[RE-wrenches] Busbar 120% rule

[Brian Mehalic]

The "120% rule" in 2014 is 705.12(D)(2)(3)(b).  Ugh.  Anyway, the inverter
current is "125 percent of the inverter(s) output circuit current", aka the
minimum OCPD and conductor ampacity value per 705.60(B) and 690.8(B)(1).

This is actually a bit of wiggle room: in 2011 the 120% rule was based on
the inverter output circuit OCPD size, which is often larger than 125% of
the inverter output circuit current.

Cheers,


Brian Mehalic
NABCEP Certified Solar PV Installation Professional(tm) R031508-59
IREC ISPQ Certified Affiliated Instructor/PV US-0132

PV Curriculum Developer and Instructor
Solar Energy International
http://www.solarenergy.org


On Thu, Mar 27, 2014 at 2:34 PM, Troy Harvey <taharvey at heliocentric.org>wrote:

> I am wondering about the busbar 120% rule, and if there is any wiggle room
> in the 2014 NEC.
>
> Fundamentally I don't understand the 120% rule. If my solar breaker is
> installed properly at the bottom of the busbar, and the grid-tie breaker is
> installed at the top, and the busbar itself is rated for 120% of the panel
> rating, I don't see any means by which a solar breaker of a size
> substantially larger than 120%  could cause a problem. There can be no
> place on the busbar under any situation (that I can think of) that would
> exceed 120% because the supply current is coming from opposite ends of the
> bus bar - even in the worst case load situation. So even if I had a huge PV
> system (100A), backfeeding the bottom of a 200A panel, I don't see a
> situation where there is more than 200A over any one section of busbar. Am
> I wrong, or is the NEC just too prescriptive for its own good?
>
> Also would you say that the 120% is based on the inverter max output or
> backfeed breaker size?
>
>
> thanks,
>
> Troy Harvey
> ---------------------
> Principal Engineer
> Heliocentric
> 801-453-9434
> taharvey at heliocentric.org
>
>
>
>
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