Tapping Existing Feeders for New Subpanel

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BenjiF23

Member
Location
NY
Occupation
Electrical Engineer
Hi all, just joined here and happy to have found this forum.

I have a project where a building has (1) 400A, 120/208, 3Ph (400A main circuit breaker) panelboard supplying the space. The existing load is probably ~100A-150A max.

We are installing (2) new HVAC units, totaling in about 75A of load. I would like to tap off the existing 400A feeders to a new subpanel for the mechanical units as the existing panel is completely full and there are no spares or spaces.

Would I be right in that for a new 100A subpanel, I can tap off the existing feeders and use #1's (Table 310.15(B)(16), 60deg column per 110.14(C)) to my new panel? I used the downstream OCPD (100A subpanel) for conductor sizing, conductors aren't extending beyond the equipment they supply, conductors are in a raceway, and the ampacity of the taps meets the 10% rating of the upstream OCPD. (All rules found in 240.21(B)(1) for taps not over 10')

Question: is there anything in the NEC prohibiting me from tapping the feeders before the MCB? The upstream OCPD has not yet been verified, but per 408.36, we know that the upstream OCPD is 400A max... so any breaker smaller would still meet the 10% rating requirement (240.21(C)(1)(4)).

I just want to make sure I am not missing anything with this approach. Any advise is greatly appreciated in advance.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
BenjiF23 said:
Hi all, just joined here and happy to have found this forum.

I have a project where a building has (1) 400A, 120/208, 3Ph (400A main circuit breaker) panelboard supplying the space. The existing load is probably ~100A-150A max.

We are installing (2) new HVAC units, totaling in about 75A of load. I would like to tap off the existing 400A feeders to a new subpanel for the mechanical units as the existing panel is completely full and there are no spares or spaces.

Would I be right in that for a new 100A subpanel, I can tap off the existing feeders and use #1's (Table 310.15(B)(16), 60deg column per 110.14(C)) to my new panel? I used the downstream OCPD (100A subpanel) for conductor sizing, conductors aren't extending beyond the equipment they supply, conductors are in a raceway, and the ampacity of the taps meets the 10% rating of the upstream OCPD. (All rules found in 240.21(B)(1) for taps not over 10')

Question: is there anything in the NEC prohibiting me from tapping the feeders before the MCB? The upstream OCPD has not yet been verified, but per 408.36, we know that the upstream OCPD is 400A max... so any breaker smaller would still meet the 10% rating requirement (240.21(C)(1)(4)).

I just want to make sure I am not missing anything with this approach. Any advise is greatly appreciated in advance.
Click to expand...
I think it looks okay, I just don't quite follow your reasoning of the upstream breaker because of 408.36. if your panel has a main breaker then that protects it at it's rating so the upstream breaker could be anything. Assuming everything was done correctly and there were no feeder taps involved, you could look at the ampacity of the conductors supplying that 400 amp panel board to get your (max) upstream breaker size.

Just one thing you might want to check which might make things a little easier, can you get feed through lugs for that panel instead of having to physically tap the conductors? I know Siemens panel boards come standard with holes for feed through lugs, can't speak for other brands though.
 
J

jap

Senior Member
Occupation
Electrician
BenjiF23 said:
Hi all, just joined here and happy to have found this forum.

I have a project where a building has (1) 400A, 120/208, 3Ph (400A main circuit breaker) panelboard supplying the space. The existing load is probably ~100A-150A max.

We are installing (2) new HVAC units, totaling in about 75A of load. I would like to tap off the existing 400A feeders to a new subpanel for the mechanical units as the existing panel is completely full and there are no spares or spaces.

Would I be right in that for a new 100A subpanel, I can tap off the existing feeders and use #1's (Table 310.15(B)(16), 60deg column per 110.14(C)) to my new panel? I used the downstream OCPD (100A subpanel) for conductor sizing, conductors aren't extending beyond the equipment they supply, conductors are in a raceway, and the ampacity of the taps meets the 10% rating of the upstream OCPD. (All rules found in 240.21(B)(1) for taps not over 10')

Question: is there anything in the NEC prohibiting me from tapping the feeders before the MCB? The upstream OCPD has not yet been verified, but per 408.36, we know that the upstream OCPD is 400A max... so any breaker smaller would still meet the 10% rating requirement (240.21(C)(1)(4)).

I just want to make sure I am not missing anything with this approach. Any advise is greatly appreciated in advance.
Click to expand...

Are you sure you're not dealing with service conductors rather than feeders?

JAP>
 
C

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
Why not move 2 small circuit out of the existing panel into your new panel then use the open spaces to add a 100 amp breaker?
 
J

jap

Senior Member
Occupation
Electrician
curt swartz said:
Why not move 2 small circuit out of the existing panel into your new panel then use the open spaces to add a 100 amp breaker?
Click to expand...

Hopefully he would move "3" small circuits to make room for the subfeed breaker to feed the new panel. :)

JAP>
 
B

BenjiF23

Member
Location
NY
Occupation
Electrical Engineer
electrofelon said:
Just one thing you might want to check which might make things a little easier, can you get feed through lugs for that panel instead of having to physically tap the conductors? I know Siemens panel boards come standard with holes for feed through lugs, can't speak for other brands though.
Click to expand...
Good point thank you - I will look into this.

electrofelon said:
2 more things:. Remember if you use a wire EGC it must be sized for the feeder being tapped. Also you can very likely use 75° for your conductor sizing.
Click to expand...
Do you know the Code reference for this? Would like to go over that section. And yes good point about the 75deg

jap said:
Are you sure you're not dealing with service conductors rather than feeders?

JAP>
Click to expand...
90% sure... waiting for that to be field confirmed soon.

curt swartz said:
Why not move 2 small circuit out of the existing panel into your new panel then use the open spaces to add a 100 amp breaker?
Click to expand...
I am assuming you meant "3" (120/208, 3 phase :)), but yes I agree this was my initial approach and then I wanted to provide a design that gave them the capacity for future work since they're existing panel has no more space. Your suggestions seems to require less time/labor/materials involved for the contractors as well.

Most of their existing ckts are lighting, recept, a few unit heaters, and some specialty equipment.

Thank you all for responses!
 
J

jap

Senior Member
Occupation
Electrician
BenjiF23 said:
Now I am confused - could you explain please?

I assumed (3) as well, and install (1) new 100A/3P breaker for my subpanel.
Click to expand...

He's being technical,, :) ,, you can move (3) individual branch circuits, or, (1) 3 phase circuit to make room for the new 3p breaker.

JAP>
 
C

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
The other thing you could do is move 9 spaces worth of lightly loaded circuits to your new panel. Put the 2 new HVAC loads in the 400 amp panel along with the 100 amp breaker feeding the new panel. This will allow for more future loads to be added to the new panel.
 
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