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-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@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 <k...@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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