Transformer sizing

[deetz]

We are replacing a new CNC machine and the transformer we had for the old one is 45kva. The new machine is 123 amps at 220 volts. It's also rated for 205-235 volts. That equates to 46.86 kva. Will the 45 kva transformer be okay? I know the usual machines tend to run lower than the actual amps. Also what about NEC compliancy? I suppose if you have it fused right you would be?

 

[waross]

It depends:
The 45 KVA transformer should give good service but NEC compliance is another matter.
Under the Canadian code, you must use either 208 Volts or 240 Volts when determining Watts or KVA.
What voltage does your transformer supply?
If your existing transformer is rated at 45 KVA at 208 Volts, that is 125 Amps rated current and you should be OK.
If your existing transformer is rated at 45 KVA at 240 Volts, that is 108 Volts rated current and you have a problem.
If your transformer is rated at other than 208 Volts or 240 Volts, check that it is approved under the NEC.
But don't take my word for it.
It doesn't matter what I say, the UHJ has the final say.
Even if the UHJ is wrong, many people have found out over the years that it is cheaper in the long run to comply with the wishes of the AHJ.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[deetz]

Good point. Thanks waross.

 

[wattyeng]

Your basic analysis shows no, it isn’t from a max load perspective
Your new CNC machine will have a design loading profile, which may have diversity and therefore will not take full load, all the time
Check the transformer design spec, including the oil - all transformers allow some overloading profile - the more the overload, the less time it is sustainable, but if your new machine only requires occasional overload, it may well accommodate it safely - you may also retro fit force cooling to the transformer to improve its loading capability (if it hasn’t already got ONAF build), or change the oil to higher op temp capability eg DuPont Nomex
Suggest you consult the CNC OEM for power requirements and if you need 46.86kva, that you upgrade the existing transformer (to also future proof) or put a second smaller kva transformer (matched impedance & ratio characteristics) in parallel with original to accommodate the load - this will be cheaper than changing to one larger unit, allowing existing kit to be used and a smaller capital outlay to cover loading - the second smaller transformer could be of higher kva than needed for the CNC alone, again allowing extra capacity if needed in future

I.Eng MIMMM MIET MIPowerE AIOSH