700A breaker to feed 300A Fused Disconnect?

[handymann]

Hello everyone. I am new to Mike Holts forum but I was lucky enough to watch his video series on the 2005 NEC and he definitely made the NEC entertaining for me since I could not read through a few pages without drifting off. Anyway...

Here are the details
An 800A main breaker for the MCC.
A 700A breaker in the MCC that currently feeds a compressor that will be decommishioned.

My boss is suggesting we use the existing feed from the 700A breaker and place a new 300A fused disconnect that ends where the old drive for the compressor is. This is all so we don't have to run more wire and install a new bucket/breaker in the MCC and we can run a 150HP pump.

I don't have much experience but it seems a little odd to me to use 300A fuses in order to create the feed for the 150Hp pump and go from 700A at the disconnect, then have 300A fuses to ensure we will have proper overcurrent protection.


Please share your thoughts.

Thank you

 

[Dennis Alwon]

If the wire from the MCC is protected at 700 amps with the appropriate wire then there is no reason that I know that would prohibit this install. The lugs for the disconnect may not be able to support the wire from the MCC. At 700 amps I think you have parallel conductors so you will have a problem with the install.

Let me ask you are you an engineer or a handy man?

 

[infinity]

What's the distance between the 700 and 300 amp equipment? You may be able to apply one of the tap rules.

 

[don_resqcapt19]

Is the device in the MCC a breaker or is it a motor starter?

 

[handymann]

Dennis Alwon

Dennis Alwon

If the wire from the MCC is protected at 700 amps with the appropriate wire then there is no reason that I know that would prohibit this install. The lugs for the disconnect may not be able to support the wire from the MCC. At 700 amps I think you have parallel conductors so you will have a problem with the install.

Let me ask you are you an engineer or a handy man?

Dennis Alwon-

I am an engineer, but new and certainly not seasoned enough to have seen it all. But you did just address one of the concerns that I would not have noted immediately since I am unseasoned, and that is I would have to install a rated disconnect for 700A rather than the 300.

I will answer the rest of the threads below and hopefully that will help paint a better picture of what this install is.

Thanks

 

[handymann]

infinity

infinity

What's the distance between the 700 and 300 amp equipment? You may be able to apply one of the tap rules.

The approximate distance is 50-75 ft from the MCC in the electrical room, out of the MCC into the cable tray and then up onto the loft above the electrical room where the proposed disconnect would be placed.

I just brushed up on some of tap rules a short while ago for this in 230/240 and couldn't find anything I could relate to but maybe you see something else?

Thank you

 

[handymann]

Don

Don

Is the device in the MCC a breaker or is it a motor starter?

The MCC is a model 6 square D 5 section MCC, the first section is the 800A main, the second section is open for whatever to install. the 3rd section has the 700A breaker for the soon to be decommishioned compressor(250Hp) and the last two sections are for (2) 200Hp Glycol chillers, only one will ever run at once. So at most the MCC ever sees a constant load of the Compressor and (1) glycol chiller.

The Device in the MCC is a 700A breaker.

Thanks

 

[petersonra]

I don't think it is a problem in general. It may be an issue with physical sizes of wires.

 

[Dennis Alwon]

But you did just address one of the concerns that I would not have noted immediately since I am unseasoned, and that is I would have to install a rated disconnect for 700A rather than the 300.

Thanks

I know that a panelboard cannot be protected by an overcurrent protective device greater than the panelboard itself however the overcurrent protective device inside the disconnect would be compliant way to address this. Also I am not sure if this rule follows thru for a disonnect but either way I don't see where you need a 700 amp rated disconnect.

 

[iceworm]

... A 700A breaker in the MCC that currently feeds a compressor that will be decommishioned. ...

700A is not a standard frame size. So I suspect this is an 800A frame with a 700A trip unit. If so, can you get a 300A trip unit from SQD? If available, trade out the trip unit, put a power distribution block in an enclosure at the other end of the existing conductors. Come out of the PDB with 300A conductors to the starter for the 150HP pump. The starter will likely have it's internal disconnect maybe even a mag-only CB.

BTW, 300A is a bit low for a thermal-mag CB feeding a 150Hp motor. I would think 450A - 500A would be the minimum. Either way, if the only load is the motor, the conductors only need to be 180 x 1.25 = 225A.

ice

 

[don_resqcapt19]

...

The Device in the MCC is a 700A breaker.

Thanks

I was concerned that the overcurrent protective device in the MCC might be a motor circuit protector and not a circuit breaker. If it was a motor circuit protector, it could not be used for your proposed installation.

As long as it is a circuit breaker and as long as the conductors leaving that breaker have an ampacity of at least 700 amps you can use them to supply the line side of your 300 amp disconnect. There many be an issue with terminating 700 amps of conductor in lugs designed for a 300 amp circuit, but other lugs may be available for the disconnect, or you could install a junction box near the disconnect and use 300 amp tap conductors.

The conductors, if they were feeding a remote motor starter, may not have an ampacity of 700 amps.

 

[handymann]

petersonra

petersonra

I don't think it is a problem in general. It may be an issue with physical sizes of wires.

Thank you I will pay attention to the wire size when running through everything.

 

[handymann]

I know that a panelboard cannot be protected by an overcurrent protective device greater than the panelboard itself however the overcurrent protective device inside the disconnect would be compliant way to address this. Also I am not sure if this rule follows thru for a disonnect but either way I don't see where you need a 700 amp rated disconnect.

I will be sure to look into the proper configuration to mesh the two. Thank you for your comments

 

[handymann]

700A is not a standard frame size. So I suspect this is an 800A frame with a 700A trip unit. If so, can you get a 300A trip unit from SQD? If available, trade out the trip unit, put a power distribution block in an enclosure at the other end of the existing conductors. Come out of the PDB with 300A conductors to the starter for the 150HP pump. The starter will likely have it's internal disconnect maybe even a mag-only CB.

BTW, 300A is a bit low for a thermal-mag CB feeding a 150Hp motor. I would think 450A - 500A would be the minimum. Either way, if the only load is the motor, the conductors only need to be 180 x 1.25 = 225A.

ice

Ice- thanks for your help. I will look into what square D can supply and the configuration your proposing sounds like a good option.

I will look back into the tables that I used for the breaker sizing and see if I was off a column or two.

Thank you for the comments

 

[handymann]

I was concerned that the overcurrent protective device in the MCC might be a motor circuit protector and not a circuit breaker. If it was a motor circuit protector, it could not be used for your proposed installation.

As long as it is a circuit breaker and as long as the conductors leaving that breaker have an ampacity of at least 700 amps you can use them to supply the line side of your 300 amp disconnect. There many be an issue with terminating 700 amps of conductor in lugs designed for a 300 amp circuit, but other lugs may be available for the disconnect, or you could install a junction box near the disconnect and use 300 amp tap conductors.

The conductors, if they were feeding a remote motor starter, may not have an ampacity of 700 amps.

Don- Thank you for insight. I will have to see what is available from our supplier once I start ording parts but I will certainly take all that into consideration.

I check again on the ampacity of the wires.

Thanks for the comments.