Interesting 2 Section Panel connection

[infinity]

Nobody has really brought up performance issues. If the phases in the two panels are unevenly loadced, and differently, the jumpers could easily carry currents that might not balance out, and can create eddy currents that cause the nipple to heat up. That's a definite no-no.

Actually I did on page 1:

Also, if one panelboard had a large load and the other had no load the effective length of the two sets of conductors would be different, which would cause one set to carry more current than the other.

 

[RUWired]

I think in wiring method 1, the conductors feeding the second panelboard is actually a tap conductor.Just like the reading of article 240.21

240.21 Location in Circuit
Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (G). No conductor supplied under the provisions of 240.21(A) through (G) shall supply another conductor under those provisions, except through an overcurrent protective device meeting the requirements of 240.4.

This setup looks something like a closed loop where the 400a panelboards are being tapped off the loop.The only problem is the loop conductors are only 3/0 and on a 400a brkr. Any one of the two panels can exceed that capacity.
For someone to take on the liability to say one panelboards draw will be spread out on both runs in parallel in this setup,is taking a huge risk.
Rick

 

[frenchelectrican]

i can see you guys have good points but one thing it did catch my attetion i dont know if you look at the photo on right side aka option 2 did you count the conductors between the panels ?? i did see it say " 8 " if so how the heck you can " Bridge it " ??

but before ya yell at my deaf ears i did see the photo saying on option 2 there is two conducts 2 1/2 inch each but when to termated the panelboard this get little conflect here will all conductors land at one panel board and jump over to other panel board ?? i was looking the photo twice and went over fine combe it never mention the main ocpd or secondary ocpd to the enxt panel so where is the code land into ??


thanks for your debates and have nice week


Merci , Marc

 

[petersonra]

This is an interesting exercise. Most of us seem to feel a certain unease about this installation, but can't quite find anything in the code that absolutely prohibits it.

The objection that seems to have the most validity involves the jumpers between the two MLO panels.

If for the time being, we assume the OCPD feeding the conductors coming to the MLO panels is properly protected, what happens if ALL the current flows through just one of the MLO panels, and it is 400 A.

If the two parallel conductors are exactly the same length, we would have a current division between those two conductors of 200A each. However, the jumpers add a slight, maybe negligible, amount of resistance to the current path for one set of conductors, meaning there is a slight imbalance in current flow between the two conductors. In the real world, this is no different than the difference in current flow that results from having the two parallel paths not being exactly equal, since it is near impossible to make them exactly equal.

Would it make a difference in your thinking if the jumpers were made so they could handle the full 400A load?

As a practical matter, I did some calculations to determine just what might be the situation if all 400A went to just one MLO panel.

3/0 has a dc resistance of .0766 ohms/1000 ft according to the NEC.

I made a little chart

The first column is the length in feet of the parallel conductors
The second column is the length of the jumpers
The third is the resistance of the parallel conductors
The fourth is the resistance of the jumpers
The last two columns are the current division between the sets of conductors

(For reasons that are not evident, the forum removes extra white space and formatting so inserting a table is near impossible)

100 05 0.00766 0.008043 204.8780488 195.1219512
100 10 0.00766 0.008426 209.5238095 190.4761905
200 05 0.01532 0.015703 202.4691358 197.5308642
200 10 0.01532 0.016086 204.8780488 195.1219512
300 05 0.02298 0.023363 201.6528926 198.3471074
300 10 0.02298 0.023746 203.2786885 196.7213115

As you can see, there is some uneven current distribution.

If one were to double up the jumpers it comes out like this:

100 05 0.00766 0.0078515 202.4691358 197.5308642
100 10 0.00766 0.0080430 204.8780488 195.1219512
200 05 0.01532 0.0155115 201.2422360 198.757764
200 10 0.01532 0.0157030 202.4691358 197.5308642
300 05 0.02298 0.0231715 200.8298755 199.1701245
300 10 0.02298 0.0233630 201.6528926 198.3471074

I am not convinced either situation presents a serious hazard, but one could make a pretty convincing argument that the current draw in worst cases would not be the same on both sets of parallel conductors. The code does not seem to require equal current division between parallel conductors however, so I am not sure it would be an enforceable observation.

 

[steve66]

What happens 3 years down the road when an electrician takes the cover off those panels and says "What the ****". He will probably remove the jumpers thinking it was installed by someone who didn't have a clue.

Steve

 

[petersonra]

What happens 3 years down the road when an electrician takes the cover off those panels and says "What the ****". He will probably remove the jumpers thinking it was installed by someone who didn't have a clue.

Steve

The code does not concern itself with potential future improper actions.

 

[LarryFine]

The code does not concern itself with potential future improper actions.

Sure it does. For example, recessed fixtures have to be load-calculated at their maximum bulb capacity.