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

Thu Dec 22 13:48:04 PST 2011

But wait, there are no loads at the 200 A disconnect. 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] 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
> Cell. 760-556-8170
> Toll Free 888-SUN-ACDC
> CA Lic. #933648
> NABCEP Certified PV Installer # 032611-174
>
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