Request guidance on a 3 phase 120/208V Subpanel design

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I stopped doing electrical design for 15 years and I’m rusty. I’ve been asked to add a small critical panel in a Building to supply some computer network equipment in a Telco Room. I’m calling this Panel B. I’ve seen this called a Subpanel.

The closest upstream 3 phase panel is a 120/ 208 VAC 100A MLO 42 circuit breaker panel 170’ away in the same Building. I’m calling this Panel A. This panel is lightly used and has 8 KVA of load on it spread against all 3 phases equally and 10 spares.

In my design for the new panel B, I did the following and I respectfully request your feedback as to whether I am doing these things correctly.
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  • I used a 3 phase 100A panel for Panel B even though a single phase panel was sufficient. This is for future considerations and unanticipated technology changes.
  • I installed a Main Breaker on Panel B since its 170’ away from Panel A and there are Security Doors restricting access.
  • I derated the new 100A Panel B to 60A by installing a 3 pole 60 Amp Main Breaker at Panel B and using a 3 pole 60 A breaker to feed Panel A. I did this to protect Panel A should it ever have more loads put on it.
  • I carried the Ground from Panel A to Panel B as an Equipment Ground and bonded it to the enclosure.
  • I installed a TVSS at Panel B. Panel A has a TVSS. What guidelines apply here? What is the NEC guidance in this situation? Do the TVSS’s have to be coordinated or is installing a second TVSS incorrect ?View attachment FENSPNLDesign 9-28.png

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Is panel “A” backed up by a UPC? If not, do you think the customer may eventually want panel “B” backed up by a UPC? Single phase UPC’s that small are cheaper. Just a thought.
 
Even though it is not required, I agree with putting a main breaker in Panel B. An electrician can then more easily isolate the panel for adding circuits. Or an operator can kill the entire panel in case of a fault in the room.

As far as having a UPS, I think it better to have them just where required to keep the comms equipment going and not include other devices in the room like the coffee pot.

And yes, run a neutral. Pick a panel with a grounding screw that can be removed since this is a sub panel.
 
Oh, and for the TVSS, I like individual power conditioners for the comms power circuits where needed instead. They can carry through a momentary power loss.
 
TwoBlocked said:
Even though it is not required, I agree with putting a main breaker in Panel B. An electrician can then more easily isolate the panel for adding circuits. Or an operator can kill the entire panel in case of a fault in the room.
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Ive never understood that argument:. The line side of the main breaker will still be hot so if you want a de-energized panel, you'd have to kill the source OCPD anyway. And won't the branch OCPD clear any fault in the room?
 
electrofelon said:
Ive never understood that argument:. The line side of the main breaker will still be hot so if you want a de-energized panel, you'd have to kill the source OCPD anyway. And won't the branch OCPD clear any fault in the room?
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Newer panels are finger safe. Yes, you are "supposed to" dress up to open the dead front and test for voltage. Even so, after that, you can do what you want as long as you don't mess with the incoming line. You don't even have to lock out the feeder in the other panel because you have the required disconnect under your control. AND you KNOW you have the right disconnect. If it's in another panel, well, all sorts of mistakes are made.

And yes, the breaker for the feeder in the other room will clear a fault if it is large and long enough. But if a coffee maker starts arcing on a 20 amp circuit with a #18 cord, probably not. Same with a lighting ballast that is going berserk.

Anyhoo, that's how I look at it. Thanks for asking. :)
 
Last question. Where can I find NEC guidance on multiple TVSS’s? Hypothetically if B was 4 foot from A instead of 170 feet, wouldn’t a TVSS be unnecessary? Trying to relearn, thanks everyone
 
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