xfmr feed and breaker

[sparkey00169]

I am installing a 75kva xfrm which draws 90 amps and it's output is 208 @ 208/120. The drawings call for a 250 amp breaker with 1 set of 1/0 aluminum xhhw. Is this permissible and if so can you explain it to me. Thanks.

 

[charlie b]

The rated secondary current will be 75,000 VA divided by 208 volts and divided again by the square root of three. That gives you about 208 amps. You are allowed to size the secondary overcurrent device as high as 125% of the rated current. My reference is table 450.3(B). That results in 260 amps. So using a 250 amp breaker is acceptable. So far, so good.

However, per table 310.15(B)(16), an aluminum 1/0 XHHW is only good for 120 amps. So that will not work. You need at least 300 MCM in that type of wire. So someone made an error on the drawings.

Welcome to the forum.

 

[augie47]

charlieb, if I;m not mistaken, even a 300 kcmil Al would not suffice in this case (assuming 75? terminations) since 240.21(C) won't allow "next size up" for transformer secondaries.

 

[charlie b]

You are right, Gus. I misread the table. A minimum 350 MCM would be needed here. Mea culpa.

 

[iwire]

I believe the OP is talking about the primaries.

 

[augie47]

If he is (speaking of primary) then the answer is a 250 amp breaker would not be allowed. If you use the 250% rule the primary current X 250% is less than 250 amps and there is not provision in 450.3 for "next size up" for primary protection.
The conductor ampacity would need to match the overcurrent device.