With that logic, why bother with any smaller breakers in a load center
at all? We could just run the whole house on the 200 amp main OCPD.
Sure make installations easier. Actually though, there is over a
hundred years of evidence to show that over sizing breakers and fuses
leads to fires. 1000's of people have died. It's why the National Fire
Protection Association started printing the NEC, and why they keep
updating it. (we're on the 52nd edition)
You're right, the difference between 200A and 240A is relatively
negligible, they're both unsafe. One could feed a short with 48 KW and
the other with 57.6 KW. In this case, I'm advocating for that alleged
#6 to be on its own 60 amp breaker, and fix the mess, not add 40 amps to
it. Ultimately though, it's not so much what we think we could get away
with, but proper application of the current code as licensed electrical
professionals.
On my electric vehicles, I do all sorts of things that aren't code
compliant (vehicles are covered by the SAE), but no one's house is going
to burn down either. If you have points that you can back with code
references, that's what this list is all about. We're always open to
new views.
Ray
If there's evidence that shorts or ground faults commonly develop with
a resistance this high, causing sustained arcing, and that the
marginal difference in resistance in our example, and in most system
scenarios, makes a statistical difference, I'm willing to concede the
point.
Jeffrey Quackenbush
------------------------------------------------------------------------
*From:* Ray Walters <r...@solarray.com>
*To:* RE-wrenches <re-wrenches@lists.re-wrenches.org>
*Sent:* Friday, December 23, 2011 10:29 PM
*Subject:* [RE-wrenches] Fwd: Re: Landing into a sub-panel without a
main service panel, just a main switch
-------- 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> <mailto:r...@solarray.com>
To: JRQ <quackkc...@yahoo.com> <mailto: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> <mailto:r...@solarray.com>
*To:* JRQ <quackkc...@yahoo.com> <mailto:quackkc...@yahoo.com>;
RE-wrenches <re-wrenches@lists.re-wrenches.org>
<mailto: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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