[RE-wrenches] Busbar 120% rule

Bill Brooks billbrooks7 at yahoo.com
Fri Mar 28 19:38:41 PDT 2014


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
<mailto: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%20
Connections.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 <mailto:taharvey at heliocentric.org> 









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