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

Thu Apr 20 06:45:35 PDT 2017

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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