[RE-wrenches] conductors and the 120% rule

[Ray Walters]

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
>
> NABCEP^TM 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.
>
> Jason Szumlanski
>
> Fafco Solar
>
>
> Description: Image removed by sender.
>
> 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^TM Certified Inaugural Certificant
>
> NYSERDA-eligible Installer
>
> VT RE Incentive Program Partner
>
> 802.863.1202 <tel:802.863.1202>
>
>
>
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