I've been banging my head against a wall for almost two years on this issue as microinverters have gained traction and combining several circuits in a PV subpanel is common, even on residential jobs. I'll take another run at it and see how it goes. We've been doing lots of supply side connections because of this issue, which adds a fusible disconnect to the design.
Thank you all very much for the feedback. Jason Szumlanski Fafco Solar From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of David Brearley Sent: Thursday, September 29, 2011 5:21 PM To: RE-wrenches Subject: Re: [RE-wrenches] Adding on to existing PV System Agreed. The intent of the change to 690.64(B) in NEC 2008 was not to restrict the use of a dedicated inverter aggregation subpanel for amp-shaving purposed (meaning the panel is being used to reduce the PV CB size ultimately used for 120% rule calculations). The intent was to clarify the that there is no need to apply the 120% rule to progressively larger upstream breakers when the interconnection happens in a subpanel. A reasonable inspector or jurisdiction will interpret the Code consistent with its intent, not according to an unintended consequence. However, you may have to educate them about the distinction between the two possible interpretations. David Brearley, Senior Technical Editor SolarPro magazine On 9/29/11 1:50 PM, "Mark Frye" <ma...@berkeleysolar.com> wrote: OK, this subject has come up before and this is a gray area in the Code. What is the definition of "the output of the inverters." I have sucessfully argued that the output of the inverters is the last point in the circuit where the circuit is free of premise load circuits. In this case it would be the 40A breaker connecting the dedicated PV load center to the busbar of the main panel. So...it is between you and you inspector. Tell him you are going to put a label on the subpanel that says something to the effect of "Dedicated inverter output accumulation subpanel. Add no additional circuits." I think this is a much better interpretation and method that preserves a clean and functional load side connection topology. I think most reasonable inspectors would agree as well. Good Luck. Mark Frye Berkeley Solar Electric Systems 303 Redbud Way Nevada City, CA 95959 (530) 401-8024 www.berkeleysolar.com <http://www.berkeleysolar.com/> ________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Jason Szumlanski Sent: Thursday, September 29, 2011 1:33 PM To: RE-wrenches Subject: Re: [RE-wrenches] Adding on to existing PV System I understand what you are saying about the 40A breaker, but according to 690.64(B)(2), "...with panelboards connected in series, the rating of the *first* overcurrent device connected *directly* to the output of the inverters shall be used in the calculations for *all* busbars and conductors." The way I have seen that interpreted is that the main distribution panel needs to be rated to support 120% of the two inverter circuits plus the main breaker, regardless of the fact that there is a 40A backfeed breaker in the main panel. 200+30+15 > 200 x 1.2 I know it doesn't make much sense, but that's the way it's been interpreted around here. It stands to reason that this applies only to the PV subpanel busbar and the conductors between the panels, and that the calculation for those should be: (30 + 15 + 40) / 1.2 = 70.83A required ampacity ...and the calculation for the main distribution panel should be: (200 + 40) / 1.2 = 200A required ampacity ... but that's not what I am being told. Jason Szumlanski Fafco Solar From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Mark Frye Sent: Thursday, September 29, 2011 4:19 PM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Adding on to existing PV System Jason: SB 5000 = 21A @ 240V, 21A x 1.25 = 26.3A needs 30A breaker M215 = 0.9 A @ 240V, 0.9A x 9 = 8.1 A, 8.1 x 1.25 = 10.1A needs 15A breaker. (Or that rounded to 10A and use a 10A breaker)? 26.3A + 10.1A = 36.4 A needs 40A breaker So, get a new 100A main lug subpanel. Connect it to a 40A breaker in the existing main panel. Move the existing 30A breaker into the new subpanel and install the new 15A breaker into the same. Mark Frye Berkeley Solar Electric Systems 303 Redbud Way Nevada City, CA 95959 (530) 401-8024 www.berkeleysolar.com <http://www.berkeleysolar.com/> ________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Jason Szumlanski Sent: Thursday, September 29, 2011 12:55 PM To: RE-wrenches Subject: Re: [RE-wrenches] Adding on to existing PV System That's not a bad idea, except that 690.64(B)(2) requires that the breaker directly connected to each inverter(s) must be used for the calculation. I have an existing SB5000 with a 30A breaker and a new string of nine Enphase M215's requiring a 15A breaker. If you combine these in a 100A or higher rated subpanel with a 40A main breaker, you still calculate the total current supplying the busbar in the main distribution panel based on 200A + 30A + 15A = 245A (with 30A and 15A being the overcurrent protection directly connected to the inverters). This exceeds the 120% rule by 5A. It also requires that the ampacity of the conductors between the two panels be 85A / 1.2 = 70.83A (assuming a 40A backfeed breaker in the main distribution panel). If your interpretation is different, please let me know! If my interpretation is correct, I like the idea of just adding a fusible disconnect and using #10 conductors all the way to the tap. That would be the most cost effective solution I believe, and since there is no foreseeable further system rating increases, the advantage of a subpanel is reduced. Thanks for all the other suggestions provided. Unfortunately I can't put in smaller main breaker, and the busbar in the main panel is definitely rated 200A. Jason Szumlanski Fafco Solar From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Glenn Burt Sent: Thursday, September 29, 2011 2:40 PM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Adding on to existing PV System How about feeding both systems into an AC combining subpanel before feeding the main load center? Often the calculations required will allow you to do more backfeeding than the simple backfeeding a load center with individual breakers. -Glenn From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Jason Szumlanski Sent: Thursday, September 29, 2011 11:49 AM To: RE-wrenches Subject: [RE-wrenches] Adding on to existing PV System We've been contracted to add on to an existing PV system in a residence. The existing system has a 30A 2P Breaker on a 200A rated panel with a 200A main breaker. The new system would require a 15A breaker, which would exceed the limit in NEC 690.64(B)(2). The existing system has a fusible AC disconnect mounted next to the meter. This was originally intended as the utility external disconnect. The utility no longer requires a disconnect, but if we do a supply side connection, we would need a fusible AC disconnect anyway for the new system. I would collocate the new disconnect with the old. Does anyone see any problem with doing a supply size connection for the new system? Any labeling snags that I might need to consider? Jason Szumlanski Fafco Solar ________________________________
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