William, Wow - you're a master at whipping up drawings!
OK, I get the concept, and it's just as I described. The scenarios in drawings one and two are OK as long as both busbars are rated 150A (commercial) or 125A (residential. You then show that the reason for the upsized feeders is to handle a short. But wait! Even if the short could produce 140A, as you have indicated, the conductors will still not carry more than the 100A that they're rated to carry. One would carry 100A from one panel and 40A from the other, Nowhere would that conductor carry 140A, and the mate to it would carry at most 60A. Increasing conductor size wouldn't change anything in that (or any) scenario. So why would you have to increase conductor size? Thanks, Allan _____ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of William Miller Sent: Saturday, August 01, 2009 11:13 PM To: RE-wrenches Subject: Re: [RE-wrenches] down-sizing main breaker Allan: I whipped up three drawings that indicate three overload scenarios possible with incorrectly sized distribution equipment. They are on our web site at: http://millersolar.com/case_studies/case_studies.html Click on Point of connection. Click on any drawing to see a larger version. I hope this helps with understanding the concept. William Miller At 06:19 PM 8/1/2009, you wrote: William, I have been following this back-and-forth thread and need you to please explain one thing: Your 140A point makes perfect sense (from the point of 690.64 in requiring that the busbar to which both utility and PV feeds are connected. No question there. But you write: "It is my understanding that if one is going to install a load side tap, the back feed calculations need to work for every link of the distribution system upstream to the meter." It seems to me that there's still no way to exceed main disconnect current rating. 1. 100A utility (using 100A subpanel main) + 40A PV tied into subpanel busbar = 140A, which is OK if the subpanel busbar is rated at least 125A (residential) or 150A (commercial). 2. 100A + 40A tapped into subpanel ahead of 100A main breaker: 1. maximum current passing through 100A main breaker = 100A. Adding 40A of PV only reduces amount through conductors from main AC panel to 60A. 2. no AC load in daytime = 40A of current back to main AC load center 3. some subpanel loads in daytime reduces 2.b. above 3. At the main panel, maximum current flow through breaker feeding subpanel is 100A, as any PV reduces load at breaker. However, main panel must itself meet 690.64, as it's still possible to feed 40A of PV (daytime with no subpanel load) into main panel. So same busbar requirements apply in the main panel as in the subpanel. But that's all. The subpanel feeders don't need to be upsized. What am I missing here, please? Allan Sindelar al...@positiveenergysolar.com NABCEP Certified Photovoltaic Installer EE98J Journeyman Electrician Positive Energy, Inc. 3201 Calle Marie Santa Fe, New Mexico 87507 505 424-1112 www.positiveenergysolar.com _____ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of William Miller Sent: Saturday, August 01, 2009 6:11 PM To: RE-wrenches Subject: Re: [RE-wrenches] down-sizing main breaker Dick: Your logic is correct but incomplete. Consider it like this: The buss bars in the service panel can handle 100 amps (they are rated for that amount). The main breaker feeds 100 amps into that buss, maximizing it's capabilities. If you feed an additional 40 amps into that buss bar, you have the potential of exceeding the ampacity of that buss assembly. If no other loads are fed from that buss bar, there is no over current scenarios. Say, however, someone puts more breakers on that buss bar and draws 140 amps from them. 100 amps comes in from the utility, 40 amps comes in from the PV and the buss is overloaded. This is the scenario for which 690.64(B) was written. This will occur only if the overload amperage is not drawn from a space on the buss bar between the utility feeder and the PV feeder. If, however, the PV feeder breaker and the utility feeder are both on one end of the buss and the load is on the other end, then the loads are additive on the buss assembly. Some want the code passage re-written to say that if the feeder is on one end of the bus assembly and the PV is on the other, there is no possibility of an overdraw on the buss bar and the installation is legal. This has not happened yet. William
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