[RE-wrenches] conductors and the 120% rule

[William Korthof]

Even in a feeder with line-neutral loads, the neutral still only carries the imbalance between the phase loads: the case of maximum possible neutral current is the hypothetical situation of the phase with largest line-neutral load current, with all those loads on, and both other phases having all line-neutral loads off---in order to get neutral current, you have to reduce the phase current in two legs--- 
So normally in 4-wire star feeders, you only need to rate for 3 current carrying conductors. 

If the neutral conductor connects to a solar inverter, the neutral load is either zero (3 phase inverter) or non-additive if you have a bank of 1 phase line/neutral (120 or 277 volt) inverters 

/wk

William Korthof
714.875.3576
Sustainable Solutions
#956904

On Mar 4, 2014, at 1:31 PM, Allen Frishman <amfrish at aeonsolar.com> wrote:

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 - cell
(888) 460-2867
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] 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.
>  

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