Thanks August... these images of PVC strapped to the roof, with a joint totally failing after only 4 years, are a great reminder to follow the code (some of this is new for 2008):
NEC 352.20 (fasten PVC such that movement from thermal effects is permitted) NEC 352.44 (use expansion fittings to compensate for thermal expansion/contraction) NEC Table 310.15/B/2/c (ambient temp adjustment for conduits exposed to sun / rooftops) These are _minimum_ requirements. We're installing these systems to last 40+ years. I beg to differ that PVC is easer/cheaper to install on a rooftop when compared to EMT, if we're looking at a long-term installation. Between summer highs and winter lows, that PVC is going to be moving all over the place, and the only thing holding it together is glue and straps. It takes a lot of patience and know-how to install PVC correctly in that kind of environment, using expansion couplings with straps that allow movement in the right places and hold the conduit tight where you don't want it to move (e.g. where it enters a box). Here again is the link to the Carlon guide to expansion fittings. Definitely read this if you're installing PVC conduit where it will see large temperature swings: http://www.carlon.com/Installation_Training/IT-ISEXPJT.pdf I'm not really comfortable with the idea of standing behind a rooftop PVC installation for anything close to the lifespan of a PV system. Given the overall cost of a PV system, I think it's incredibly short sighted to save a few bucks this way on the initial installation, as evidenced by August's photos of the Marin County system. If you have to run around the roof with EMT, you can protect it from rust with a coat of paint. Regards, -Hans ________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of August Goers Sent: Friday, October 09, 2009 12:57 PM To: RE-wrenches Subject: Re: [RE-wrenches] Rooftop wiring methods between multiple subarrays All - PVC doesn't hold up well in the Bay Area - Here's a photo of some ~4 year old PVC browned PVC from Marin County: PVC (Small).jpg You can see that the straps melted resulting in the conduit sliding down. Thank goodness we didn't install this. PCV seems like a good option for shaded areas. As Bill pointed out, if we use EMT we need to bond both ends of the conduit to comply with NEC 250.97. My issue is that it is hard to find outdoor rated bonding bushings - that's why we've just been installing cast metal boxes with threaded connections (complies with NEC 250.92(B)(2)). As William point out, EMT is robust and reliable. I still think that there must be more efficient way to go between arrays. Maybe that's why commercial low profile racking systems are often fully integrated systems with built in wire raceways. -August August Goers Luminalt Energy Corporation O: 415.564.7652 M: 415.559.1525 F: 650.244.9167 aug...@luminalt.com _______________________________________________ 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
