[RE-wrenches] conductors and the 120% rule

Wed Mar 5 08:04:34 PST 2014

I thought one of the the rationale for the upsized conductor between
panelboards in series was to account for the possibility that the conductor
could be later tapped with other loads or backfed from another PV source in
theory. I never thought that rationale was reasonable - any future changes
of this significance would need to be addressed at that time anyway.

I have found that AHJs are lenient and reasonable on this point if
addressed in advance. It should only be an issue in NEC 2008 and 2011 if
using a load side connection. If your jurisdiction allows it, a supply side
connection should bypass this issue altogether. When using a subpanel to
combine multiple inverter outputs, it's often the case that a load side
connection is not possible in retrofit situations, at least in my
experience. Even on a 1600A commercial main bus where the 120% rule is a
non-factor, there are often prohibitive issues that make a supply-side
connection much more cost effective. For example, I've run into manual
generator switches or situations where it is impractical to move breakers
to accommodate the proper backfed breaker location.

By the way, we've had this discussion before. Search the archives for:
"sizing a sub-panel used to combinemultipleinverter outputs"

(there are several prior discussions on the subject)

Jason Szumlanski

Fafco Solar

On Wed, Mar 5, 2014 at 10:30 AM, Dave Click <daveclick at fsec.ucf.edu> wrote:

>  I think there's plenty of room to interpret the 2008 and 2011 NEC such
> that you don't need to upsize this conductor either. 2008 690.64(B)(2) and
> 2011 705.12(D)(2) are both titled "bus or conductor," and as Brian points
> out below the currents aren't additive. As I understand it the 120% rule
> was because panelboard manufacturers weren't comfortable with exceeding
> 120% due to heat concerns under continuous operation (assuming a fully
> loaded panelboard). As for conductors, I wonder if that 120% was just to
> keep it consistent with the panelboard requirement, even though it would be
> safe at levels >120% as long as you're feeding from opposite ends.
>
> Also, since you seem to be talking about a good amount of PV with your 2
> SolarEdge 20s, make sure your upstream breakers are OK to be backfed (up
> through and including the main, unless you've got a large, guaranteed
> baseload).
>
> DKC
>
> On 2014/3/5, 10:01, Brian Mehalic wrote:
>
> A very clarifying change is what I'd call it!
>
>  First off, rather than being based on the actual breaker size on the
> inverter output circuit, calcs are instead based on 125% of the inverter
> rated output current.
>
>  705.12(D)(2)(1) addresses "Feeders" - but only applies when the inverter
> output circuit connection is made somewhere other than the opposite end of
> the feeder from the utility supply.  This addresses concerns about whether
> the feeder conductor needs to be larger due to the presence of the
> additional source of supply, and so long as the inverter isn't connected to
> the feeder in the middle of it then the existing conductor size should be
> okay (because if it is at the opposite end of the feeder than there is
> nowhere where the utility and inverter current will be additive).
>
>  705.12(D)(2)(3) addresses "Busbars" and allows several options,
> including the familiar "120% rule" as you stated in your original post.
>  Also check out 705.12(D)(2)(3)(c) - depending on the load breakers in the
> subpanel, the 120% rule may not even need to be used (if the sum of the
> inverter and load breakers is less than or equal to the busbar rating).
>
>  And remember, even if your AHJ hasn't adopted 2014 yet it is worth
> having a conversation with them to see if they'll allow you to design the
> system based on the new Code - after all, in a certain sense, the 2014 NEC
> is what "they" meant the 2011 NEC to say!
>
>  Cheers,
>
>
>  Brian Mehalic
> NABCEP Certified Solar PV Installation Professional(tm) R031508-59
> IREC ISPQ Certified Affiliated Instructor/PV US-0132
>
>  PV Curriculum Developer and Instructor
> Solar Energy International
> http://www.solarenergy.org
>
>
> On Tue, Mar 4, 2014 at 5:21 PM, Kirk <kirk at vtsolar.com> wrote:
>
>>  Is this a code change in 2014 vs 2011 or merely a clarification? Vt has
>> not adopted 2014 yet. What was the original rationale for the 120% rule to
>> apply to conductors in addition to a panel bus?
>>
>> Kirk Herander
>> VSE
>>
>> On Mar 4, 2014, at 6:20 PM, Brian Mehalic <brian at solarenergy.org> wrote:
>>
>>   If the subpanel is at the end if the feeder, and there are no taps in
>> between the main and the sub then I don't see any reason that the
>> conductors need to be any larger than 200 A as there is no where on the
>> feeder conductors where grid and PV current will be additive.
>>
>>  The changes in 705.12 in 2014 address this in large part.
>>
>> Brian
>>
>> On Mar 4, 2014, at 2:41 PM, "Kirk Herander" <kirk at vtsolar.com> wrote:
>>
>>   Solaredge 20 kw, 480 3-phase. Good point, but that may be irrelevant.
>> The feed-in subpanel is also powering unrelated loads, which use the
>> neutral as a conductor from the main panel. So 4 conductors from the main.
>>
>>
>>
>> Kirk Herander
>>
>> VT Solar, LLC
>>
>> dba Vermont Solar Engineering
>>
>> NABCEPTM Certified Inaugural Certificant
>>
>> NYSERDA-eligible Installer
>>
>> VT RE Incentive Program Partner
>>
>> 802.863.1202
>>
>>
>>
>> *From:* re-wrenches-bounces at lists.re-wrenches.org [
>> mailto:re-wrenches-bounces at lists.re-wrenches.org<re-wrenches-bounces at lists.re-wrenches.org>]
>> *On Behalf Of *Allen Frishman
>> *Sent:* Tuesday, March 04, 2014 4:32 PM
>> *To:* RE-wrenches
>> *Subject:* Re: [RE-wrenches] conductors and the 120% rule
>>
>>
>>
>> what inverter(s) are you using?    In many cases the Neutral is not
>> considered a Current Carying Conductor by the Manufacturer and therefore
>> you only have 3 CCC.
>>
>> *Al Frishman*
>> AeonSolar
>>
>> *(917) 699-6641 <%28917%29%20699-6641> - cell*
>> *(888) 460-2867 <%28888%29%20460-2867>*
>> *www.aeonsolar.com <http://www.aeonsolar.com/>*
>>
>>
>>
>> On Mar 4, 2014, at 4:20 PM, Kirk Herander wrote:
>>
>>
>>
>>   Approx.. 50 - 60ft.
>>
>>
>>
>> Kirk Herander
>>
>> VT Solar, LLC
>>
>> dba Vermont Solar Engineering
>>
>> NABCEPTM Certified Inaugural Certificant
>>
>> NYSERDA-eligible Installer
>>
>> VT RE Incentive Program Partner
>>
>> 802.863.1202
>>
>>
>>
>> *From:* re-wrenches-bounces at lists.re-wrenches.org
>> [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Ray
>> Walters
>> *Sent:* Tuesday, March 04, 2014 4:05 PM
>> *To:* RE-wrenches
>> *Subject:* Re: [RE-wrenches] conductors and the 120% rule
>>
>>
>>
>> What is the length of the conduit to the subpanel? That will determine
>> whether to apply the derates.
>>
>>
>>  R.Ray Walters
>>
>> CTO, Solarray, Inc
>>
>> Nabcep Certified PV Installer,
>>
>> Licensed Master Electrician
>>
>> Solar Design Engineer
>>
>> 303 505-8760
>>
>>  On 3/4/2014 1:34 PM, Kirk Herander wrote:
>>
>>  Whether or not a further derate has to be applied is the killer here,
>> as I am working with existing panels and conductors. In an old Code
>> Corner(HP140) J. Wiles goes through a similar scenario and calls out the
>> allowable current rating and conductor in 310.15, but makes no mention of
>> applying additional derate factors. The .8 derate for 4-6
>> conductors(l1,l2,l3, & n) will put the existing 4/0 cable between feed-in
>> and main panel at 208 amps, less than the allowable 217. I'd hate to need
>> to upsize the wire to 250 mcm.
>>
>>
>>
>> Kirk Herander
>>
>> VT Solar, LLC
>>
>> dba Vermont Solar Engineering
>>
>> NABCEPTM Certified Inaugural Certificant
>>
>> NYSERDA-eligible Installer
>>
>> VT RE Incentive Program Partner
>>
>> 802.863.1202
>>
>>
>>
>> *From:* re-wrenches-bounces at lists.re-wrenches.org [
>> mailto:re-wrenches-bounces at lists.re-wrenches.org<re-wrenches-bounces at lists.re-wrenches.org>
>> ] *On Behalf Of *Jason Szumlanski
>> *Sent:* Tuesday, March 04, 2014 2:57 PM
>> *To:* RE-wrenches
>> *Subject:* Re: [RE-wrenches] conductors and the 120% rule
>>
>>
>>
>> Both the bus and conductors need to be rated for 217 amps minimum. As you
>> mentioned, the bus is not a problem. The way I interpret it, the conductor
>> size required would be after derate factors are applied. The rating of the
>> conductor is ultimately dependent on the derate factors.
>>
>>
>>
>> If you can locate your subpanel adjacent to the main distribution panel,
>> you may be able to use Exception #3 to 310.15(B)(2) by connecting the
>> panels with a short nipple. I assume you are just looking at a number of
>> conductor derate and not an ambient temperature derate.
>>
>>
>>
>> Jason Szumlanski
>>
>> Fafco Solar
>>
>>
>>
>>
>>
>>
>> On Tue, Mar 4, 2014 at 12:05 PM, Kirk Herander <kirk at vtsolar.com> wrote:
>>
>> Hello,
>>
>> I have a 225 amp 3-phase main lug sub-panel protected by a 200 amp
>> breaker. My inverter breaker feeding the sub panel is 60 amps. So 225 a bus
>> x 1.2 = 270 amps. That's less than the sum of the two breakers of 260 amps,
>> so no issue there. The conductors between sub and main panel have to be
>> rated for at least 260/1.2 = 217 amps, correct? Is this 217 amps before or
>> after derating the conductor?
>>
>>
>>
>>
>>
>> Kirk Herander
>>
>> VT Solar, LLC
>>
>> dba Vermont Solar Engineering
>>
>> NABCEPTM Certified Inaugural Certificant
>>
>> NYSERDA-eligible Installer
>>
>> VT RE Incentive Program Partner
>>
>> 802.863.1202
>>
>>
>>
>>
>>
>>
>>
>
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