[RE-wrenches] Busbar 120% rule

Phil Forest pforest at southmountain.com
Thu Apr 3 09:18:07 PDT 2014


Troy,
This is my first post. 
We install 32 amp rated output inverters, reduce the main breaker to 175amp, or line side tap. 

Phil Forest
South Mountain Company

> On Apr 2, 2014, at 1:40 PM, Troy Harvey <taharvey at heliocentric.org> wrote:
> 
> Thanks Bill, Makes sense to me too. Splitting your current over two sections of busbar, as calculated by P=I^2R, does seem like it will in reality reduce the heat load in the load center.
> 
> However, no good AHJ solutions today. We are finding the most customers have been moving towards larger systems as panel prices have fallen. If typical service is 200AMP, and that average american household usage is ~32kWh/day, that is approx. a 8kW system in most areas. And 8kW inverter * 125% means a 43AMP backfeed. Next size up is 50amp. That is bigger than 120%. This is most every project we work on these days is over 120%. 
> 
> So how is everyone else dealing with this?
> 
> thanks,
> 
> Troy Harvey
> 
> 
> 
> 
>> On Mar 28, 2014, at 8:38 PM, Bill Brooks <billbrooks7 at yahoo.com> wrote:
>> 
>> Dave and Troy,
>>  
>> I don’t think JW had his facts correct on this. The standard test for a busbar is to place the highest allowable breakers directly below the main breaker to test for overtemperature of the busbar. With the requirement for Article 220 compliance of the panel, a panel that actually complies with Article 220 could go to 200% and will likely run cooler than a panel only fed by the utility.
>>  
>> Devil’s advocates state that people violate Article 220 all the time so we need to be conservative.
>>  
>> Make a proposal at the meeting in Golden on April 9-10 and you may become famous.
>>  
>> Bill.
>>  
>> From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Troy Harvey
>> Sent: Friday, March 28, 2014 1:35 PM
>> To: RE-wrenches
>> Subject: Re: [RE-wrenches] Busbar 120% rule
>>  
>> Very interesting. 
>>  
>> So, it is not a overcurrent risk, but a heat issue that may lead to a nuisance breaker tripping issue?
>>  
>>  
>> On Mar 27, 2014, at 12:46 PM, Dave Click <daveclick at fsec.ucf.edu> wrote:
>> 
>> 
>> I had a nice response all typed up before rediscovering my original source. Simple answer: there's still a thermal load to deal with even though there's no point on the bus seeing a current above the busbar rating. I am a linking machine today:
>> http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/690.64(B)(2)Load%20Side%20Connections.pdf
>> 
>> While this situation of connecting supply overcurrent devices at opposite ends may be 
>> safe for restricted conductors, it may not be suitable for busbars in panel boards, even 
>> though this allowance is in the 2008 NEC. Panel boards are subject to busbar current 
>> limitations and are also subject to thermal limitations due to the heating associated with 
>> the thermal trip elements in the common thermal/magnetic molded case circuit breakers. 
>> For example a 100-amp, 120/240V panel board is tested during the listing process with a 
>> 100 amp main breaker and two 100-amp load breakers (one per phase) mounted directly 
>> below the main breaker. The ambient temperature is raised to 45 degrees Celsius, the 
>> input and output currents are set at 100 amps, the temperature is allowed to stabilize, 
>> and the panel must pass this test with no deformation of any parts. If we add a backfed 
>> PV breaker pair, for example 50 amps, at the bottom of the panel, and if the loads on the 
>> panel were increased to 150 amps, no breakers would trip, no busbars would be over 
>> loaded, but the thermal load in the panel would be that associated with 300 amps, not the 
>> 200 amps the panel was designed and listed for. Panel manufacturers have stated that 
>> these panels cannot pass UL listing tests with those excessive thermal loads.
>> 
>> On 2014/3/27, 14:34, Troy Harvey 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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