Step up step down transformers.

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Dusty1987

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Augusta MT.
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Self employed electrician.
Good morning. Can anybody help me? On a step up, step down. Transformer. Is it absolutely necessary to use a step up transformer? Versus step down transform. And what are the benefits?
 
What I am looking at is? 208 volt, 2- 500 KCMIL Parallel=1000 KCMIL 400 amps Breaker. And the run is 750 feet.
So if my calculations are correct to step up. From 208 to 480. 150 KVA transformer should do the trick.
But my question is, do I absolutely need a step up transformer to do this? Or can I reverse the voltage in a normal transformer?
 
Dusty1987 said:
What I am looking at is? 208 volt, 2- 500 KCMIL Parallel=1000 KCMIL 400 amps Breaker. And the run is 750 feet.
So if my calculations are correct to step up. From 208 to 480. 150 KVA transformer should do the trick.
But my question is, do I absolutely need a step up transformer to do this? Or can I reverse the voltage in a normal transformer?
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The NEC requires a transformer to be labeled for reverse connectability. Some manufacturers have handled this by removing the terms primary and secondary from their nameplates, using just high voltage and low voltage.

A reverse connected transformer will have higher than normal in rush current, often requiring you to go to the maximum OCPD sizing allowed by code.
A delta-wye transformer connected in reverse will present some challenges, the low voltage source side X0 bushing cannot be connected to ground or neutral as it will cause circulating currents. The high voltage delta winding will need to be corner grounded or ground detectors will need to be employed.
 
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Post #4 pretty much covers it.

Transformers work in reverse. But the coils _intended_ to be primary are designed to have voltage selection taps (to compensate for different supply voltages) and to have greater leakage inductance (to reduce inrush current when the transformer gets energized).

If your incoming '208V' is hot you might have problems with excessive transformer losses or even worse inrush. If your incoming '208V' tends to run low then this should be fine.

The other big problem with transformers used in reverse is grounding of the secondary; but for your application that isn't an issue. Corner grounding works just fine for a step up-step down situation.

Also when you go step up-step down you are introducing transformer operating losses for the life of the project. It might be cheaper initially but those costs add up over time. The transformers themselves have losses, and also have internal impedance which causes voltage drop.

-Jon
 
winnie said:
Also when you go step up-step down you are introducing transformer operating losses for the life of the project. It might be cheaper initially but those costs add up over time. The transformers themselves have losses, and also have internal impedance which causes voltage drop.
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Not just the losses, but heating power.
 
750 feet is unlikely to be cost effective for a step up step down scheme. Step up step down is really an awful solution and should be avoided if at all possible. You will have a lot of voltage drop with two transformers, and losses will be costly.
 
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