Transformer Primary Side OCP

[Npstewart]

If I have a 75 KVA transformer, I typically feed it with a 150A/3P breaker if the primary side voltage is 480-3. A designer I work with is insisting a 100A breaker, I kind of think its a little too close to the load. Anyone have a ruling?

 

[Cold Fusion]

If I have a 75 KVA transformer, I typically feed it with a 150A/3P breaker if the primary side voltage is 480-3. A designer I work with is insisting a 100A breaker, I kind of think its a little too close to the load. Anyone have a ruling?

As long as the xfm wasn't something strange (like a fully enclosed) I'd probably go witha a 125A and #2.

But there is nothing wrong with 150A and 1/0. And you could as high as a 225A and 4/0.

cf

 

[Npstewart]

OK, that sounds exactly right now that you say that. Also, What section do you get the #2 ground from?

 

[Cold Fusion]

OK, that sounds exactly right now that you say that. Also, What section do you get the #2 ground from?

Not a ground, primary conductors.

cf

 

[Npstewart]

Thats right, its grounded for the secondary conducts per 250.66 Service Ground

 

[barclayd]

See Table 450.3(B)
If there is secondary overcurrent protection, the primary can be protected up to 250% of rated current.
If there is no secondary overcurrent protection, then 125% of rated current (to next higher standard rating).
FLA = 90.2 amps x 1.25 = 112.75 amps. Next higher standard rating = 125 amps.
90.2 x 2.5 = 225.5 amps, 225 amp breaker, max.

This is probably a separately derived system, rather than a service.
db

 

[Mike01]

If there is no secondary overcurrent protection, then 125% of rated current (to next higher standard rating).
FLA = 90.2 amps x 1.25 = 112.75 amps. Next higher standard rating = 125 amps.
90.2 x 2.5 = 225.5 amps, 225 amp breaker, max.

This is probably a separately derived system, rather than a service.
db

Just remember to refer to section 240 as there are only a couple of circumstances where you can use primary protection only...