Taps for Transformers

[Mr. Spinosa]

I would like to know how NEC 240.21(B)(3) is applied using real world values and ratios. - (Especially 240.21(B)(3)(2) - What do they mean by "less than the primary-to-secondary voltage ratio multiplied by one third of the rating of the overcurrent device..." I would like to see something like this actually worked out or explained if someone is willing to do so. - Thanks so much in advance.

 

[rcwilson]

Imagine two 45 kVA, 3 phase 480-208V transformers tapped off a single 480V #2/0 feeder fed from a 175 A breaker.

Each transformer has 208V secondary conductors, #1/0, run 15 feet from transformer to a panel with a 150A main breaker. Criteria (5) is satsified.

Primary tap from the #2/0 in the gutter to the transformer is #4 Awg, 8 feet long.

Total length is 8'+15' which is less than 25' so criteria (3) is good.

Primary protection is 175A, 1/3 of 175A = 58 A.

#4 awg, 75 C rating is 85 A. 58 A is less than the 85 A rating so condition (1) is OK. (Tap conductor ampacity > 1/3 overcurrent device).

#1/0, 75 C = 150 A. 150A times the voltage ratio must be more than 58 A, 1/3 of the primary protection:

150 A x 208V/480V = 65A. 65A > 58A so condition (2) is satisfied. (150 A at 208 = 65 Amps at 480V)

Another way to do it is 65 A x 3 = 195A. Any 480V protection less than 195A will comply.