-------- Original Message --------
Subject: Re: [RE-wrenches] Landing into a sub-panel without a main
service panel, just a main switch
Date: Fri, 23 Dec 2011 09:41:24 -0700
From: Ray Walters <r...@solarray.com>
To: JRQ <quackkc...@yahoo.com>
HI Jeffrey;
Electricity certainly can flow in both directions on a conductor (unless
they've installed huge diodes). If there is a short circuit in the
smallest conductor feeding one of the subpanels (I'm guessing #6) The
short circuit would be fed by up to 260 amps without tripping an OCPD
anywhere. In this particular case 705.12 actually makes more sense than
when just applied to a bussbar. Tapping the solar into the load side
makes a dangerous situation even worse.
You're right, it's not a problem if everything is operating normally,
but breakers don't do anything normally either. It's when something bad
happens (like a rodent chewing through some wiring) that the breakers
and their ratings suddenly become the difference between a power outage
or a structure fire.
Allan Sindelar taught me the importance of exactly wording your permit
to limit the scope of work and your potential liability.
In this case, I would replace the 200A main with a larger load center
(as I said before) and purposely word the permit to not take
responsibility for any distribution wiring beyond the new load center.
A line side tap would do the same thing, but is the coward's way out.
If there is an electrical problem later, they could still blame the new
10 KW PV system on the roof. Whether its making some electrical
improvements or reroofing, I always try to make things better when I add
a PV system.
Ray
On 12/22/2011 8:05 PM, JRQ wrote:
The flow of electricity isn't two-way traffic along a conductor. If
there are no loads on the conductor between the main system disconnect
and the main breaker of a subpanel, in this scenario, there can only
be up to 200 A coming from the utility OR up to 60 A coming from the
solar system backfed through the subpanel. , logically it follows that
the sum of the OCPDs supplying that conductor and the main disconnect
is 200 A /or/ 60 A, but not 260 A.
Furthermore, the alternate interpretation misunderstands the rule in
this context.
Jeffrey Quackenbush
------------------------------------------------------------------------
*From:* Ray Walters <r...@solarray.com>
*To:* JRQ <quackkc...@yahoo.com>; RE-wrenches
<re-wrenches@lists.re-wrenches.org>
*Sent:* Friday, December 23, 2011 4:45 AM
*Subject:* Re: [RE-wrenches] Landing into a sub-panel without a main
service panel, just a main switch
I'm not a huge fan of this 120% rule myself, but if you're going to
apply it properly, it applies to all of the conductors, buss bar,
cables, etc. between the main disco to all of the sub panels. This
rule is not limited just to a load center buss(read 705.12D2), it
applies to all the conductors, and clearly says the sum of breakers
feeding in (200 + 40) can't exceed 120% of the rating of any of the
conductors between the main and the subpanel main breakers.
This is an interesting situation, where apparently the original
electrical work took liberal advantage of the tap rules in 240.21.
705.12, however, doesn't have any exemptions that include the tap
rules, so actually it would be applied to the smallest conductor. If
any of the conductors between the 200 amp main and sub panels is less
than 200 amp rating, you're off to a bad start.
Here's how I would fix it: Charge extra to put in a new 250 amp rated
load center, with a 200 amp main breaker, feed all the subpanels with
breakers properly sized for the various conductors, and then leave
yourself a nice 60 amp breaker on the far end of the buss for your 10
KW PV system. It's not a service upgrade (you're still at 200 amps),
you've made the house much safer, and you've fixed your PV intertie
issues as well. Besides being PV installers, we ARE electricians, and
we should be fixing bogus wiring when it also benefits the PV
install. Then all of us could sleep better.
Ray
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