[RE-wrenches] Busbar 120% rule

[Nathan Charles]

Also in 2014 we get 705.12(D)(2)(3)(c) which should be very useful in
distribution panelboards.

-N


On Thu, Mar 27, 2014 at 3:00 PM, Brian Mehalic <brian at solarenergy.org>wrote:

> 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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-- 
Nathan Charles
Engineer
NABCEP Certified PV Installation Professional #042013-20
Paradise Energy Solutions
(717) 283-2021 direct
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