Dry type transformer loading

[Npstewart]

We designed a new construction project a while back and assumed a 208/3 service was available, turns out it is but it would cost $30,000 to go from 480/3 to 208/3 on the utility side (not sure why). The electrician drew a sketch for us with a 300 kVA dry type transformer being installed outside. The problem is that our load is only around 150 kVA and we could probably run some stuff on 480 which would reduce the required transformer size to maybe even 112.5 kVA. The reason why he wants to use a 300 KVA is because they have one they are trying to get rid of and can sell it to the owner really cheap. Does anyone see any problems with running a transformer at such low load conditions?

Chances are, this wont go ahead anyway because the sketch they drew did not have secondary over current protection. They were under the impression they could use the smaller 200A disconnects fed from the wire way that the transformer is feeding for secondary protection, essentially protecting the transformer with parallel disconnects. Once we show the secondary disconnect and feeders, they will probably change their mind.

 

[templdl]

We designed a new construction project a while back and assumed a 208/3 service was available, turns out it is but it would cost $30,000 to go from 480/3 to 208/3 on the utility side (not sure why). The electrician drew a sketch for us with a 300 kVA dry type transformer being installed outside. The problem is that our load is only around 150 kVA and we could probably run some stuff on 480 which would reduce the required transformer size to maybe even 112.5 kVA. The reason why he wants to use a 300 KVA is because they have one they are trying to get rid of and can sell it to the owner really cheap. Does anyone see any problems with running a transformer at such low load conditions?

Chances are, this wont go ahead anyway because the sketch they drew did not have secondary over current protection. They were under the impression they could use the smaller 200A disconnects fed from the wire way that the transformer is feeding for secondary protection, essentially protecting the transformer with parallel disconnects. Once we show the secondary disconnect and feeders, they will probably change their mind.

From what I could sort out from your post I understand that the transformer proposed is a 300kva transformer with a 208y/120v secondary in lieu of a 150 kva. The operating cost should be compared first. The 300 kva will have a higher No load loss than the 150kva that is the losses will be greater with the 300kva just sitting there unloaded. The, you must compare the losses AF each transformed at the load anticipated. Just for grins in you load a 150kva transformer at 100% loaning a 300kva at 150kva would be loading loading it at 50%. How would the operating costs compare. By installing a 300kva transformer you electric bill may end up to be higher for the same load when NL and FL losses are considered which is an important cost to consider. The 300kva transformer may appear to be a good deal but it's going to cost more to run it.
The other aspect is that the secondary protection of either transformer could be the same. There is no need to see increase the secondary protection of the 300kva transformer should the actual load be the same as if you were to use a 150kva transformer. Remember that the secondary protection can be less than the secondary ration of the transformer. Why incur the higher cost for a larger secondary breaker in order to match the secondary capacity of the 3o?kva transformer when you only need 150kva.

 

[Npstewart]

agreed, I dont like the idea of increasing the size either. It will also be very intrusive in its foot print.