Size the transformer for (30) Apt. Units

[zemingduan]

Let's say you have a high rise building and the incoming electrical service of the building is 480Y/277V 3ph 4wire. You will need a 480V 3ph 3wire - 208Y/120V 3ph 4wire transformer to supply (30) Apt. units on 10th & 11th floors. The NEC calculated load for these (30) Apt. units is 327 KVA based on NEC 220.84.

1. Are you allowed to use a 300 KVA transformer for the (30) Apt. Units by NEC? My thought is yes, since I think NEC Article 450 doesn't require you to size the transformer for these Apt. based on 220.84.

2. Will you specify 300KVA transformer for this case? The next standard transformer is 500KVA which is overkill for the (30) Apt. units. In practical, the actual KVA demand maybe just around 0.4-0.5 of the NEC calculated load. 300 KVA transformer should be enough. But as an engineer, is it good to specify 300 KVA for this case?

Let me know you thoughts fellows! Thanks!

 

[infinity]

Typically for these types of buildings the apartments are fed with single phase 120/208. Are the apartment panels single phase?

 

[electrofelon]

I agree with you and would use the 300KVA

 

[junkhound]

poco here would only use a 150 kVA transformer at most, but poco can ignore NEC

 

[JoeStillman]

220.84 calculations give a total non-continuous load (all resi loads are non-continuous except maybe EV and they're hard to find on the 10th and 11th floor). So your 327 kVA at 208V 3Ø means you need 908 Amps. The closest size gear is 1000A. If we take that as 125% of the FLA of the transformer feeding the load, you get 288 kVA so 300 kVA is the right size.

 

[petersonra]

220.84 calculations give a total non-continuous load (all resi loads are non-continuous except maybe EV and they're hard to find on the 10th and 11th floor). So your 327 kVA at 208V 3Ø means you need 908 Amps. The closest size gear is 1000A. If we take that as 125% of the FLA of the transformer feeding the load, you get 288 kVA so 300 kVA is the right size.

Where is any of this in the code? Sizing a transformer is a design decision, not a code issue.

 

[JoeStillman]

Where is any of this in the code? Sizing a transformer is a design decision, not a code issue.

I can't prove that transformer sizing calcs are NOT in the code. But whatever design you decide on is subject to 220.80, 240.4 and 450.3.

 

[jim dungar]

I can't prove that transformer sizing calcs are NOT in the code. But whatever design you decide on is subject to 220.80, 240.4 and 450.3.

Those are sections related to conductor and overcurrent protection sizing. They do not relate to sizing the source, be it a utility or a transformer. The NEC does have source sizing requirements for backup/emergency sources.

 

[infinity]

So what's the answer? Even if not required a good design would use a 300 kva transformer?

 

[jim dungar]

So what's the answer? Even if not required a good design would use a 300 kva transformer?

I would.

 

[wwhitney]

Where is any of this in the code? Sizing a transformer is a design decision, not a code issue.

Joe is pointing out that the NEC specifies the minimum size OCPD based on the NEC calculated load, as well as the maximum allowable OCPD for a given transformer rating. The result is a minimum transformer size of at least 80% of the calculated load, possibly more based on OCPD size granularity. In the OP's case, for example, there would be no way to use a 150 kVA transformer and comply with all the NEC sections subsequently cited.

Cheers, Wayne

 

[zemingduan]

Typically for these types of buildings the apartments are fed with single phase 120/208. Are the apartment panels single phase?

The aprtment unit is 120/208V 1ph. The distribution panel for the 30 unit apt. is 3ph.

 

[wwhitney]

For example, from the OP, the NEC calculated load is 327 kVA. Best case that is balanced on the 3 phases. In which case for a 480V : 208Y/120V transformer, the primary side current would be 393A, and the secondary side current would be 908A.

If both the primary and secondary side OCPDs are standard sizes, the smallest they could be would be 400A and 1000A, respectively, per NEC 215.3. 400/393 is less than 1000/908, so the primary side OCPD will control the minimum transformer size. So in order to comply with NEC 450.3 transformer needs to have a rated primary current of at least 400/125% = 320A. Which means the transformer needs to be rated at least 266 kVA; any smaller and it will be impossible to comply with both NEC 450.3 and NEC 215.3 simultaneously.

Cheers, Wayne

 

[junkhound]

Is there a separate high voltage step down xfmr supplying the 480 to only your 30 apt transformer?
No reason for the 480 to 120/208 xfmr to be any larger.

 

[JoeStillman]

The apartment unit is 120/208V 1ph. The distribution panel for the 30 unit apt. is 3ph.

Sounds balanced to me.