William, I think we have it all out on the table. Below you cite 2008 690.64 (B) (2). But the requirements of (B) (2) do not come into play if it is excluded by the provisions of 690.64 (B). And as you already pointed out, this all revolves around the interpretation of the Code vis-a-vis the word "capable". What is the intent of the Code? What reflects reality? In reality, you cannot find any point in the entire feeder circuit including the load center itself where the response to a ground fault or short circuit failure would not overwhelming be controlled by the feeder breaker. On one side of the feeder breaker you have some inverters, whose ability to produce excess fault current is limited short term discharge of it's capacitors. On the other you have an almost unlimited source of excess current and uncontrolled rise in the rate of energy flowing into a fault. I suggest that in this case, it will always be the feeder breaker that will open in response to a fault and to that extent it is perfectly reasonable to size the load center and feeder to the feeder breaker sufficient to carry the maximum continous ortput of the inverters. So what was the intent of the Code makers? Why did they include the language "capable of supplying multiple branch " if their intent was other than to allow the use of dedicated load centers?
Mark Frye Berkeley Solar Electric Systems 303 Redbud Way Nevada City, CA 95959 (530) 401-8024 <http://www.berkeleysolar.com/> www.berkeleysolar.com _____ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of William Miller Sent: Sunday, January 22, 2012 11:28 AM To: RE-wrenches Subject: Re: [RE-wrenches] Paralleling Multiple Inverter Mark: I would respectfully disagree, based on my reading. If you have code citations that inform me otherwise, I would be very grateful to expand my knowledge. Pending hearing otherwise from you, here is what I know, based on 2008 code, and assuming load side connection (2008 citations in italics): 1. 690.64(B)(1) Dedicated Overcurrent and Disconnect. Each source interconnection shall be made at a dedicated circuit breaker or fusible disconnecting means. Each inverter will require a 40 amp circuit breaker. The value of that breaker is calculated by adding a 25% continuous duty rating to the maximum AC output, which is 25 amps: 25 * 1.25 = 31.25. The next breaker size up is 40 amps, so you need 40 amp breakers. 2. 690.64(B)(2) Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor. The designer must add the values of the circuit breakers to determine the back-feed value. We just calculated the circuit breaker size in step 1, above. Four inverters means four 40 amp circuit breakers, therefore: 4 * 40 = 160. 3. In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility interactive inverter(s) shall be used in the calculations for all busbars and conductors. Therefore the designer must use 160 amps as the total back-feed value for all panels and feeders in series all the way back to the service. Mark, I would really appreciate it if you could reply today with any information I am missing. I could use my new-found knowledge to modify the permit application I am submitting tomorrow morning. Sincerely, William Miller PS: I used to think, erroneously, that I need only consider the actual maximum AC amperage from a given inverter. Some time ago I bid on and started a job based on that fallacy. Mid-way into the job the AHJ informed me that my calculations were incorrect, that I needed to use the breaker value. This job used SB6000 inverters and the value required was 40 amps. I researched this thoroughly and discovered they were right. In order to comply with 690.640(B), I had to downgrade the main breaker at my own expense. The breaker was not inexpensive, so this is a lesson I learned the hard way. This is why I am very interested in any knowledge that might prove otherwise in this scenario. Wm
_______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
