Disconnect

[hhsting]

I have 200A breaker feeding 100A disconnect. The 100A disconnect is 45kva transformer primary disconnect. I am not sure if the disconnect is fused or non fuse.

In either case fused or non fuse Should the 100A disconnect be 200A since upstream breaker is 200A?

 

[JoeStillman]

200A if non-fused. 100A is ok if it's fused. What is the transformer voltage? The 200A CB may be too large to serve as the primary overcurrent protection.

 

[Fred B]

At least 2 conditions present to account for.
1 wire sized to the breaker protecting it.
2 equipment protection at the capacity it listed for.
If the 100A is not a breaker then the equipment is not listed for anything more and needs to be protected at that rate equipment is listed for, 100A.
If 100A is a breaker then feeder wire would need to be sized for 200A capacity as that is the feeder breaker and the 100A breaker/disconnect is adequate. (If no other considerations are present.)

 

[hhsting]

200A if non-fused. 100A is ok if it's fused. What is the transformer voltage? The 200A CB may be too large to serve as the primary overcurrent protection.

480V primary to 120/208V. Secondary breaker size is 150A

 

[infinity]

If the 100 amp disconnect is fused and the conductors feeding it is less than 200 amps then tap rules would apply.

 

[JoeStillman]

So the transformer primary current is 54 amps. Per table 450.3(B), with primary and secondary overcurrent protection, the maximum primary protection is 250% or 54 x 2.5=135A. The 200A CB upstream is too big to be the only overcurrent protection on the primary.. That 100A switch needs a fuse. Being fused, the tap rule applies so you don't need a 200A switch.

And like Fred says, you need to consider the conductor ampacities.

 

[Fred B]

Transformer supply input amperage? You question sounds related to the input side.

 

[augie47]

As noted, the 200 amp breaker would be oversized for that transformer overcurrent protection so the switch would need to be fused. That fuse could be any size up to 150 amp (450.3, next size up) but more commonly in the 100 amp range.
It would make sense if your plans showed a 100 amp FUSIBLE switch with the supply conductors full size or sized per Art 240 tap rules.