A commercial office building is to be supplied service at 208Y/120 V from the utility company 12.47 kV distribution feeder (12.47 kV line-to-line and 7.2 kV lineto-neutral). The estimated demand load is 85 kW at 0.9 lagging power factor. Determine the appropriate apparent power rating and voltage rating of the threephase, pad mounted transformer required to serve this load. The transformer is connected delta-grounded wye. Assume the load is continuous and allow a 25% factor for load growth.

Apparent power rating is: 120 kVA

Apparent voltage rating is: 0.208 kV

Explanation:

Given demand load of commercial office building P = 85 kW @ pf = 0.9 lagging

Allow load growth of 25% = 0.25 * 85 = 21.25 kW

Maximum load is Pm = 85 + 21.25 = 106.25 @ pf = 0.9 lagging

Apparent power demand S = Pm/pf = 106.25/0.9 = 118.05 kVA

This apparent power should be meet by 3-phase pad mount transformer at secondary voltage of 208 V (L-L) and secondary should grounded star .

Apparent Power rating of 3-phase pad mounted transformer is ST = 120 kVA

Volatage rating of 3-phase pad mounted transformer is Delta - grounded star V1 : V2 = 12.47 : 0.208 kV

0 = 0.40 / 0 = 0

= 0.20 = 9.80 ⁄2 =?

=0 , = = 0.20 =0.50 0 0.40 ⁄

=0−122 =0−12(9.80 ⁄2)(0.50)2 =1.2