[RE-wrenches] Fwd: Re: Landing into a sub-panel without a main service panel, just a main switch
JRQ
quackkcauq at yahoo.com
Sat Dec 24 09:57:00 PST 2011
I'm not sure what "#6" refers. I didn't see any additional documentation from the original poster beyond the text of his query.
Running a house from a single 200 A main OCPD has nothing to do with what I'm talking about. On the contrary, my point is that if you have a conductor with OCPDs on either side and no load in the middle, electricity can flow in only one direction at one time. Under normal operation it can't be fed by two sources from different directions simultaneously, and therefore the current on that conductor will never exceed one or the other OCPD.
Now, it is true that a short is basically a load. However, my argument is that as a load, a short will only overload the conductor by the sum of the OCPDs under a very narrow range of resistances, which themselves are significantly higher than what should be expected from a short-circuit or a ground fault. So I'd say there is a burden of proof on empirical tests to determine how often those conditions would develop.
Where you have a main disconnect that feeds a load center that also has a main breaker (not an unheard of configuration), you get exactly this situation: a conductor with OCPDs on both sides, no load on the conductor and no parallel circuits. Neither the conductor nor the main disconnect will maintain a current in excess of the main OCPD size under normal operation.
I specifically said that this does not apply if there is any kind of a functional load on the conductor in an earlier email. Maybe I didn't spell this out explicitly enough, but if there are parallel circuits coming off this conductor, say, for instance, a tap, this also would not apply.
Jeffrey Quackenbush
________________________________
From: Ray Walters <ray at solarray.com>
To: JRQ <quackkcauq at yahoo.com>; RE-wrenches <re-wrenches at lists.re-wrenches.org>
Sent: Saturday, December 24, 2011 11:29 AM
Subject: Re: [RE-wrenches] Fwd: Re: Landing into a sub-panel without a main service panel, just a main switch
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 <ray at solarray.com>
>To: RE-wrenches <re-wrenches at 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 <ray at solarray.com>
>To: JRQ <quackkcauq at 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 <ray at solarray.com>
>>To: JRQ <quackkcauq at yahoo.com>; RE-wrenches <re-wrenches at 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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