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

Thu Dec 22 15:15:38 PST 2011

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

On 12/22/2011 2:48 PM, JRQ wrote:
> But wait, there are no loads at the 200 A disconnect.
Yes there are. The 5 subpanels are all fed from there. That's plenty of 
loads.

> The idea behind the limitation in the "20% rule" is that if you have 
> power coming from a main to a set of loads on a bus and also power 
> from a backfed PV breaker to the same bus, that bus could 
> theoretically draw the sum of the main and the PV breaker. Which could 
> overheat the bus if the amount is more than the bus rating. If there 
> are no loads, the circuit will only draw power downline, and there is 
> no way the disconnect or the conductors to and fro could overheat. So 
> as long as there are main breakers on each subpanel (i.e. they are not 
> "pass-through" subpanels with only lugs feeding their bus), there will 
> be no loads between the backfed PV breaker and that main disconnect. 
> In that scenario, if you're backfeeding power across the load circuit 
> coming directly from the 200 A disconnect, that means the PV system is 
> meeting all the house's loads, and therefore no power is being drawn 
> from the utility side (much less 200 A worth).
>
> So more information is needed about the subpanels wiring in relation 
> to the main disconnect. How do you have 5 subpanels connected to one 
> disconnect without some kind of distribution bus? If they are strung 
> in some series/parallel manner, what is the configuration? What is the 
> size of the main breaker on each subpanel? Which subpanel do you want 
> to interconnect to?
>
> Honestly, if the subpanels are newer, I'd stay as far away from the 
> Zinsco switch as you can.
>
> Jeffrey
>
> ------------------------------------------------------------------------
> *From:* Jason Szumlanski <Jason at fafcosolar.com>
> *To:* RE-wrenches <re-wrenches at lists.re-wrenches.org>
> *Sent:* Friday, December 23, 2011 2:44 AM
> *Subject:* Re: [RE-wrenches] Landing into a sub-panel without a main 
> service panel, just a main switch
>
> Yep, you need to be on the service side of the 200A main switch. Without
> seeing it, it makes sense that you could use a fused disconnect and
> insulation piercing taps like Ray suggests on the supply side of the
> main disconnect. Keep your conductors from the fused disconnect to the
> taps as short as possible. Ask you AHJ and utility for the desired
> location of the new fused disconnect.
>
> To answer question #3, that depends on your utility company to a large
> extent. For example, FPL in Florida limits the gross power rating of an
> interconnected system to 90% of the service rating. The gross power
> rating is defined as 85% of the DC rating (STC, presumably). Therefore,
> the most you can put on a 200A service would be 200A x .9 x 240V / .85 =
> 50,823W DC-STC. That's a pretty large residential system, so I doubt you
> will run up against a restriction.
>
> While on that subject, FPL considers the gross power rating to determine
> the Tier level. That is important, because it determines the
> interconnection fee. There is no fee for under 10kW gross power, but
> $400 for 10kW to 100kW. You can install up to 11,764W before reaching
> Tier 2. This 0.85 factor doesn't make too much sense to me... you could
> install 51 x 230W modules for a total of 11,730W with 51 Enphase M215
> microinverters and still be Tier 1 even though the AC rating of the
> system would be 10,965W because the gross power rating as defined by FPL
> would be 9,971W.
>
> Jason Szumlanski
> Fafco Solar
>
>
>
>
> -----Original Message-----
> From: re-wrenches-bounces at lists.re-wrenches.org 
> <mailto:re-wrenches-bounces at lists.re-wrenches.org>
> [mailto:re-wrenches-bounces at lists.re-wrenches.org 
> <mailto:re-wrenches-bounces at lists.re-wrenches.org>] On Behalf Of Ray
> Walters
> Sent: Thursday, December 22, 2011 3:08 PM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] Landing into a sub-panel without a main
> service panel, just a main switch
>
> I'd consider a fused disconnect on the line side of the 200 amp main.
> The wires on the load side are treated just as a bussbar. 690.64 (which
> is now 705.12) reads conductor or bussbar.
> You could use a 100% duty rated 40 amp fused disconnect, and get up to
> 9600 watts through on the load side, but a regular disconnect is going
> to be limited by 690.8 to 80% of 40 amps, (7.7 KW inverter rated output
> x 125% = 40 amps)
> Since the subpanels have 125 A bussbars, but only 60 amp mains, you
> could add there without further penalty, but the bottom line is: you're
> not going to get 10KW on the load side and be code compliant.
>
> Ray Walters
>
> On 12/22/2011 11:34 AM, Pekka Laine wrote:
> > All,
> >
> > We have been contracted to install a 10kW-ac system into a 45-year old
> house that has no main service breaker panel, only a 200A fused main
> switch (Zinsco). There are five 125 A rated sub-panels throughout the
> house, some of which have 50A&  60A breakers in them.  I have been told
> that we are not limited to 40 A max. PV as per 690.64 (B) since there is
> no main bussbar. I have three questions:
> > 1. Can we land 60A into one of these existing sub-panels
> > 2. If not, how would you recommend "tapping" in the old 200 A fused
> main switch wires?
> > 3. What do you think is the maximum amperage of PV that can be backfed
> in to this service?
> >
> > Any other recommendations welcome.
> >
> > Respectfully,
> >
> > Pekka Laine
> >
> > President
> > Photon Solar Power Inc.
> > www.PhotonSolarPower.com <http://www.PhotonSolarPower.com>
> > Cell. 760-556-8170
> > Toll Free 888-SUN-ACDC
> > CA Lic. #933648
> > NABCEP Certified PV Installer # 032611-174
> >
> > _______________________________________________
> > List sponsored by Home Power magazine
> >
> > List Address: RE-wrenches at lists.re-wrenches.org 
> <mailto:RE-wrenches at lists.re-wrenches.org>
> >
> > Options&  settings:
> > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
> >
> > List-Archive:
> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
> >
> > List rules&  etiquette:
> > www.re-wrenches.org/etiquette.htm 
> <http://www.re-wrenches.org/etiquette.htm>
> >
> > Check out participant bios:
> > www.members.re-wrenches.org <http://www.members.re-wrenches.org>
> >
> >
> >
> >
>
> _______________________________________________
> List sponsored by Home Power magazine
>
> List Address: RE-wrenches at lists.re-wrenches.org 
> <mailto:RE-wrenches at lists.re-wrenches.org>
>
> Options & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>
> List-Archive:
> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
>
> Check out participant bios:
> www.members.re-wrenches.org
>
> _______________________________________________
> List sponsored by Home Power magazine
>
> List Address: RE-wrenches at lists.re-wrenches.org 
> <mailto:RE-wrenches at lists.re-wrenches.org>
>
> Options & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>
> List-Archive: 
> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
>
> Check out participant bios:
> www.members.re-wrenches.org
>
>
>
>
>
> _______________________________________________
> List sponsored by Home Power magazine
>
> List Address: RE-wrenches at lists.re-wrenches.org
>
> Options&  settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>
> List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>
> List rules&  etiquette:
> www.re-wrenches.org/etiquette.htm
>
> Check out participant bios:
> www.members.re-wrenches.org
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20111222/7b9abbdf/attachment-0003.html>