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

JRQ quackkcauq at yahoo.com
Fri Dec 23 19:00:36 PST 2011 

I meant it doesn't flow in two directions simultaneously and in the same location. Electricity flows from a negative point to a positive point by means of a load (which allows its energy to change forms and thus be conserved). Obviously with an AC circuit, the electricity is constantly switching directions, but not in coeval overlap.

Granted, a short circuit is a load. However, if there is a short on the load conductor leading from the main disconnect, it's going to draw a lot more than 200 A from the utility, and the main OCPD will trip. For a 240V source, the resistance of the short could be no less than 0.9Ω before it starts drawing more than 260 A. Also note that if we follow the 20% rule, 240 A would be an "acceptable" overload; a 240 V source will deliver 240 A at 1Ω -- with a difference of 0.1Ω from our scenario. 

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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