Blake:
Thank you for the reference from Schneider. I think the point being missed here is that the ampacity rating of each device needs to be evaluated in light of the power flow in either direction. If a 40 Amp breaker is supplied between the utility and the disconnect in question, in case of a fault, it could supply more current from the grid than a 30 Amp disconnect is rated to handle. This is particularly relevant considering the grid is not nearly as much a current limited source as an inverter. Sincerely, William Miller Miller Solar 17395 Oak Road, Atascadero, CA 93422 805-438-5600 www.millersolar.com CA Lic. 773985 Quote of the month: “As they age, batteries transition from energy storage devices to energy consuming devices.“ W. Miller *From:* Blake Gleason [mailto:bl...@sunlightandpower.com] *Sent:* Thursday, August 22, 2019 12:38 PM *To:* will...@millersolar.com; RE-wrenches *Subject:* Re: [RE-wrenches] 30 amp disconnect/40 amp breaker Hi William, Yes, the bulletin I attached was just an example of a 100% rated disconnect (which happened to be DC since that's the one I had handy!). My understanding is that most of the NON-fused Square D switches are rated for 100%. See this FAQ from the manufacturer: https://www.schneider-electric.us/en/faqs/FA237949/, quoted below: *Resolution:* All General Duty, Heavy Duty or Double Throw Safety Switches maximum current rating, either 80% or 100% of the nameplate rating, depends on if the switch is Fusible or NON-Fusible. o Fusible switches are rated to carry up to 80% of the amp rating of the fuses installed in the switch o Non-fusible (unfused) switches are rated to carry 100% of the switch nameplate ampere rating Regarding this point: If, hypothetically, you could find a 30A disconnect that was continuous duty rated, if it is fed by a 40 Amp breaker, as I believe would be required, then the 30 Amp disconnect is still not suitable. You can not feed a 30 Amp rated device with more than 30 Amps. I believe the original poster was asking about a 6000W inverter with 25A max output. So in that case you could use a 100% rated non-fusible disconnect, because the max current is only 25A. (The required 40A breaker is sized larger because commonly available breakers are only 80% rated, not 100%. But the max current is still only 25A.) See 690.8(A) for calculation of maximum circuit current, specifically section (3) for inverters: "The maximum current shall be the inverter continuous output current rating." (Not "the inverter continuous output current multiplied by 1.25.") Best, Blake On Wed, Aug 21, 2019 at 10:59 PM William Miller <will...@millersolar.com> wrote: Blake: Thanks for sharing the data sheet. The document applies to DC PV systems, however, and I believe the discussion pertains to the AC inverter output. Therefore the bulletin you attached does not apply here. (Note also that this data sheet indicates you need to use two poles for any application, ungrounded or grounded. I believe that requirement was usurped later on for Square D Heavy Duty switches, but again, that is for a different application.) I looked again at Square D general and heavy duty safety switches rated at 30 Amps and I could not find any reference that these switches are continuous duty rated. I still believe that for the application described, the 125% rating has to be applied. If, hypothetically, you could find a 30A disconnect that was continuous duty rated, if it is fed by a 40 Amp breaker, as I believe would be required, then the 30 Amp disconnect is still not suitable. You can not feed a 30 Amp rated device with more than 30 Amps. Bottom line, a 30 Amp disconnect will not meet code on the described project. William Miller Miller Solar 17395 Oak Road, Atascadero, CA 93422 805-438-5600 www.millersolar.com CA Lic. 773985 Quote of the month: “As they age, batteries transition from energy storage devices to energy consuming devices.“ W. Miller *From:* RE-wrenches [mailto:re-wrenches-boun...@lists.re-wrenches.org] *On Behalf Of *Blake Gleason *Sent:* Wednesday, August 21, 2019 4:20 PM *To:* RE-wrenches *Subject:* Re: [RE-wrenches] 30 amp disconnect/40 amp breaker Typically a fused disconnect can only handle 80% of it's nominal current on a continuous basis. Unfused disconnects are often 100% rated. For example, see attached Square D bulletin #3110DB0401R0410. So, a 100% rated (unfused) 30A disconnect (at least, from Square D) should be fine here, as long as your OCPD is provided elsewhere (eg, the breaker you mentioned). NEC 690.8(B)(1)(Exception) explicitly allows a 100%-rated device to be used without multiplying the inverter output current by 125%. On Wed, Aug 21, 2019 at 3:22 PM Greg <g...@remotepowerinc.com> wrote: Fellow Wrenches, I'm having a discussion with a utility engineer regarding the following situation: Installing a 6kW inverter that has a full rated output of 25 amps. 80% of 30 is 24. That means to me I need to go up to a 40 amp breaker. I'm running #8 (continuous load 25 x 1.25 etc.) The engineer believes I should use a 60 amp disco since it's on a 40 amp breaker. My thinking is I can use a 30 amp disco since it can not produce more than 25 amps ever. Do any of you have a reference that shows what I'm doing is okay? Not okay? Thanks again, Greg Egan Remote Power Inc. _______________________________________________ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org -- *Blake Gleason, PE* | Employee-Owner | *Director of Innovation & Technical Excellence* 510.845.2997 x128 Follow us on Facebook <https://www.facebook.com/pages/Sun-Light-Power/119675658118322?ref=hl>, LinkedIn <https://www.linkedin.com/company/sun-light-and-power> or our website <https://sunlightandpower.com/>. _______________________________________________ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org -- *Blake Gleason, PE* | Employee-Owner | *Director of Innovation & Technical Excellence* 510.845.2997 x128 Follow us on Facebook <https://www.facebook.com/pages/Sun-Light-Power/119675658118322?ref=hl>, LinkedIn <https://www.linkedin.com/company/sun-light-and-power> or our website <https://sunlightandpower.com/>.
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