system expansion [RE-wrenches]

[John Berdner]

Sky/Wrenches:
 
During the last code cycle we were trying to address the issue of bus
bars and cables and the 120% rule.
Basically we were discussing the fact that if the breakers for the PV
were installed at the bottom of the panel (farthest from the main lugs)
you can not create an over current situation anywhere on the bus.
The current from the grid flows down from the top and the PV current up
from the bottom.
 
The Square D guy had a very interesting point that I had not
considered.
One of the issues in a distribution panel is thermal loading caused by
the breakers themselves.
When you add PV (or other generation sources) to a panel you are adding
the heat load of the PV breakers (40 Amps on a 200 Amp residential
panel).  You also create a situation where you can supply 240 Amps of
load breakers in the same panel.
The total thermal loading of the panel is potentially 480 Amps worth of
breakers (200 main, 240 load, 40 PV).
Square D was saying that the plastics that support the bus bars can get
too hot and deform in this situation especially if the panel is exposed
to direct sun.
He did not go so far to say this would create a hazard but he did say
this was something they do not test for so it potentially could be an
issue.  
At minimum I can see how it could lead to increased tripping of
breakers in hot climates.
 
Best Regards,
 
John Berdner
 

-----Original Message-----
From: Sky Sims [mailto:sky at ecologicalsystems.biz] 
Sent: Monday, August 20, 2007 9:46 AM
To: RE-wrenches at topica.com 
Cc: david sims
Subject: RE: system expansion [RE-wrenches]



Replace the main breaker in the main panel with a 150 amp breaker.
Then
you will be well within the 120% of the bus rating rule by any ones
interpretation of the code.

This raises an issue that has been on my mind for quite some time
though.

A while back I was speaking to an engineer at square d and he informed
me that the bus rating is not actually a bus rating it is a tab
rating.
The tab is the point that the breaker attaches to on the bus.
The bus is actually rated for far more than the tab.

This means that the NEC is being misinterpreted by nearly everyone in
the country. It is impossible for the individual tabs to be overloaded
unless the breaker on that tab has a higher rating than the tab.

If a panel has a 200amp rating then that means each tab is rated for
200
amps and the bus itself is actually rated for far more (10x?? or more)
than the actual tab rating. Additionally the breakers are thermally
activated, so if the bus or the tab actually begins to approach
capacity
(and heat up) the breakers will derate and trip off.

I look forward to more discussion on this topic. It is possible that I
am missing something here but I don't think so. If what I said above
is
correct then there is no issue (aside from NEC misinterpretation by
the
local inspector) with putting a 200amp solar back feed on a panel with
a
200amp tab rating and a 200amp main breaker feeding from the utility
company.

Sky Sims
Ecological Systems
www.ecologicalsystems.biz 
220 County Road 522
Manalapan, NJ 07726
ph)732-462-3858 fax)732-462-3962




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