480 Delta to 120/208 Wye Calculations

[sblair]

It's been a while since I've dealt with this issue, so wanted to make sure my calculations are correct.

I need a 200A secondary feed per phase off a transformer. The load will be a large number of 120v servers for a temporary installation.

The primary side is 480v delta.

Right now we're looking at a 75kVA transformer which by:

75000 / (208 x 1.73) = 208A. That seems like we are right on the edge here. Given the type of load does 75kVA sound large enough? This is for a temporary installation so pushing the envelope a little bit is okay, but I want to make sure it is safe and capable.

Thanks.

 

[iwire]

In my area a 75 KVA will usually have a 100 to 150 amp primary breaker and a 225 amp secondary breaker. Usually the transformer will feed a 225 amp main breaker panel.

 

[BAHTAH]

Servers

Servers

Its been a long time since I delt with any computer rooms but if all your loads are servers at 120V will harmonics be an issue? You may want to consider a K-rated transformer and oversized neutral for this temp installation.
Just a thought.

 

[bhsrnd]

.... Given the type of load does 75kVA sound large enough? ....

That's kind of a trick question. I have two PDUs at 200A each (each fed by a 150kVA transformer with enough capacity for one additional 200A PDU on each transformer) powering a 4,000sq. ft. data center but that doesn't mean that my setup will work for every data center of the same size.

Being this is temporary I will assume it is not a large data center and it will probably work fine. The amount of equipment you're looking to power will determine the load on the transformer.

 

[sblair]

It will be running about 130 Servers for a short term test (2 months). When I said we need 200A per leg, it is because we will easily be drawing at least 175A per leg most of the time.

I had thoughts about whether it should be a K-rated transformer too, but that isn't what was purchased. If nothing else I guess the top of the transformer will make a great place for baking cookies.

 

[weressl]

It will be running about 130 Servers for a short term test (2 months). When I said we need 200A per leg, it is because we will easily be drawing at least 175A per leg most of the time.

I had thoughts about whether it should be a K-rated transformer too, but that isn't what was purchased. If nothing else I guess the top of the transformer will make a great place for baking cookies.

Since it is a temporary installation you may want to consider the ambient rating of the unit. That may allow you some room for overload. However if your breaker can not handle the overload, then you're cooked. Since these are non-linear loads, you may be in trouble anyway.:grin:

I would but another transformer and split the load. You can wait to buy after you see how the initial installation is running, but I guess they may not give you the time after they up-and-running.

 

[sdv]

k-factor

k-factor

Too close for comfort, especially if you actually use 200A continuously and your loads are 120 VAC servers. Harmonic heating will be an issue. You can use a higher K-factor rated transformer. I'll cost little more