Eric,
Your reasoning sounds logical, but it is missing the point. Transformers have to be protected by overcurrent devices, similar to wire. Inverters are current limited devices so they only have to be able to withstand a certain size circuit breaker. The current from the inverter is considered continuous so most circuit breakers are designed to operate at 80% of their rating continuously (therefore the requirement to multiply inverter output current by 125%). Does that make any more sense? Bill. Bill Brooks, PE Principal Brooks Engineering 3949 Joslin Lane Vacaville, CA 95688 707-332-0761 (office and mobile) b...@brooksolar.com (email) www.brooksolar.com (web) From: RE-wrenches [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of e...@harvesthesun.com Sent: Monday, October 31, 2016 3:34 PM To: RE-wrenches <re-wrenches@lists.re-wrenches.org> Subject: [RE-wrenches] Undercurrent Protection? I realize this is potentially a stupid question, and one that is long overdue, but here goes: Why in 690(B) are we setting a minimum threshold on OCPD when we're protecting against over current? Shouldn't we be setting a maximum threshold...as is done in pretty much all other cases? The way I see it, we should take the array/inverter max output AC current and multiply by 125% to set our min ampacity on wire, and then calc down to find the breaker trade size that lands between the max AC output current and max AC output current times 125%. That way, we avoid nuisance tripping but achieve the max overcurrent protection possible. By following 690(B) we're actually decreasing OCP: Literally, 690(B) is stating that we can have a breaker 100x over the circuit rating, so long as it's not under the circuit rating times 1.25. To me this seems antilogical. Mr. Brooks? 690.9(B) Overcurrent Device Ratings. Overcurrent device ratings shall be not less than 125 percent of the maximum currents calculated in 690.8(A). 455.7(B)...The overcurrent protection determined from this section shall not exceed 125 percent of the phase converter nameplate single-phase input amperes. 450.4(A)...Such overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer. 450.5(A)(2) Overcurrent Protection. An overcurrent sensing device shall be provided that will cause the main switch or commontrip overcurrent protection referred to in 450.5(A)(1) to open if the load on the autotransformer reaches or exceeds 125 percent of its continuous current per-phase or neutral rating. 450.5(B)(2)(b) Ampere Rating. The overcurrent protection shall be rated or set at a current not exceeding 125 percent of the autotransformer continuous per-phase current rating or 42 percent of the continuous-current rating of any seriesconnected devices in the autotransformer neutral connection. Eric Stikes Founding Director & CEO Good Sun Solar, A CA non-profit corp. www.goodsun.life <http://www.goodsun.life> (530) 559-5023
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