Series OCPD - Help finding in NEC?

[DustyRusty]

Hello,

Working on a project and I have come across a question I am having a hard time finding an answer too, hoping someone can point me to the code that might speak to this.

I am replacing an AC unit that is currently run to a 400A Disc (not fused) then to a 250A breaker. the new units ratings are smaller and call for a 175A OCPD. I want to replace the disconnect with a 200A fused disconnect with 175A fuses and just leave the 250A breaker. I dont want to remove the breaker because I would have to take an outage on the MCC its in and the other circuits are in service. any direction on this would be great.

Thank You

 

[david luchini]

In order to do what you suggest, the conductors from the breaker to the disconnect would have to be protected by the 250A breaker. They would have to be #4/0 or larger (assuming no adjustment or correction factors.)

 

[petersonra]

Are you saying the 400 A switch feeds a 250 A CB in an MCC? Or does the 250 A CB in the MCC feed the 400 A unfused?

 

[DustyRusty]

Are you saying the 400 A switch feeds a 250 A CB in an MCC? Or does the 250 A CB in the MCC feed the 400 A unfused?

right now its a 250A CB feeding a 400A switch (unfused) I want to feed a 200A FDC with 175A fuses.

 

[DustyRusty]

In order to do what you suggest, the conductors from the breaker to the disconnect would have to be protected by the 250A breaker. They would have to be #4/0 or larger (assuming no adjustment or correction factors.)

wouldn't the 175A fuses protect those conductors also?

edit: the existing conductors from the old system are already sized for the 250A breaker.

Really looking for a section in the NEC that speaks to this configuration, Its off though, and wouldn't normally find it in any new installation. is the rule of thumb for this "if it doesnt exist its not permitted" or does this go on an individual AHJ basis?

 

[david luchini]

Really looking for a section in the NEC that speaks to this configuration, Its off though, and wouldn't normally find it in any new installation. is the rule of thumb for this "if it doesnt exist its not permitted" or does this go on an individual AHJ basis?

You would have a feeder from the c/b to the fused disconnect switch...See 215.3.

You would have a branch circuit from the fused disconnect switch to the AC unit...See 440.22.

 

[DustyRusty]

You would have a feeder from the c/b to the fused disconnect switch...See 215.3.

You would have a branch circuit from the fused disconnect switch to the AC unit...See 440.22.

OK, so just treat them separately. that makes sense. Thank you

 

[Sea Nile]

My understanding is this. The Over current protection device determines the rating of the circuit. To exaggerate what I'm thinking, here is an example not related to your specific question. You could have a 15 amp breaker feeding a 200 amp disconnect. That's fine. The 200 amp disconnect would then function as a simple disconnecting means and never trip.

You could do it the other way, breakers have a fault rating of 10000 amps? So you could feed a smaller breaker with a larger one. Disclaimer, Im not a qualified electrician.

 

[jap]

Sounds like the existing is a 250 amp feeder to a 400 amp NF disconnect to an integral 250 amp OCPD in the existing AC unit.

The new unit seems to require 175 amp Overcurrent protection.

Maybe look into changing out the existing 400a NF disconnect to a 400 amp Fused Disconnect with 400a to 175a fuse reducers and leave everything to the line side of the new fused disconnect as is.

JAP>

 

[petersonra]

right now its a 250A CB feeding a 400A switch (unfused) I want to feed a 200A FDC with 175A fuses.

So essentially you want to replace a 400 A unfused switch with a 200 A fused switch. I can't see any downside as long as the wires are not so big they won't fit on the lugs of the 200 A fused switch. Worst case you have to change out the wires or at least add some smaller tails to them.

 

[DustyRusty]

Sounds like the existing is a 250 amp feeder to a 400 amp NF disconnect to an integral 250 amp OCPD in the existing AC unit.

The new unit seems to require 175 amp Overcurrent protection.

Maybe look into changing out the existing 400a NF disconnect to a 400 amp Fused Disconnect with 400a to 175a fuse reducers and leave everything to the line side of the new fused disconnect as is.

JAP>

Thank you, that was my initial approach but we happen to already have the 200A FDC on site, so Im trying to figure out if code will let me use it without changing out the 250A breaker.

 

[jap]

If you change out the existing disconnect to a 400 amp fused, your existing conductors will fit under the lugs, you can reduce the 250 amp feeder down to 175 amps to feed the new unit, and, you haven't choked down the 250 amp feeder to the unith

Thank you, that was my initial approach but we happen to already have the 200A FDC on site, so Im trying to figure out if code will let me use it without changing out the 250A breaker.

That's going to cause you a lot of grief.

JAP>

 

[DustyRusty]

Thanks Everyone! I think you have helped me to figure out what to do. going to just order the 400A disconnect. appreciate everyone's time.

 

[jap]

This is just a suggestion of an option.

I'd be sure to check on the availability of all the components needed before making a final decision as to what would work out the best.

JAP>