Sizing buck boost transformer

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Crossboss

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I need to wire a temp . 480v AHU package unit and only have 120/208. Can I use a buck boost transformer? Where do I find the formula for sizing? Tia!
 
Buck/boost is the common term for an 'autotransformer'. What this means is that instead of having separate primary and secondary circuits you connect the secondary so that it adds to (boosts) or subtracts from (bucks) the primary voltage. They are particularly useful for small changes in voltage, say if you have 208V and need 240V.

You can make this work to go from 208V to 480V, but you lose most of the benefit of the approach, because the voltage change is so large. IMHO the only reason to use an autotransformer configuration in this particular case is if it let me use transformers that I already had.

The best way to size a 'buck/boost' transformer is to go to a manufacturer catalog and use their selection tables, eg. https://temcoindustrial.com/product-guides/transformers/autotransformer-selection-guide (Note that these guys don't even have a 208 to 480V configuration listed!)

If you want to calculate your own configuration, your first step is to figure a connection that will work. Then calculate the current flowing through the 'secondary coils' of the transformer (remember, this is not really a secondary circuit, since the transformer secondary coils are connected in series with the primary supply). 'Secondary current' * 'secondary voltage' gives you the VA rating of the transformer in question.

Since autotransformers are sized based on the load current * delta, when you have a small change a small transformer can supply a huge load. For the 208V to 240V change, a 1 kVA transformer can reasonably supply 7 kVA of load. But when you have a big voltage change you don't get this benefit. 208V to 480V means that 1kVA of transformer can supply perhaps 1.5kVA of load, not enough benefit to be worth the added complexity in most cases.

For a temp job the deciding factor is most likely which transformer you can get most easily and dispose of after the job is done.

Jon
 
ptonsparky said:
No. Use a step up transformer.

Usually current * the change in voltage, I believe
50 * 272 = 13600 and not sure if you would need three or not. Never attempted it.
Click to expand...
Ok so using a step up transformer should I use a 208 delta to 480/277 wye? And the Ahu has 60amp MOP. As of now I have only a 100 amp breaker on a bus duct. Would I be correct at determining the amps at 208/480=.433x100= 43.3 amps, so I would need a larger breaker? And what formula would I use to size the step up transformer? Been awhile since dealing with transformers but I learn really fast so thanks for the help guys!
 
Normally, you have to pick a standard size transformer. I would pick one that has more current output than the AHU requires. Probably that will not be real close. If it is, I would be inclined to select the next standard sized xfmr.
 
LarryFine said:
Absolutely.
Click to expand...
Ok so using a step up transformer should I use a 208 delta to 480/277 wye? And the Ahu has 60amp MOP. As of now I have only a 100 amp breaker on a bus duct. Would I be correct at determining the amps at 208/480=.433x100= 43.3 amps, so I would need a larger breaker? And what formula would I use to size the step up transformer? Been awhile since dealing with transformers but I learn really fast so thanks for the help guys!
 
IMHO the ideal transformer for this job is a 208V delta to 480/277V wye step up transformer. Probably 45 or 75 kVA.

The current at 208V must be greater than the current at 480V, so you are correct, 100A at 208V is the equivalent of of about 43A at 480V.

This might _barely_ be sufficient to run the AHU, the MOP (maximum overcurrent protection) of the AHU is certainly greater than the operating current. Does the AHU have an MCA (minimum circuit ampacity) rating?

Regarding using 2 single phase step up transformers...this is one of the possible autotransformer arrangements which could be made to work. My recommendation is to avoid this unless you already have suitable transformers.

The only autotransformer setup that I would consider is a packaged wye 3 phase autotransformer. This gets you something which can take any input (208, 240, 400, 480, 600) and give you any of the others as output...but depending upon the situation might require a primary neutral brought to the transformer. Might be a better choice for a temporary install because it gives you a tool that has value elsewhere. For example:

I have no experience with the autotransformer linked.

Jon
 
At best, you have a tight situation. Once you know the MCA you can get a better idea,
At 60 MOCP you are possibly looking at a MCA of 45 or so or 38 kw on 480.
A 45 kva transformer will have a primary current of 125 amps on 208 so your 100 amp buss breaker will be marginal at best.
When you consider inrush currents on the HVAC and the transformer, IMO, a 100 amp buss breaker and 45 kva transformer is risky.
 
augie47 said:
At best, you have a tight situation. Once you know the MCA you can get a better idea,
At 60 MOCP you are possibly looking at a MCA of 45 or so or 38 kw on 480.
A 45 kva transformer will have a primary current of 125 amps on 208 so your 100 amp buss breaker will be marginal at best.
When you consider inrush currents on the HVAC and the transformer, IMO, a 100 amp buss breaker and 45 kva transformer is risky.
Click to expand...
Mca is 49 amps
 
winnie said:
IMHO the ideal transformer for this job is a 208V delta to 480/277V wye step up transformer. Probably 45 or 75 kVA.

The current at 208V must be greater than the current at 480V, so you are correct, 100A at 208V is the equivalent of of about 43A at 480V.

This might _barely_ be sufficient to run the AHU, the MOP (maximum overcurrent protection) of the AHU is certainly greater than the operating current. Does the AHU have an MCA (minimum circuit ampacity) rating?

Regarding using 2 single phase step up transformers...this is one of the possible autotransformer arrangements which could be made to work. My recommendation is to avoid this unless you already have suitable transformers.

The only autotransformer setup that I would consider is a packaged wye 3 phase autotransformer. This gets you something which can take any input (208, 240, 400, 480, 600) and give you any of the others as output...but depending upon the situation might require a primary neutral brought to the transformer. Might be a better choice for a temporary install because it gives you a tool that has value elsewhere. For example:

I have no experience with the autotransformer linked.

Jon
Click to expand...
MCA is 49amps
 
MCA of 49 amps at 480V corresponds to 113A at 208V. Actual consumption will be less than this, but you clearly have negative margin here. You will need to figure something out for the 208V supply.

-Jon
 
Update: Ok I do have a 200amp source off the bus duct. Now should i have a 45kva or 75 kva transformer and need to find size of wire for secondary side to the AHU. MOP IS 60 MCA is 49
 
With all the engineering factors involved I certainly am not qualified to size the transformer but a SWAG I;'d think at 49 MCA a 45 kva would suffice but hopefully better qualified folks will chime in.
You secondary conductors, IMO, would need to be rated at 49 amps to meet the MCA.
(FWIW I'd feed the transformer with a 150 amp OCO at the buss)
 
Crossboss said:
I need to wire a temp . 480v AHU package unit and only have 120/208. Can I use a buck boost transformer? Where do I find the formula for sizing? Tia!
Click to expand...
Many good answers, but a thought. Many (most?) 480 systems can be connected for 240, PERHAPS 208. Have you contacted the manufacturer?
 
augie47 said:
With all the engineering factors involved I certainly am not qualified to size the transformer but a SWAG I;'d think at 49 MCA a 45 kva would suffice but hopefully better qualified folks will chime in.
You secondary conductors, IMO, would need to be rated at 49 amps to meet the MCA.
(FWIW I'd feed the transformer with a 150 amp OCO at the buss)
Click to expand...
Wasn't thinking clearly I'll be sizing the secondary to the 60 MOP. Thanks for the input👍🏻 haven't found the calculation yet to see if 45kva would suffice though.
 
GeorgeB said:
Many good answers, but a thought. Many (most?) 480 systems can be connected for 240, PERHAPS 208. Have you contacted the manufacturer?
Click to expand...
No it would usually say on nameplate if it could run on 208v I'll double check from the manufacturer but I'm sure it will not! Thanks for the input. Great advice!
 
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