FW: system expansion [RE-wrenches]
Sky Sims
sky at ecologicalsystems.biz
Wed Aug 22 10:59:11 PDT 2007
"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."
The lack of testing on the plastic supports does not even begin to make
sense to me. I find the concept ridiculous. Every material in a panel
has to have been tested (to failure) and rated at some point in the
manufacturing and component selection process. The actual temperature at
which the plastic supports melt or lose their integrity must be
documented by the manufacturer, otherwise how would they get it listed
for use in a temperature sensitive environment.
Regardless this concern remains a non issue because of the thermal
nature of the breakers which will result in breakers tripping off before
heat build up becomes an actual hazard to the integrity of the bus or
the plastic standoffs. (please correct me with manufacturers data if I
am mistaken)
"The total thermal loading of the panel is potentially 480 Amps worth of
breakers (200 main, 240 load, 40 PV)."
As for this point, so what? Is someone trying to imply that the
resistive loads of breakers feeding loads is so high that it will result
in the panel reaching a dangerous temperature before the thermal nature
of the breakers causes them to trip? In this scenario the bus still only
sees 240 amps. And as long as the solar fed breaker is at the opposite
end of the bus from the main breaker (something I've always encouraged)
no point on the bus will ever see more than 200amps, even if the solar
fed breaker was a 200amp breaker instead of the 40amp in this example.
These points that are being raised as reasons to limit the allowable
size of a solar fed breaker to anything less than the tab rating of the
panel are ridiculous.
In any scenario that I can think of the solar breaker either offsets the
power being fed from the utility fed breaker or breakers start tripping
and it becomes very apparent very quickly that the panel needs to be
upgraded. (let me know if I'm missing something)
I am very concerned about standards raisers adding costs to systems for
ridiculous reasons. This industry really needs to focus more on how to
reduce costs so that everyone can obtain a system.
If we applied the same philosophy to the wiring in our homes as we do to
solar we would all be living in padded cells. Our TV's would be hard
wired to an inaccessible power supply and the wires would be encased in
conduit so that children couldn't bite through the wires when we aren't
looking or stick their fingers in the outlets. Every time we decided to
move the TV or replace it a skilled professional would have to come out
and do it for us. And the only people with electricity would be the
wealthiest 5 percent of society because the cost would be so high that
no one else could afford it.
There must be a happy middle ground between safety and affordability.
People use gas in their homes in spite of the "reasonable" risks
associated with
carbon monoxide poisoning and the fact that nearly 100,000 people in
this country are negatively affected by its use annually many actually
killed. Solar isn't at all like that.
Plug and Play should be our shining goal. Allowing inverters to be
simply plugged into an exterior or interior outlet is something that we
should be pushing for (European countries already allow this). Getting
the tab rating versus bus rating issue addressed is worth pushing for
now.
At this point it may be time to get some materials engineers from some
of the bigger panel manufacturers like square d, siemens, cutler hammer
etc.. to weigh in on this issue.
My 2 cents,
Sky Sims
Ecological Systems
www.ecologicalsystems.biz
ph)732-462-3858 fax)732-462-3962
-----Original Message-----
From: John Berdner [mailto:jberdner at sma-america.com]
Sent: Tuesday, August 21, 2007 1:30 PM
To: RE-wrenches at topica.com
Subject: RE: system expansion [RE-wrenches]
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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