[RE-wrenches] Fwd: Re: Landing into a sub-panel without a main service panel, just a main switch

[Ray Walters]

-------- 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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