conduit vs tray cable [RE-wrenches]

Tue Apr 16 12:21:42 PDT 2002

Howdy Guys!

As for Kelly. Yes I still use this method.
Inspectors love it.

And Bill I agree in what you say.

I do not mean to denegrate any particular manufacture but Siemens J
boxes are bomb proof and I can understand why some others have not taken
the time or expense to get theirs UL listed. Some in my opinion are
actually pathetic.
In 20 years of installing and designing and selling at least 1000
modules perhaps 1/2% has been something other than Siemens.
Maybe that is why I have had zero problems with conduit and J boxes.

Now a little unlisted but  well controlled and scientific info on this
subject.

As an engineer and natural born curmudgeon I just had to do the
following.

And Bill I realise that this would all have to be done at UL to be valid
but hear me out.

Using CarFlex brand 1/2" flex conduit 6" long I glued two 1/2" PVC male
ends using the following method.
I constructed  two identical units.

NO primer  and clear PVC cement and both conduit and ends allowed to sit
in the fridge  at 42 degrees  for one hour.

In a 70 degree F ambient condition I removed the items from the fridge
and within  no more than one minute I glued the ends to the conduit.

Glue was applied liberally to both the end and conduit and the conduit
was twisted at least 1/4 turn.

After allowing the glue to set for 24 hours I performed the following
test.

The conduit assemblies were  placed in a pot of water and set on a
standard laboratory heater. Using a thermistor probe and a control I was
able to heat this assembly to 200 degrees F. They were  allowed to stay
at this temperature for 24 hours always covered with at least 3 inches
of water. When replenishing the water lost to evaporation the water
never dropped below 160 degrees F.

I then removed the assemblies and let them dry.

The following tests were performed.

Using the J box and section of glass from a broken SP75 module (I won't
go into the details of how this was broken;-) ) I inserted one end into
the conduit box and the other was attached to a fixture that allowed me
to attach a spring scale. The scale was calibrated against a standard
prior to the test and showed deviation of less than 5% full scale. Full
scale was 200 lbs.

Using a plastic nut assembly on both ends and securing the panel and box
to a rigid fixture I began a controlled pull using a "comealong"

At 54 lbs the side of the box began to bulge slightly and the lid of the
J box began to seperate somewhat from the box.

At 125 lbs the box showed quite a bit of distortion but the plastic held
and the RTV/EPDM bond held.

At this point I decided to disconnect the conduit and instead try to
pull the box from the backing simply using the comealong and cable.

With the scale fully deflected to 200 lbs the box was still attached to
the module but showed stress and distortion.

I then took the untested conduit assembly and simply tried to pull the
ends off using the spring scale and comealong.

One end finally came off with a pop at 145 lbs.

Carefully checking the bond between end and conduit showed that the
failure was a shear line directly through the conduit parent material.

Since PVC gluing  is more of a fusion method than an adhesive bond this
was consistent with expectation.

The rigid PVC has a higher elastic modulus than the flex so the failure
was in the conduit itself. No where did the glue line show any signs of
stress or failure.

Now I realise that this test did not include thermal cycling over time
or fatigue due to mean cyclic stress due to wind loading,etc but it does
show one thing.

Highly elevated tempertures and high wet conditions had little if any
affect on the carflex or PVC end.
The J box on a Siemens is one tough (son-0 gun) .

I think the controversies generated between UL requirements and the PV
industries requirements is more a function of the unique environment
that the PV lives in than any conspiracy to restrict PV usage.

I just wish the manufactures and UL would get together and do realistic
controlled tests on the conduit and J boxes.

That would go much further to promote the technology than arguing about
the validity of things that have never been tested.

I for one have little desire to drop the use of conduit.
It works, looks very neat and professional and allows more versatility
in wire routing.

As to PVC/ultra violet degradation.
I never allow the conduit to be exposed to direct sun except for the
small amount running down the pole to the combiner box. And then I
always try to secure it on the north side of the pole.

Now you guys know what I do in my limited spare time besides playing
with hydrogen and fuel cells.

The Eternal Curmudgeon

> Kelly and Larry,
>
> Just a little note on conduit. Larry mentioned the problem of not
having the
> right temperature rating, but the bigger problem in my opinion is that
many
> manufacturers do not rate their junction boxes for conduit. It must be
> explicitely called out in the installation instructions otherwise it
> violates the UL listing of the module. Those cute little 0.75" holes
on most
> junction boxes are not designed for conduit but for cable glands.
>
> Larry, I'm sure that your conduit installations are of the highest
quality,
> but field experience with average installations on rooftops is far
from
> encouraging. I have seen conduit pull out of connectors, pull out of
> junction boxes, and pop the whole junction box off the back of the
module.
> Siemens sells their module as "conduit rated" and they are one of the
few
> companies that has gone through UL to get their modules approved for
> conduit. There are very few things more dangerous in a PV system than
an
> open junction box laying on a roof.
>
> Bill.
>
>

> ==^================================================================
>
>
>
>

- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://www.topica.com/lists/RE-wrenches/

List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm

Check out participant bios: www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com

==^================================================================
This email was sent to: michael.welch at homepower.com

EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9
Or send an email to: RE-wrenches-unsubscribe at topica.com

T O P I C A -- Register now to manage your mail!
http://www.topica.com/partner/tag02/register
==^================================================================