[RE-wrenches] NEC 705.12(B)(2)(3)(c) intention

[Dan Fink]

Esteemed Wrenches;

Because I have to distill this topic into simple working knowledge for my
college students being hired for the start of their PV career, I would
really appreciate any input on my talking points below, to make sure I am
not spreading BS to my students.

1) The NEC 2011 120% rule on load side interconnects prevents overloading a
main panel buss from PV providing the potential overcurrent loads not
regulated by the panel main breaker, but limits PV input amps thanks to a
rounding up error by calculating this rule based on main breakers upsized
to the next available OCPD size;

2) The NEC 2014 updates were intended to allow more PV amperage on a
load-side main panel interconnect via 3 different options by using inverter
FLA instead of main breaker size; so If you just follow NEC 2011 120% rule
you won't violate NEC 2014 except in your paperwork;

3) NEC 2014 705.12(B)(2)(3)(a) now allows the PV OCPDs to be located
anywhere on the panel, not just the bottom, also a good option for
center-fed main panels;

4) NEC 2014 705.12(B)(2)(3)(b) gives you a bit of boost to your main panel
rating, again to allow a bit more PV input, but still restricts PV OCPD
location to the bottom of the panel;

5) NEC 2014 705.12(B)(2)(3)(c) , the sum of all OCPDs, seems more suited
for microinverter subpanels that do not have any actual loads.

Have I made any errors in these basic talking points for students here? Any
input appreciated.

Dan Fink
Professor of Solar Energy Technology, Ecotech Institute
IREC Certified Instructor™ for:
~ PV Installation Professional
~ Small Wind Installer
Executive Director, Buckville Energy
NABCEP Registered Continuing Education Providers™
970.672.4342



On Fri, Apr 28, 2017 at 8:45 AM, Jason Fisher (STC) <stc.jason at gmail.com>
wrote:

> Ah, I better understand your question now August. You are basically
> pointing out that in any loadcenter, if the total sum of all loads can be
> limited then why would we limited the sum of all sources that feed that
> panelboard? I've certainly heard this questioned before and I understand
> where it is coming from. The challenge with this argument is that it really
> applies to all types of connections between multiple sources and loads,
> including commercial/industrial and residential and any source such as
> utility, solar, wind, rotating generators and even storage. In order to
> modify any NEC language for the situation you mention, all other types of
> interconnections would need to be considered. In general, there must be a
> single OCP to prevent conductors or busbars from overload. It is a rare
> case in the Code where a bus is allowed to be protected through the use of
> multiple smaller OCPs that sum together. Our PV combiner boxes can be one
> of those rare cases. The old split-bus service panel would be another.
> Within standard AC loadcenters it is a particularly problematic argument
> since additional breakers can be added or increased in size very easily. A
> simple warning label may not be adequate by many people to protect the bus
> from overload conditions. Because of that the Code has allowed us some room
> to size the sum of all sources higher than the bus rating (the 120% rule),
> and perhaps there is a good argument to allow even more room, but I think
> it would be hard to say that you could feed a 200A loadcenter with two 200A
> sources (which is how others will view your example) without creating some
> solid argument for how the bus is protected from overload. Perhaps with
> smarter panelboards in the future containing other protections such as
> thermal protection, load control, source control, etc. we would have the
> substantiation needed to pursue this type of design but until we have those
> products, standards, and new Code allowances (which probably will come as
> Microgrids develop) we are most likely stuck with the relatively
> straightforward requirements found in 705.12. Which I would say have
> substantially improved in NEC 2014 and 2017.
>
> If you are motivated I would suggest you take a stab at writing up some
> new language that you like and to think through how an argument would be
> made to support it. We have entered the NEC2020 cycle and public inputs are
> due in September so you do have a window to get your ideas into the
> discussions. If you are not aware of it SEIA is helping to coordinate an
> effort as part of the group we call "the PV Industry Forum" to develop
> broad consensus proposals for the next edition. If you want to join this
> effort send an email to NEC2020 at seia.org or let me know.
>
> Cheers,
>
> Jason Fisher
> NEC CMP-4 Principal Rep for SEIA
>
>
> On Apr 20, 2017 6:45 AM, "August Goers" <august at luminalt.com> wrote:
>
> Hi Jason and All –
>
>
>
> Thanks for the responses. I think it is fair to say that the 200 A of
> loads plus the 200 A of solar (albeit through a feedthrough lug) equals 400
> A as Jason mentions below.
>
>
>
> I guess my real question is whether limiting the loads to 200 A plus
> allowing up to 200 A of solar (using my 200 A meter/main example) is a safe
> setup even if it is not currently NEC compliant. Perhaps future editions of
> the NEC could be expanded to allow this? Or is there a reason that NEC
> 705.12(B)(2)(3)(c) is written the way it is and we must include the PV too?
>
>
>
> Cheers,
>
>
>
> August
>
>
>
> *From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On
> Behalf Of *Jason Fisher (STC)
> *Sent:* Thursday, April 20, 2017 6:10 AM
>
> *To:* RE-wrenches <re-wrenches at lists.re-wrenches.org>
> *Subject:* Re: [RE-wrenches] NEC 705.12(B)(2)(3)(c) intention
>
>
>
> Hi August.
>
>
>
> If the main panel had a 400A rated bus and the sub was 200A then I think
> you could make a good argument but it appears the main is a 200A bus so
> therefore the sum of all load and supply breakers on that bus is 400A
> (loads plus solar excluding the main breaker), which does not comply with
> the Code language nor intent. The feed through lugs do not create a single
> bus, you now just have two buses and a feeder conductor to evaluate. Once
> the solar is connected you there are two sources feeding the main panel
> bus, not just the utility. You can exclude one but not both when evaluating
> the main panel bus.
>
>
>
> Hope I correctly assessed your situation.
>
>
>
> Jason Fisher
>
>
>
>
>
> On Apr 19, 2017 6:01 PM, "Mark Frye" <markf at berkeleysolar.com> wrote:
>
> True, but, the intent of the Code is not to always allow the
> interconnection of the maximum amount of PV power possible, so long as an
> over-current event cannot occur.
>
> Here we see Code that prevents over-current events in all cases. I guess
> you could say that that is it's intent.
>
> Sadly, the case you show would not be one of those allowed.
>
> Happily, many that would not have been allowed in the past are now allowed.
>
> This is called progress.
>
>
>
> On 4/19/2017 4:31 PM, August Goers wrote:
>
> Right, but if we limit the load breakers to the bus rating per NEC
> 705.12(B)(2)(3)(c), then there will never be an overcurrent event.
>
>
>
> August
>
> *From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On
> Behalf Of *Mark Frye
> *Sent:* Wednesday, April 19, 2017 4:18 PM
> *To:* RE-wrenches <re-wrenches at lists.re-wrenches.org>
> *Subject:* Re: [RE-wrenches] NEC 705.12(B)(2)(3)(c) intention
>
>
>
> If your sketch shows that there are 200A of breakers in the sub panel and
> 200A of breakers in the main, then, no, that would not be the intent.  The
> lug kit makes the two separate bars into a single bar.
>
> Mark
>
> On 4/19/2017 3:29 PM, August Goers wrote:
>
> Hi Wrenches –
>
>
>
> Now that we’re in the 2014 NEC, we’ve increasingly been using the NEC
> 705.12(B)(2)(3)(c) interconnection method:
>
>
>
> [image:
> imap://markf@mail.lmi.net:143/fetch%3EUID%3E.Re-Wrenches%3E12200?header=quotebody&part=1.1.2&filename=image001.png]
>
>
>
> This is a great way of putting lots of PV on a panel when there are few
> loads. As long as the total breaker rating (excluding the supply breaker)
> equals the bus rating or less, we’re good to go. However, we recently ran
> into a panel that had a lug subfeed kit installed in it.
>
>
>
> Do you think the intention of the code is to allow something like shown in
> this sketch?
>
>
>
>
>
> Cheers,
>
>
>
> August
>
> Luminalt
>
>
>
>
>
>
>
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