[RE-wrenches] Landing into a sub-panel without a main service panel, just a main switch
JRQ
quackkcauq at yahoo.com
Thu Dec 22 19:05:18 PST 2011
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. The intent of limiting the solar system size plus the utility source to 120%, or even 100%, of a bus or conductor rating is when that bus or conductor can potentially see current from both sources simultaneously, it could overheat if these sources exceed the bus or conductor rating. That's not physically possible unless there is a load in between both sources.
Going back to NEC 2008 690.64B2 (I don't have my 2011 codebook on hand), it states that this rule applies to "The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor...". If you think of this in terms of time, the language is fuzzy, but I think an argument can be made in support of my point. In our scenario, the two sources of power cannot supply power simultaneously; because power is an instantaneous concept (the article doesn't say "supply energy"), 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
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] 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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