120% rule???

[Davis White]

Hearing opposite things from two electricians whom I respect.

Background

Main panel:
125 amp busbar
100 amp overcurrent / single phase
Conductors from the Powco feed into the centerfed busbar, not lugs or breaker on top.

Inverter:
4000 Watts/50 amp OPD, so
50 amp Breaker for inverter on critical loads/subpanel

One gent says I have to derate/replace main panel/install lineside tap because the inverter breaker cannot be on the opposite end of the busbar from the main breaker.

The other gent says no problem/no need to change anything.

Trying to make sense of this....

Which is correct? (AHJ will not communicate prior to inspection.)
https://www.electricallicenserenewa...ation-Courses/NEC-Content.php?sectionID=190.0
https://www.electricallicenserenewa...ation-Courses/NEC-Content.php?sectionID=190.0

Thank you!!!

 

[svetz]

NEC 705.12(D) (the 120% rule) through me for a loop at first too, but it's a good rule to follow and is explained nicely here.

The gist of it is that the main might not trip when there's a short that exceeds the busbar's capacity - which can happen because both solar and grid are supplying power. Say the main is rated for 100 amps and has a 100 amp breaker and solar a 60 amp breaker. Solar is providing 60 amps and the grid 90 with a 150 amp short. The grid breaker won't trip as it's less than 100 and the solar breaker won't trip. But the backplane is carrying 150 amps.

 

[Davis White]

705.12(D)(2)(3)

(d). Connections shall be permitted on multiple-ampacity busbars or center-fed panelboards where designed under engineering supervision that includes fault studies and busbar load calculations.

The following subsection is not present in the printed NEC textbook but was added by TIA 14-12:

(e). A connection at either end, but not both ends, of a center-fed panel board 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.