Transformer for UPS Sizing

1 reply [Last post]
cgs5028's picture
Joined: 2017-05-03

Hello everyone,

I am looking for some opinions on how to go about sizing a transformer for a UPS. Any applicable code references are appreciated, though, other than transformer protection, I see no code dictating transformer sizing. Please correct me if I'm wrong.

We are replacing a UPS with a new unit. The current installation is as follows:

  • 30kVA transformer, 480V to 208Y/120V, fed from (3) 40A fuses
  • 80A fuses in 100A rated disconnect fed from transformer
  • UPS fed from disconnect

The UPS will be replaced with a new 30kVA unit, unity PF, 30kW. It's Data Plate information is as follows:

  • Nominal Current: 89A
  • Maximum Input Current: 102A
  • Rec. OCPD: 150A

The UPS can be de-rated to a 20kVA unit with new Data Plate information:

  • Nominal Current: 59A
  • Maximum Input Current: 68A
  • Rec. OCPD: 90A

Since the method will be the same for both UPS ratings, let's just use the 20kVA for this question. Do I need to size the transformer so the rated secondary current is equal to or greater than the maximum input current, or the recommended overcurrent protection device size, or for that matter, either?

For Max Input current: 68A x 1.732 x 208V = 25kVA, meaning the existing transformer is large enough for this installation.
For Rec. OCPD: 90A x 1.732 x 208V = 32kVA, meaning the existing transformer is not large enough for this installation.

From the NEC, I believe I can use the 30kVA transformer, primary protected only, at 125% [(30,000 ÷ (1.732 x 480)] x 1.25 = 45A, next fuse size up 50A. No secondary protection per Sec. 450.3(B). Then use the 90A fuses (manufacturer recommended) in the existing disconnect and size the secondary conductors accordingly.

The other opinion is the transformer needs to be sized so the rated secondary current is equal to or greater than the Rec. OCPD. This would mean we need to install a 45kVA transformer since the calculation required a 32kVA, and the next size up is 45kVA. If we do not de-rate the unit (leave it at 30kVA), this opinion would mean we need to install a 75kVA transformer for a 30kVA UPS. That seems very oversized to me, which kicked off this discussion and caused me to look into de-rating the UPS in an effort to use the existing transformer.

This transformer serves one dedicated load, the UPS. No possibility of expansion.

For those with more knowledge and experience than myself, I am curious to hear your thoughts.

Thank you!

ekras's picture
Joined: 2013-11-08

The real question is: what is the load connected to the UPS? If it is the same load as is currently connected to the old UPS, then derating will work and so will everything connected to it since it is running on 40A/80A fuses now. 45A fuses are available for the primary side of the transformer protection, so that should not be sized up. 90A on the secondary side will keep the UPS manufacturer happy. Size wire at 125% to match.

A side issue not addressed (which should be when cutting it close) is the harmonic content presented to the transformer by the UPS. This may require a K-rated transformer, or a larger transformer to handle the circulating currents. This may not be an issue since, once again, we do not have the actual load or the harmonic content spec. for the UPS.

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