Parallel feeder - intercepted but not tapped

[Dark Sparky]

I'm reviewing a concept proposed by an electrical contractor. There is an existing 480V, 3 phase, 400A distribution panel DP1, fed from switchboard MDP via a 400A breaker in MDP. Proposal is to intercept (more below) that feeder via a new J-box just below DP1. Then run 2 sets of 4-#3/0+G in 2-1/2" conduit immediately to a 400A enclosed circuit breaker (located near the intercept). Continue from that breaker about 175' to a new 400A distribution panel DP2.

1. Because the ampacity of the feeder (2x200A=400A) isn't exceeded by the rating of the upstream 400A overcurrent device, I understand that the feeder conductors are NOT considered to be tap conductors, per the definition of tap conductor in 240.2. (The intercept below DP1 is not considered a tap.)
2. Because the feeder intercept (point of supply) isn't considered a tap, I understand that there is no rule regarding additional overcurrent protection, and the new 400A enclosed circuit breaker could be anywhere convenient (in other words, it does not need to be a given distance from the feeder intercept).
3. I have no issue with the volt drop, even at a full 400A load.
4. The proposed ground wire size for the parallel feed was unclear. Per 250.122(G), etc., I would supply a #3 ground in each conduit (full-sized ground in each conduit, sized for 400A upstream breaker.)

Please let me know if anything above needs more clarification, is incorrect, or questionable. Thanks in advance as always!

 

[d0nut]

Assuming the panels are all in the same building, the 400A enclosed breaker can be wherever you want it. In fact, it doesn't need to exist at all if you don't want to have it.

 

[petersonra]

I don't see why you need to put in the 400 A enclosed CB, especially if the new DP has a main CB.

I agree with your #3 EGC, although if it is metal conduit I would probably just skip it altogether.

Is there a possibility of using feed through lugs instead of the junction box?

 

[augie47]

I would at least look at the possibility of installing double lugs in DP_1 and connect my feed there rather than adding a j box.

 

[hillbilly1]

Don’t see a reason to add a breaker at the tap point, or at the new panel as others have said. Feed thru lugs as suggested by others would be the cleanest look, but may not be possible depending on the panel style and manufacturer.

 

[Dark Sparky]

Great insight, all.

I wondered about the need for the breaker also, but the new panel DP2 is MLO (main lugs only) thus we need a breaker somewhere. Since DP1 is that ~175' away (in a separate part of the building), I'll check if the building Owner would prefer panel DP2 to have a main breaker (more local to the loads) - or if they prefer the upstream enclosed breaker near the feeder intercept (which renders DP2 entirely de-energized for any work on it).

I will check into the feed through lugs also. DP1 is an existing panel and I have no info on the style/mfg, but I could give the electrical contractor the option, as an alternate way to feed DP2.

 

[hillbilly1]

Great insight, all.

I wondered about the need for the breaker also, but the new panel DP2 is MLO (main lugs only) thus we need a breaker somewhere. Since DP1 is that ~175' away (in a separate part of the building), I'll check if the building Owner would prefer panel DP2 to have a main breaker (more local to the loads) - or if they prefer the upstream enclosed breaker near the feeder intercept (which renders DP2 entirely de-energized for any work on it).

I will check into the feed through lugs also. DP1 is an existing panel and I have no info on the style/mfg, but I could give the electrical contractor the option, as an alternate way to feed DP2.

The only main required is back at the MDP, but the owner may want another main past DP1, as you said, to keep from having to de-energize both to add or work on DP2. Just not code required.