Transformer for UPS Sizing

[cgs5028]

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 if wrong.

We are replacing a UPS with a new unit.

The current installation is as follows:
30kVA transformer, 480 to 208Y/120, 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.
Data Plate Information:
Nominal Current: 89
Maximum Input Current: 102
Rec. OCPD: 150A

The UPS can be de-rated to a 20kVA unit with new
Data Plate Information:
Nominal Current: 59
Maximum Input Current: 68
Rec. OCPD: 90A

Since the method will be the same for both UPS ratings, lets 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 over current protection device size, or for that matter, either?

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

From the code, I believe I can use the 30kVA transformer, primary protected only, at 125% ((30,000/(1.732*480))*1.25) = 45A, next fuse size up 50A. No secondary protection per 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 in 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 what your thoughts are.

Thank you

 

[petersonra]

3 phase transformers tend to come in certain sizes. I would select the next size up from the maximum input current on the nameplate of the UPS and not worry about it any more than that. the cost difference between the two sizes of xfmrs and the ocpd is going to be less than screwing around trying to save a few bucks.

the secondary protection on the transformer normally cannot exceed 125% of the rated current (with allowance to go to the next highest rating of OCPD) so i don't know why you think you need to upsize the transformer.

 

[cgs5028]

3 phase transformers tend to come in certain sizes. I would select the next size up from the maximum input current on the nameplate of the UPS and not worry about it any more than that. the cost difference between the two sizes of xfmrs and the ocpd is going to be less than screwing around trying to save a few bucks.
the secondary protection on the transformer normally cannot exceed 125% of the rated current (with allowance to go to the next highest rating of OCPD) so i don't know why you think you need to upsize the transformer.

Thank you for the response.

For clarification:
I do not think I need to upsize the transformer. I think I can allow the manufacturer to de-rate the unit to 20kVA and use the existing 30kVA transformer that is installed, based on the maximum input current of 68A as shown on the data plate for a 20kVA UPS. My question is I am I correct in thinking this?

I would like to de-rate the unit to 20kVA, leave the 30kVA transformer, use primary only protection of [30000/(1.732*480)]=36A * 1.25 = 45A see NEC Table 450.3(B), and install the 90A fuses and #2 conductors (4W) that UPS manual recommends. Secondary protection would not be required for the transformer, the secondary conductors will be protected.

The reason I brought this up is because I am being told that I need to size the transformer so its rated secondary current is equal to or higher than 90A, which is the listed REC. OCPD as shown in the UPS manual. Using the 90A value, I would need to increase the size of the transformer to a 45kVA.

 

[petersonra]

I don't get the benefit of derating the UPS. if you don't need the larger UPS, why not just buy a smaller one in the first place?

I am not entirely sure the UPS can be protected by the xfmr primary OCPD.

In any case, I don't think you can use the primary OCPD to protect the secondary conductors.

240.4 (F) Transformer Secondary Conductors. Single-phase
(other than 2-wire) and multiphase (other than delta-delta,
3-wire) transformer secondary conductors shall not be considered
to be protected by the primary overcurrent protective
device.

You do not have a delta-delta configuration.

 

[cgs5028]

I don't get the benefit of derating the UPS. if you don't need the larger UPS, why not just buy a smaller one in the first place?

I am not entirely sure the UPS can be protected by the xfmr primary OCPD.

In any case, I don't think you can use the primary OCPD to protect the secondary conductors.




You do not have a delta-delta configuration.

1. I have the UPS described at that is what I am working with.
2. The UPS would not be protected by the transformer primary OCPD, the transformer would be protected by the transformer OCPD.
3. The UPS would be protected by the 90A fuses in the disconnect between the transformer and the UPS.
a. That fuse size is per the UPS manufacturer.
b. The secondary conductors would be sized accordingly and protected by the 90A fuses.
4. I am not using the transformer primary OCPD to protect the secondary conductors, the 90A fuses in the disconnect would protect those conductors.