[RE-wrenches] Rooftop wiring methods between multiple subarrays

Hans Frederickson hans at fredelectric.com
Fri Oct 9 14:14:53 PDT 2009


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-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at 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

august at luminalt.com

 





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