Re: [RE-wrenches] Cable Sizing - revisited, Ambient Temp
John, 240.4(B) allows the ampacity of a wire to drop below the OCPD rating so you could argue that wouldn't be a safety issue. It's always irritated me that these NEC derating factors have two decimal places so that you feel your calculations are extremely exact... and then your carefully-derated-to-126.39A wire has no problem at all on a 150A breaker. But that's the way it is. It takes a lot of heating to exceed the 90C rating of a wire, but yes theoretically that would damage the insulation and cause a fault. As for voltage drop of your wires, I don't think that's as big an issue as you think. Heating from 40C to 80C, a wire is going to see its resistance increase by about 15%, so your voltage drop may go from 1.5% to 1.73%. Your module voltage drop is going to be the more pressing concern. If on your drawings you state something like "conduits to be minimum 3.5 inches above the roof" (or for residential, put the run in the attic) then that puts you at a 17C adder which is pretty manageable for most areas of the US (worst case, you're in the 61-70C temp derate range). And again, in some cases where you have a very hot section of conduit, the 10'/10% rule may let you ignore that localized heating. Since you're the one stamping these drawings, you have to stay within your comfort zone- add another 10C if you're worried about it and add some extra expansion joints. Or you can specify that your contractors shade all rooftop conduit, but I imagine that would limit your repeat business. Dave John Wadley wrote: Dave, Thanks for responding in Mr. Brooks place. Since ASHREA 2% is not the very worst case, it seems like it might be possible for the ampacity of the wire chosen to dip below the rating of the OCPD protecting it, if there is not much margin. I've been trying to rationalize whether this would become a safety issue. I don't think it would since the OCPD protects the wire from a current source increasing beyond the expected design output. I don't think there is much chance of that for a PV module (unless there was a short between two strings). I think the increased heating would more likely increase wire resistance/voltage drop and lower production. With enough voltage drop, the inverter might shut off. I guess my new concern is in the most severe case where there is solar concentration on a short section of conduit. Here, the heating effect of both the elevated ambient temp and reduced wire resistance might lead to premature failure of the wire insulation. If the combined heating effects exceed the 90C rating of the wire, does the insulation embrittle or melt? In either case, I foresee a grounding fault, and if the GFCI failed, it could spark a fire. I know the best solution is to keep conduit shaded and avoid these worst case solar concentrating conditions. Sometimes. when I design a system for a new contractor, I don't always know exactly where they plan to run conduit on a roof (nor can I control it) and I start "what-if'ing" whether my design numbers will be conservative enough to prevent a system failure or a fire. Thanks and regards, John Wadley, PE NABCEP Certified Solar PV Installer (TM) Wadley Engineering > Date: Tue, 25 Jan 2011 15:37:20 -0500 > From: Dave Click > To: re-wrenches@lists.re-wrenches.org > Subject: Re: [RE-wrenches] Cable Sizing - revisited, Ambient Temp > Message-ID: <4d3f3480.9030...@fsec.ucf.edu> > Content-Type: text/plain; charset=UTF-8; format=flowed > > John, > > The 2% "ambient temperature" from ASHRAE is the appropriate starting > point to use for these calculations. For some additional background I'll > quote Bill: > > ** > ASHRAE bases its ?warm?season temperature conditions? for each city on > annual percentiles of 0.4%, 1.0% and 2.0%. As an example, the June 2.0% > dry?bulb design temperature for Atlanta is 91.7?F. Therefore, based on a > 30?day month (i.e. 720 hours), the actual temperatures can be expected > to exceed 91.7?F a total of 14 hours a month. The corresponding 1.0% > design temperature (93.1?F) can be expected to be exceeded for 7 hours a > month; while the 0.4% design temperature (94.6?F) can be expected to be > exceeded for 3 hours a month (column 2). > ** > > In Jim Dunlop's example it sounds like he's starting with the summer > ambient high (likely around 90F / 32C) and adding the 310.15(B)(2)(c) > 33C figure to reach the 61-70C range. > > IMHO, ASHRAE 2% high temperature should be the "standard practice" for > these conditions when calculating your base ambient temperature before > additional adders. There are going to be site-specific conditions like > your example where the conduits may heat up more than 310.15(B)(2)(c) > requires; in that case I think y
Re: [RE-wrenches] Cable Sizing - revisited, Ambient Temp
David B., Thanks for the reference to this excellent article by Mr. Brooks. I'm a subscriber and fan of SolarPro and have adopted use of this article on several projects already. In fact I'm butting heads with a customer now over this min string size check. I've shown them the calculations, referenced this article directly and I'm sure they are aware of the weight behind Mr. Brooks expertise, but they are still willing to implement the shorter string size. I plan to make sure they sign something in writing stating they have been informed of the potential consequences of their decision. If I see a situation where I think the contractor might install conduit that does not meet standard practices or may experience abnormally high ambient temps, I'll use the more conservative number (ASHRAE Extreme Annual Mean Maximum Design Dry Bulb Temperature ) to protect myself and the end customer. Otherwise, I'll have to force them to show conduit runs on a site layout drawing and say any change from that has to be approved through me. Best Regards, John Wadley, PE NABCEP Certified Solar PV Installer (TM) Wadley Engineering > Date: Thu, 27 Jan 2011 09:02:31 -0600 > From: David Brearley > To: RE-wrenches > Subject: Re: [RE-wrenches] Cable Sizing - revisited, Ambient Temp > Message-ID: > Content-Type: text/plain; charset="iso-8859-1" > > John, > > Bill has written an article for SolarPro that you may find relevant: > > http://solarprofessional.com/article/?file=SP3_6_pg68_Brooks&search= > > Here?s an excerpt: > > ?...it is important for system designers to perform detailed low dc voltage > calculations for specific array configurations. Designers should use the > highest expected continuous ambient temperature for calculation purposes. > According to the Copper Development Association, the highest ASHRAE > temperature data that is likely to create a 3-hour continuous condition, per > the definition of continuous found in NEC Article 100, is the 2% Annual > Design Dry Bulb Temperature, which is also found in Appendix E of the > Expedited Permit Process for PV Systems. For designers who feel that the > ASHRAE 2% temperature is not high enough, the same table also includes > ASHRAE Extreme Annual Mean Maximum Design Dry Bulb Temperature data, which > can be used for even more conservative voltage or ampacity calculations.? > > I realize you are talking about a different set of calculations, but the > rationale for which data to use may still apply. > > David Brearley, Senior Technical Editor > SolarPro magazine > NABCEP Certified PV Installer ? > david.brear...@solarprofessional.com > Direct: 541.261.6545 > > > > On 1/27/11 1:20 AM, "John Wadley" wrote: > > > Dave, > > Thanks for responding in Mr. Brooks place. Since ASHREA 2% is not the > > very worst case, it seems like it might be possible for the ampacity of the > > wire chosen to dip below the rating of the OCPD protecting it, if there is > > not > > much margin. I've been trying to rationalize whether this would become a > > safety issue. I don't think it would since the OCPD protects the wire from a > > current source increasing beyond the expected design output. I don't think > > there is much chance of that for a PV module (unless there was a short > > between > > two strings). I think the increased heating would more likely increase wire > > resistance/voltage drop and lower production. With enough voltage drop, the > > inverter might shut off. > > I guess my new concern is in the most severe case where there is solar > > concentration on a short section of conduit. Here, the heating effect of > > both > > the elevated ambient temp and reduced wire resistance might lead to > > premature > > failure of the wire insulation. If the combined heating effects exceed the > > 90C rating of the wire, does the insulation embrittle or melt? In either > > case, I foresee a grounding fault, and if the GFCI failed, it could spark a > > fire. > > I know the best solution is to keep conduit shaded and avoid these worst > > case solar concentrating conditions. Sometimes. when I design a system for a > > new contractor, I don't always know exactly where they plan to run conduit > > on > > a roof (nor can I control it) and I start "what-if'ing" whether my design > > numbers will be conservative enough to prevent a system failure or a fire. > > > > Thanks and regards, > > John Wadley, PE > > NABCEP Certified Solar PV Installer (TM) > > Wadley Engineering > > ___ 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
Re: [RE-wrenches] Cable Sizing - revisited, Ambient Temp
John, Bill has written an article for SolarPro that you may find relevant: http://solarprofessional.com/article/?file=SP3_6_pg68_Brooks&search= Here¹s an excerpt: ³...it is important for system designers to perform detailed low dc voltage calculations for specific array configurations. Designers should use the highest expected continuous ambient temperature for calculation purposes. According to the Copper Development Association, the highest ASHRAE temperature data that is likely to create a 3-hour continuous condition, per the definition of continuous found in NEC Article 100, is the 2% Annual Design Dry Bulb Temperature, which is also found in Appendix E of the Expedited Permit Process for PV Systems. For designers who feel that the ASHRAE 2% temperature is not high enough, the same table also includes ASHRAE Extreme Annual Mean Maximum Design Dry Bulb Temperature data, which can be used for even more conservative voltage or ampacity calculations.² I realize you are talking about a different set of calculations, but the rationale for which data to use may still apply. David Brearley, Senior Technical Editor SolarPro magazine NABCEP Certified PV Installer david.brear...@solarprofessional.com Direct: 541.261.6545 On 1/27/11 1:20 AM, "John Wadley" wrote: > Dave, > Thanks for responding in Mr. Brooks place. Since ASHREA 2% is not the > very worst case, it seems like it might be possible for the ampacity of the > wire chosen to dip below the rating of the OCPD protecting it, if there is not > much margin. I've been trying to rationalize whether this would become a > safety issue. I don't think it would since the OCPD protects the wire from a > current source increasing beyond the expected design output. I don't think > there is much chance of that for a PV module (unless there was a short between > two strings). I think the increased heating would more likely increase wire > resistance/voltage drop and lower production. With enough voltage drop, the > inverter might shut off. > I guess my new concern is in the most severe case where there is solar > concentration on a short section of conduit. Here, the heating effect of both > the elevated ambient temp and reduced wire resistance might lead to premature > failure of the wire insulation. If the combined heating effects exceed the > 90C rating of the wire, does the insulation embrittle or melt? In either > case, I foresee a grounding fault, and if the GFCI failed, it could spark a > fire. > I know the best solution is to keep conduit shaded and avoid these worst > case solar concentrating conditions. Sometimes. when I design a system for a > new contractor, I don't always know exactly where they plan to run conduit on > a roof (nor can I control it) and I start "what-if'ing" whether my design > numbers will be conservative enough to prevent a system failure or a fire. > > Thanks and regards, > John Wadley, PE > NABCEP Certified Solar PV Installer (TM) > Wadley Engineering > ___ 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
Re: [RE-wrenches] Cable Sizing - revisited, Ambient Temp
Dave, Thanks for responding in Mr. Brooks place. Since ASHREA 2% is not the very worst case, it seems like it might be possible for the ampacity of the wire chosen to dip below the rating of the OCPD protecting it, if there is not much margin. I've been trying to rationalize whether this would become a safety issue. I don't think it would since the OCPD protects the wire from a current source increasing beyond the expected design output. I don't think there is much chance of that for a PV module (unless there was a short between two strings). I think the increased heating would more likely increase wire resistance/voltage drop and lower production. With enough voltage drop, the inverter might shut off. I guess my new concern is in the most severe case where there is solar concentration on a short section of conduit. Here, the heating effect of both the elevated ambient temp and reduced wire resistance might lead to premature failure of the wire insulation. If the combined heating effects exceed the 90C rating of the wire, does the insulation embrittle or melt? In either case, I foresee a grounding fault, and if the GFCI failed, it could spark a fire. I know the best solution is to keep conduit shaded and avoid these worst case solar concentrating conditions. Sometimes. when I design a system for a new contractor, I don't always know exactly where they plan to run conduit on a roof (nor can I control it) and I start "what-if'ing" whether my design numbers will be conservative enough to prevent a system failure or a fire. Thanks and regards, John Wadley, PE NABCEP Certified Solar PV Installer (TM) Wadley Engineering > Date: Tue, 25 Jan 2011 15:37:20 -0500 > From: Dave Click > To: re-wrenches@lists.re-wrenches.org > Subject: Re: [RE-wrenches] Cable Sizing - revisited, Ambient Temp > Message-ID: <4d3f3480.9030...@fsec.ucf.edu> > Content-Type: text/plain; charset=UTF-8; format=flowed > > John, > > The 2% "ambient temperature" from ASHRAE is the appropriate starting > point to use for these calculations. For some additional background I'll > quote Bill: > > ** > ASHRAE bases its ?warm?season temperature conditions? for each city on > annual percentiles of 0.4%, 1.0% and 2.0%. As an example, the June 2.0% > dry?bulb design temperature for Atlanta is 91.7?F. Therefore, based on a > 30?day month (i.e. 720 hours), the actual temperatures can be expected > to exceed 91.7?F a total of 14 hours a month. The corresponding 1.0% > design temperature (93.1?F) can be expected to be exceeded for 7 hours a > month; while the 0.4% design temperature (94.6?F) can be expected to be > exceeded for 3 hours a month (column 2). > ** > > In Jim Dunlop's example it sounds like he's starting with the summer > ambient high (likely around 90F / 32C) and adding the 310.15(B)(2)(c) > 33C figure to reach the 61-70C range. > > IMHO, ASHRAE 2% high temperature should be the "standard practice" for > these conditions when calculating your base ambient temperature before > additional adders. There are going to be site-specific conditions like > your example where the conduits may heat up more than 310.15(B)(2)(c) > requires; in that case I think you'd be on the right track to make your > own field measurements to determine an appropriate temperature. In some > cases the 10%/10ft rule may mean you can ignore short hot spots in the > wire. If you had a situation where the conduit was in direct sunlight, > plus light was being reflected off the roof and a light-colored wall > behind the conduit, I suppose that would yield more heating than what > the CDA study had found (http://www.iaei.org/magazine/?p=1743). I'm not > sure that shooting an IR thermometer is the best option here; if you > want to best replicate the study conditions, you may put a conduit > section up on the roof in the desired location, put a temperature sensor > in the conduit, and let it soak. Then compare that number with [ASHRAE > 2% + 310.15(B)(2)(c)] and pick the higher number. Or just add an > additional 10C on top of ASHRAE+B2c and be done with it... > > I think your plan of trying to measure the temperature 4" off the roof > where the conduit sits, and then adding the additional 17C (or whatever) > from the Table, will be too conservative; your initial measurement will > be affected by some of the heating that's wrapped into the > 310.15(B)(2)(c) factor and you'd be double-counting that effect. > > Hope that helps. > Dave > > Original Message > Subject: [RE-wrenches] Cable Sizing - revisited, Ambient Temp > From: John Wadley > To: RE-wrenches > Date: 2011/1/22 02:40 > > > Mr. Br
Re: [RE-wrenches] Cable Sizing - revisited, Ambient Temp
John, The 2% "ambient temperature" from ASHRAE is the appropriate starting point to use for these calculations. For some additional background I'll quote Bill: ** ASHRAE bases its ‘warm‐season temperature conditions’ for each city on annual percentiles of 0.4%, 1.0% and 2.0%. As an example, the June 2.0% dry‐bulb design temperature for Atlanta is 91.7°F. Therefore, based on a 30‐day month (i.e. 720 hours), the actual temperatures can be expected to exceed 91.7°F a total of 14 hours a month. The corresponding 1.0% design temperature (93.1°F) can be expected to be exceeded for 7 hours a month; while the 0.4% design temperature (94.6°F) can be expected to be exceeded for 3 hours a month (column 2). ** In Jim Dunlop's example it sounds like he's starting with the summer ambient high (likely around 90F / 32C) and adding the 310.15(B)(2)(c) 33C figure to reach the 61-70C range. IMHO, ASHRAE 2% high temperature should be the "standard practice" for these conditions when calculating your base ambient temperature before additional adders. There are going to be site-specific conditions like your example where the conduits may heat up more than 310.15(B)(2)(c) requires; in that case I think you'd be on the right track to make your own field measurements to determine an appropriate temperature. In some cases the 10%/10ft rule may mean you can ignore short hot spots in the wire. If you had a situation where the conduit was in direct sunlight, plus light was being reflected off the roof and a light-colored wall behind the conduit, I suppose that would yield more heating than what the CDA study had found (http://www.iaei.org/magazine/?p=1743). I'm not sure that shooting an IR thermometer is the best option here; if you want to best replicate the study conditions, you may put a conduit section up on the roof in the desired location, put a temperature sensor in the conduit, and let it soak. Then compare that number with [ASHRAE 2% + 310.15(B)(2)(c)] and pick the higher number. Or just add an additional 10C on top of ASHRAE+B2c and be done with it... I think your plan of trying to measure the temperature 4" off the roof where the conduit sits, and then adding the additional 17C (or whatever) from the Table, will be too conservative; your initial measurement will be affected by some of the heating that's wrapped into the 310.15(B)(2)(c) factor and you'd be double-counting that effect. Hope that helps. Dave Original Message Subject: [RE-wrenches] Cable Sizing - revisited, Ambient Temp From: John Wadley To: RE-wrenches Date: 2011/1/22 02:40 Mr. Brooks, You replied to Mr. Parrish back in 2009 with this example (below) on properly applying all the deratings to ampacity for wire sizing. I have a bit of confusion and a question about the definition of "ambient temperature". You define it below as the ASHREA 2% high temp. My NEC 2008 (310.15 (2) Except No. 5 (3) (b) FPN) mentions it being an "average ASHREA" number. The only definition for "ambient temp" in NEC I could find was 310.10 FPN (1) which says it "varies along the length of the conductor by time and place". In the Photovoltaic Systems by Dunlop, p. 288, he cites a sizing example without saying where ambient comes from but uses 61C-70C (142F-158F) derate factor (0.58) for a sunlit roof top conduit. He does not say how he arrives at that tempature range, but I suspect he started with the 90F rating of the USE-2 conductor in the example and added a Table 310.15 (B) (2) (c) adder of 33C. Other articles I've read talk about conditions like an unventilated attic or a sunlit jbox on a roof where ambient temps could reach 150F. I can also think of a situation where on a flat roof with a surrounding parapet wall, the sunlight shining into a corner would act like a solar oven on any conduit running close to the corner. So, given all these definitions and possible exceptions to the definition of "ambient temperature", does your original definition (ASHREA 2% high temp) still stand as standard practice for most conditions and are there situations where one should use something other than that defined value? If one is unsure of an exceptional situation, would it make sense to use an IR thermometer to measure free air temp on a sunny, calm day and then the air temp exactly where conduit might run and use the temp delta as an adder (like Table 310.15 (B) 2 (c)) to the ASHREA 2% high temp to arrive at a new, situational ambient temp before applying the other factors cited? Thanks in advance, John Wadley, PE Wadley Engineering NABCEP Certified Solar PV Installer (TM) Dallas, TX Peter, We cannot use load diversity to increase the number of conductors in a PV conduit since there generally is little diversity among the conductors, particularly on large arrays. The more traditional conduit adjustment table to use is Table 310.15(B)(2)(a). The value from this table is multiplied by the temperature adjustment factor in Table
