Brian, William & Allan, Perhaps I've misunderstand the specific distribution equipment involved. I have assumed the main SERVICE consisted of a meter and single, 100 amp main breaker, as a combo unit or as separate enclosures, but with no busbar provision for additional breakers there at the pedestal.
The service disconnect, in other words, consists of a single switch, in an enclosure that allows no other breakers. The feeder conductors to the house are terminated on the load TERMINALS of that switch / breaker. The meter load side is connected directly to the switch / breaker line side. There is no busbar in the sense that additional breakers could be installed. Dick Ratico Solarwind Electric >Brian wrote: Hi Folks, Can anyone provide code reference or advice on this question? Our customer has 100 amp service at the meter pedestal, 100 amp main breaker feeding 200 foot underground run to a 100 amp panel at the house. We need to land a 40 amp breaker, so are proposing to replace main panel at house with 200 amp panel. The AHJ says we need a 200 amp main breaker in this panel, which will work, but says this requires upsizing the feeder, main breaker at meter, and therefore utility service to 200 amps. I've read on this list and elsewhere about downsizing the main breaker in a panel to create capacity to make the 120% rule. This AHJ says you can't put in a breaker smaller than the panel size, and furthermore, everything upstream would have to be upsized. Is this from the code? Bruce Erickson Mendocino Solar Service PO Box 1252 Mendocino, CA 95460 707-937-1701 >Allan 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 tape, 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: a. maximum current passing through 100A main breaker = 100A. Adding 40A of PV only reduces amount through conductors from main AC panel to 60A. b. no AC load in daytime = 40A of current back to main AC load center c. 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? >William wrote: 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 --- end of quote --- _______________________________________________ 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
