The only thing I am aware of wrt to fused DC discos is the notion that PV runs that penetrate a building shell need over current protection. The City of Santa Monica used to (and may still) have this requirement.
The (non) issues as I understand them: (1) PV modules should be treated as any other independent source, regardless of the fact that they are inherently current limited. It would be straightforward to determine whether or not the wire used has the adequate ampacity. (2) Inverters may be capable of back-feeding the PV array . Manufacturers that continue to sell and market a specific inverter model, are usually willing to write a letter to the effect that back-feeding will not occur. The only mfgr that I know of that demurred is Sharp wrt their JH-3500, and it is no longer sold in the US. Of course the use of an unfused disco makes good sense, prior to penetrating building shell. - Peter Peter T. Parrish, Ph.D., President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 CA Lic. 854779, NABCEP Cert. 031806-26 [EMAIL PROTECTED] _____ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of ASAP POWER! 2 Sent: Friday, November 21, 2008 10:47 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] DC Rating of Disconnects In 2008, we have had to replace our standard Sq. D HU361RB used since our beginnings, with the Sq. D H361RB (U stands for unfused). Was there some sort of code change NEC or CBC maybe that made most jurisdictions in So Cal suddenly start requiring fused DC disconnects? Typically, we fill the fused Sq. D. box with 15A 5" fuses. Peter D. _____ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of William Miller Sent: Friday, November 21, 2008 9:43 AM To: RE-wrenches Subject: Re: [RE-wrenches] DC Rating of Disconnects Peter: I am resending, the first version inexplicably deleted some text: An arc is created in the process of disconnecting any current under load. DC circuits have an enhanced ability to sustain that arc. PV circuits exacerbate this problem as a short or load is disconnected because they can sustain a short but the voltage rises drastically as a circuit is disconnected. This all justifies the added expense of a DC rated disconnect. We use the Square D HU361 without hesitation. There are few alternatives available and none with any significant cost savings. In selecting a disconnect, a design-build contractor must use the highest Voc possible. Make sure you are entering the 690.7 table with the record low temperature for the location. If we used a disconnect without the correct rating we would open ourselves to unacceptable liability exposure and, worse yet, would not experience that pleasant feeling one gets when you know you've done the job right. William Miller At 09:11 AM 11/21/2008, you wrote: _____ From: Peter Parrish [mailto:[EMAIL PROTECTED] Sent: Friday, November 21, 2008 9:10 AM To: 'RE-wrenches' Cc: 'Peter Parrish' Subject: DC Rating of Disconnects I am looking into using the PV Powered PVP2000 inverter for some small, low voltage designs. The string Voc will be around 200 Vdc, worst case. Since the PVP2000 doesnt come with a DC disconnect, I am wondered what other wrenches who install the PVP2000 have been doing about this. I hate to spend $150 on a 600 V AC/DC disco such as the SD HU361RB to provide the means of disconnection. I believe that the 240 V AC discos such as the SD DU221RB have no DC rating. >From an academic point of view, I wonder why the smaller discos are not dual rated, since we all know that the real-time voltage difference in a 240 V AC system can be 41% greater than 240 V. In any event, any Code-compliant solutions would be greatly appreciated. - Peter Peter T. Parrish, President
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