Help size breaker for sub panel please

[staticcontrol]

Hello,

In a room on 3rd floor is a sub panel, 3 phase MLO. This panel is fed with #4 THHN/THWN from a disconnect on same floor in another room.

I want to take feeds off MLO and install a shunt trip breaker to re feed panel.

It's a 18 circuit 3 phase panel. It has (13) 20 amp breakers in it. Am I correct to select a 100 amp shunt trip breaker to back feed panel?

Any help is greatly appreciated. I also amp probed the feeds at the sub panel.

Black= 8 amps Red= 11 amps Blue= 6 amps This panel only serves some outlets in this room only.

Thank You.

Static

 

[charlie b]

It doesn't sound like you are planning to change anything upstream of the panel. So whatever is protecting the feeder to the panel, and whatever is protecting the panel itself, will remain in place. If that is true, and presuming the present overcurrent device(s) adequately protect the equipment, then it doesn't matter what size shunt trip breaker you use. It will not be the device that provides overcurrent protection, but rather will only serve the role of remote tripping.

That said, it should be noted that a #4 is only good for 85 amps. So I have to hope that the upstream protection is not set for 100 amps. I would suggest using a shunt trip breaker set at the same value as the upstream protection, whatever it is.

 

[augie47]

If the disconnect that feeds the panel remains in place and is properly sized, the size of main breaker you install in the panel makes no difference as it is only acting as a disconnect (in this case a shunt trip disconnect).
The conductors to the panel are protected by the existing disconnect.
For reference, a #4 THHN should be protected by no larger than a 90 amp overcurrent device under normal conditions (ampacity is 85 amps)

 

[staticcontrol]

Yes, thank you both for your response. I was unable to see at the time the disconnect size because it's behind a locked door and no one was around to let me in. I did use 310.16 but I thought I was able to go to next size breaker. I used the 75 degree column. I will check for a 90 amp. Thank you.

 

[augie47]

Yes, thank you both for your response. I was unable to see at the time the disconnect size because it's behind a locked door and no one was around to let me in. I did use 310.16 but I thought I was able to go to next size breaker. I used the 75 degree column..

correct..and the 75? column for #4 is 85 amps. next standard size breaker is 90

I will check for a 90 amp. Thank you.

again, if you have a 90 amp OCP protecting the feeder/panel you can use any size as the shunt-trip disconenct..50 amp, 200 amp any...
you may find a 100 more available than a 90

 

[staticcontrol]

Thanks again, Charlie b. and Augie. I do have another question please.

Same place different estimate: I want to install a 3 phase 12 circuit panel next to an existing panel that is packed. I will install a SQ-D MLO panel and protect it from a shunt trip breaker as well. Again- shunt trip will be located next to sub panel. Generally speaking, I will be moving existing ckts from one panel to sub panel. (total of 5) What should I charge for that?

Thank You

static

 

[charlie b]

What should I charge for that?

I can't help you there. But if you want me to design it for you, then I can give you a price for my engineering design services. :roll:

 

[staticcontrol]

Hmmm Charlie B, I hear ya. Thanks for the offer but I'll have to respectfully pass.

Thanks again for the info earlier.