calculating branch circuit for transformer feed

[PhaseShift]

When calculating the circuit ampacity for the feed to the primary of a transformer do you use the transformer primary rated current or do you have to use the transformer primary rated current + 125% to size conductors. I always thought it was just primary rated current.

So if I have primary 100kVA at 480V then transformer has primary rated current of 208A. When sizing conductor do I use 208, or must I use 208 * 1.25= 260A to determine ampacity for feeder?

Also when you have a transformer which you are including with other loads on a feeder circuit then do you use the transformer primary rating only added to other continuous loads and then apply 125% to the total continous load total only once?

 

[charlie b]

When sizing a transformer primary feeder, I use 125% of the rated primary current. When including the transformer load in a load calculation, I disregard the existence of the transformer itself. That is, if I calculated a load of 80 KVA (including, as necessary, and 125% factors for motors or continuous loads), and if I chose a 112.5 KVA transformer, then I would account for only the 80 KVA, not the entire 112.5 KVA.

 

[PhaseShift]

When sizing a transformer primary feeder, I use 125% of the rated primary current. When including the transformer load in a load calculation, I disregard the existence of the transformer itself. That is, if I calculated a load of 80 KVA (including, as necessary, and 125% factors for motors or continuous loads), and if I chose a 112.5 KVA transformer, then I would account for only the 80 KVA, not the entire 112.5 KVA.

I've also seen that you typically size breaker at 125% of transformer rating to account for inrush.

So if you say you size feeders for 125% do you add another 125% for breaker size to account for inrush, or do you simply use 125% of primary rated current to account for inrush?

 

[kwired]

Check out 215.2(B) for sizing feeders supplying transformers.

Every time overcurrent protection is mentioned in article 450 it is maximum allowed - nothing says you can't have overcurrent protection less than maximum allowed, too low to handle inrush current is a nuisance though.