PV load side connection on center-fed busbar

Status
Not open for further replies.
C

ChargedUp

Member
Location
Ohio
2017 NEC 705.12(B)(2)(3d) says, "a connection at either end, but not both ends, of a center-fed panelboard in dwellings shall be permitted where the sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar does not exceed 120 percent of the current rating of the busbar"

As long as the sizing criteria is met, can this be applied to a center fed busbar in switchgear for a commercial application?

If not, state the theory why...

Thanks all.
 
I'm sorry all, I should have said at the end, "Please state the theory why". Please someone respond, otherwise I may start backstabbing receptacles.
 
ChargedUp said:
2017 NEC 705.12(B)(2)(3d) says, "a connection at either end, but not both ends, of a center-fed panelboard in dwellings shall be permitted where the sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar does not exceed 120 percent of the current rating of the busbar"

As long as the sizing criteria is met, can this be applied to a center fed busbar in switchgear for a commercial application?

If not, state the theory why...

Thanks all.
Click to expand...

Unfortunately, extending this same rule to panelboards and switchboards in commercial applications is uncharted territory. The NEC's direct rules don't give a direct calculation, and it is up to the licensed engineer's supervision to determine whether a connection is acceptable or not.

The reason there is a concern about applying the 120% rule to center-fed panelboards in general, is as follows. Suppose we interconnect 40A (32A continuous) on the bottom half, and we have a 200A busbar, and 200A main in the center. Consider the unlikely event of "top heavy" loads, such that the loads in the bottom half diminish to zero, and simultaneously, all the loads in the top half draw as much current as possible. It would be possible for 232A to be present on the busbar, just above the main supply, and it would go "unnoticed" by the main breaker. This exceeds what a 200A busbar is rated to withstand.

The theory behind the 120% rule depends on the fact that the current flowing in opposite directions is subtractive rather than additive, which would always be the case for busbars fed from exact opposite ends. But when you introduce centerfeeding, this is no longer a fact we can rely upon. We now rely upon the statistical likelihood of the panel having either a decent distribution of its loads to both sides, or the panel mains being oversized anyway, to avoid the aformentioned problem of a "top heavy" load situation. In residential applications, it is rare that panelboards draw close to the full amps of the service anyway, due to being sized by a conservative calculation. A commercial/industrial application would require engineering supervision to account for how the loads realistically "add up", to determine the ideal location to interconnect that reduces this risk as much as possible.
 
Last edited:
There was some language in the 2014 NEC that permitted centerfed and multiple ampacity busbar connections under 'engineering supervision', with some particulars. That might be the best bet for making such a connection. I think Carultch did a good job of explaining the theory, such as it is.
 
Status
Not open for further replies.
Top