[RE-wrenches] conductors and the 120% rule
Dave Click
daveclick at fsec.ucf.edu
Wed Mar 5 10:40:30 PST 2014
Jason, this is the same rationale I have heard before-- and sadly
parroted myself before seeing the light-- but I agree that it's
inaccurate. 90.8(A) is the only "futureproofing" thing in the NEC that I
know of, but I take it as an "FYI, if you oversize now it allows for
easier expansion later" rather than a requirement. After all, no one
runs #4 for a 20A circuit within a 4" conduit on the assumption that
someone later may want to add 38 more conductors into that same raceway.
And if you trip a main breaker with the Tesla fast charger you just
installed in the garage, it's hard to blame the original electrician for
lack of foresight...
DKC
On 2014/3/5, 11:04, Jason Szumlanski wrote:
> 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
> <mailto: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
>> <mailto: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 <mailto: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 <mailto: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
>>>>
>>>> NABCEP^TM Certified Inaugural Certificant
>>>>
>>>> NYSERDA-eligible Installer
>>>>
>>>> VT RE Incentive Program Partner
>>>>
>>>> 802.863.1202 <tel:802.863.1202>
>>>>
>>>> *From:*re-wrenches-bounces at lists.re-wrenches.org
>>>> <mailto:re-wrenches-bounces at lists.re-wrenches.org>
>>>> [mailto: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 <tel:%28917%29%20699-6641> - cell/
>>>> /(888) 460-2867 <tel:%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
>>>>
>>>> NABCEP^TMCertified Inaugural Certificant
>>>>
>>>> NYSERDA-eligible Installer
>>>>
>>>> VT RE Incentive Program Partner
>>>>
>>>> 802.863.1202 <tel:802.863.1202>
>>>>
>>>> *From:*re-wrenches-bounces at lists.re-wrenches.org
>>>> <mailto:re-wrenches-bounces at lists.re-wrenches.org>[mailto:re-wrenches-bounces at lists.re-wrenches.org]
>>>> <mailto:[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 <tel:303%20505-8760>
>>>>
>>>> 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
>>>>
>>>> NABCEP^TMCertified Inaugural Certificant
>>>>
>>>> NYSERDA-eligible Installer
>>>>
>>>> VT RE Incentive Program Partner
>>>>
>>>> 802.863.1202 <tel:802.863.1202>
>>>>
>>>> *From:*re-wrenches-bounces at lists.re-wrenches.org
>>>> <mailto:re-wrenches-bounces at lists.re-wrenches.org>[mailto: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 <mailto: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
>>>>
>>>> NABCEP^TMCertified Inaugural Certificant
>>>>
>>>> NYSERDA-eligible Installer
>>>>
>>>> VT RE Incentive Program Partner
>>>>
>>>> 802.863.1202 <tel:802.863.1202>
>>>>
>>>>
>
>
>
>
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