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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:<br>
<a class="moz-txt-link-freetext" href="http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/690.64(B)(2)Load%20Side%20Connections.pdf">http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/690.64(B)(2)Load%20Side%20Connections.pdf</a><br>
<br>
<i>While this situation of connecting supply overcurrent devices at
opposite ends may be </i><i><br>
</i><i>safe for restricted conductors, it may not be suitable for
busbars in panel boards, even </i><i><br>
</i><i>though this allowance is in the 2008 NEC. Panel boards are
subject to busbar current </i><i><br>
</i><i>limitations and are also subject to thermal limitations due
to the heating associated with </i><i><br>
</i><i>the thermal trip elements in the common thermal/magnetic
molded case circuit breakers. </i><i><br>
</i><i>For example a 100-amp, 120/240V panel board is tested during
the listing process with a </i><i><br>
</i><i>100 amp main breaker and two 100-amp load breakers (one per
phase) mounted directly </i><i><br>
</i><i>below the main breaker. The ambient temperature is raised to
45 degrees Celsius, the </i><i><br>
</i><i>input and output currents are set at 100 amps, the
temperature is allowed to stabilize, </i><i><br>
</i><i>and the panel must pass this test with no deformation of any
parts. If we add a backfed </i><i><br>
</i><i>PV breaker pair, for example 50 amps, at the bottom of the
panel, and if the loads on the </i><i><br>
</i><i>panel were increased to 150 amps, no breakers would trip, no
busbars would be over </i><i><br>
</i><i>loaded, but the thermal load in the panel would be that
associated with 300 amps, not the </i><i><br>
</i><i>200 amps the panel was designed and listed for. Panel
manufacturers have stated that </i><i><br>
</i><i>these panels cannot pass UL listing tests with those
excessive thermal loads.</i><br>
<br>
<div class="moz-cite-prefix">On 2014/3/27, 14:34, Troy Harvey wrote:<br>
</div>
<blockquote
cite="mid:83463131-0E23-48AB-AF67-FB2E668BB97F@heliocentric.org"
type="cite">
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<div>I am wondering about the busbar 120% rule, and if there is
any wiggle room in the 2014 NEC.</div>
<div><br>
</div>
<div>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?</div>
<div><br>
</div>
<div>Also would you say that the 120% is based on the inverter max
output or backfeed breaker size?</div>
<div><br>
</div>
<div><br>
</div>
<div apple-content-edited="true">
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0px; ">
<div style="word-wrap: break-word; -webkit-nbsp-mode: space;
-webkit-line-break: after-white-space; "><span
style="font-size: 11px; ">thanks,<br>
<br>
Troy Harvey<br>
---------------------<br>
Principal Engineer<br>
Heliocentric<br>
801-453-9434<br>
<a moz-do-not-send="true"
href="mailto:taharvey@heliocentric.org">taharvey@heliocentric.org</a><br>
</span><br>
<br>
</div>
</span>
</div>
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