difference between 208 and 240V is 13.3% so 240+13.3% is about 272V, higher than your 260 but if we substitute 230V, you end with almost exactly 260V which is possible if taps are present.
    – Mister Tea
    Jun 25, 2018 at 23:05

2 Answers 2


That transformer is awful small

Your transformer is not a buck-boost transformer as you assumed it to be -- instead, it's a standard (isolating dry type) transformer with a 120/240V primary and a 208V secondary. As a result, its windings are handling full power, which means that since it's a 500VA unit, it can only handle 2A at 240V coming out. I'd expect it to overheat quite severely trying to handle the 8FLA@240VAC of a 1HP motor.

As to the high output

I would check the input voltage -- it sounds like the input is on the high side, and doubly so because there will usually be a bit of turns ratio compensation on the design secondary side that acts in the opposite direction from what you are describing in a backfed application such as yours. However, it should drop to a more normal value under a reference load.

  • Thanks for your helpful input. I came also to this conclusion on my own this morning while studying GE's catalog. I'm really glad to have figured this out before connecting a submersible pump, hanging ~300' underground. I have ordered a similarly sized 32v buck/boost transformer GE 9T51B0128 to suit my needs. I'm now left wondering what would be a good use-case for a dedicated 208x240 transformer that can only carry a ~2A load...
    – dewhite04
    Jun 26, 2018 at 17:58

It sounds to me that you have boosted your power by 20% (210*1.2=252). Your input voltage is constantly changing plus or minus 5% so your output will not be exactly 252V.

I don't know how sensitive your pump motor is and if it would take that high of a boost. Motors do like to run a little hot but you would need to check with the manufacturer to see if 259V would fit in the range. A tolerance might be listed somewhere in the paperwork or on the motor. Let's say your odds are about 50/50.

I have attached the buck/boost diagram from my old ugly's if you want to try one of those. The 5% increase and the 10% increase are on the far right first and second respectively. enter image description here

If this doesn't help GE should have a similar set of diagrams with the xfmr.

Good luck

  • Thanks for your response. What you're saying makes sense, but I'm afraid I've got a specialty unit that is only designed to yield a set amount of buck or boost. I'm attaching GE's diagram, and info label. It appears the taps are more limited and can only be wired in two ways (to make 240v or 120v from 208v, or of course, vice-versa). I'm now thinking that some of the windings in the transformer may be leaking voltage, causing the ratio to become skewed. Alternatively, I can hope that presenting a load to the secondary might drop the actual voltage delivered into the desired range.
    – dewhite04
    Jun 25, 2018 at 21:34
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    Edit: It seems I don't have enough reputation to post images, but I can link to them. See below: label diagram
    – dewhite04
    Jun 25, 2018 at 21:37
  • The poster doesn't have a buck-boost, just a standard 500VA control transformer.
    – Mister Tea
    Jun 25, 2018 at 23:06
  • I wanted to add one more comment to your reply, saying that I appreciate the information you provided. I thought it best to select one of the other replies as "best answer" because it could really save someone a headache if they were making the same mistake I almost did, but your post was also very informative and put my mind on the track to understanding what I was doing incorrectly. Thanks!
    – dewhite04
    Jun 26, 2018 at 18:00

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