705.12(D)(2)(3)(d) Center-fed Panelboards.
Code Change Summary: New rules pertaining to PV connections inside center-fed panelboards have been added.
For many years, PV installers and inspectors have struggled with backfed PV connections to circuit breakers installed in center-fed panelboards. In Section 705.12(D), the NEC® allows panelboard busbars supplied from more than one source to be rated less than the amount of current supplying it.
An example might be a 100-amp rated service panelboard equipped with a 100-amp main circuit breaker, supplied from the utility meter. If this service panelboard is also supplied from a backfed PV circuit breaker, then the amount of additional PV current produced onto the service panel busbars must be taken into consideration to avoid overheating the 100-amp rated busbars.
For quite some time, the NEC’s solution to avoid overheating the busbars or creating a hot spot has been to locate the backfed PV circuit breaker at the opposite end of the main circuit breaker or main lugs in order to create a bit of space between the supplies to help with heat dissipation.
For center-fed panelboards, like in the image, which end is considered the opposite end from the main circuit breaker? For many homes with similar center-fed panelboards, the only solution if a PV system was desired was to change the service panel to a type that is top or bottom fed.
Now the NEC® has code language that addresses these installations.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).
2014 Code Language:
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.