Will this fly?

[mvannevel]

Straight from the prints: 3000 ampere panel, 480/277, 3ph, 4 wire. Total CONNECTED kva=2581, total CONNECTED amps=3105, Feeder amp=2912. Customer owned primary, AND they want to use a 1500 KVA transformer to feed it. From the transformer, 7 sets of 750 kcmil THWN in 4" conduit. Any input?

[ April 22, 2003, 03:16 PM: Message edited by: mvannevel ]

 

[bennie]

Re: Will this fly?

My calculations indicate a demand factor of 58%.

This appears to be a bit low, but looks like a utility calculation.

Charlie the utility guy, and others, can give better information.

 

[charlie b]

Re: Will this fly?

(1) The statement ?feeder amps = 2912? does not correlate to 7 sets of 750. Nor does it correlate to the 1500 KVA rating of the transformer.

(2) Having a service transformer rated at 58% of the "load-as-calculated-by-NEC-methodology" is acceptable. You didn?t give us the calculated load, but it must necessarily be no higher than the connected load. Therefore, 1500 KVA should be fine.

(3) The feeder?s ampacity must be at least as high as its calculated load. A feeder ampacity of 2912 is less than the connected 3015 amps, but that may still be acceptable. I would need to see a calculated load.

 

[jmc]

Re: Will this fly?

Since the primary is customer owned, the "service point" is on the primary side of the transformer. Therefore, the installation on the load-side of the "service point" comes under the NEC. The size of the conductors on the primary as well as the secondary must sized in accordance with Article 220 and the ampacity tables in Article 310, unless you use the Neher-McGrath calculation for calculating the load and ampacity. Based on Table 310.16, 7 sets of 750Kcmil copper conductors, the load must not exceed 3325 amperes. However, the 1500 Kva transfomer is only capable of supplying 1806 amperes. The load calculation is necessary for a proper service size.

 

[bennie]

Re: Will this fly?

I was applying backward math (whatever that is) when I arrived at the 58% figure.

I should have stated "the transformer will supply 58% of the connected load".

If the demand factor is not over 58% the 1500 KVA transformer will fly.

 

[mvannevel]

Re: Will this fly?

Posted the original question in a bit of a hurry yesterday. Here's some more information. This is a large retail establishment. All fluorescent and HID lighting. A significant number of roof top units as well as refrigeration compressors for the grocery area. The prints don't show a load calculation, as such. Only panel schedules with the connected kva and amperage and feeder kva and amperage (usually 125% of the connected amperage). However, it appears to me that only the connected kva and amperage were taken into account when figuring the service size. Maybe I'm missing something here.

 

[bob]

Re: Will this fly?

You may used the connected KVA to size the transformer but it may not be smaller than the size determined by NEC caculations. Since this
transformer is now covered under the NEC you may have a larger transformer than what the actual demand will require. The utility would size it by using 100 % of the lighting and A/C plus the other
load that would determine the peak load.

 

[jmc]

Re: Will this fly?

You can size the transform for 1500kVA which is less than the load requirement. But remember you must protect the transformer in accordance with Article 450. Therefore, if the load is grater than what the overcurrent protection for the transformer, it will trip the breaker or blow the fuse. The designer in that case did not design the system properly, however,it is not a Code violation, but the system will not deliver its intended energy. Remember the Code is not a design manual!!