Another reason to dislike tray cable [RE-wrenches]

Nick Nicholas, APS APS at SBCGLOBAL.NET
Mon Apr 21 15:36:35 PDT 2003


William:
    It doesn't seem hard to run correct length MC cables to a Connect
PCB-10 combiner box and out to a disconnect.  We recently installed a system
with 10 MC cables per combiner box.  Of course, we had to punched new holes.
One interesting method a customer used:
    I have a customer who installed 20 Sharp 185s on his machine shop.  He
used the MX-60 to wire the array at 48vdc while utilizing his old 24 volt
inverter/battery.  He wanted to save MC cable costs, so he fabricated 10
j-boxes which allowed series connection of the two modules with a fuse at
the series connection.  He fabricated lugs which had a j shaped clip for
easy installation of the large 1/0 homerun cable.  The panels + and -
terminated onto the homerun cables at the j shaped lugs. He used conduit to
connect the 10 j- boxes.  This combiner system forced (allowed) him to cut
the leads off each module (not recommended to him, but he is a grown up.)
Alternative Power Systems
www.alternativepowersystems.com
aps at sbcglobal.net
135 Colfax Ave.
Grass Valley, CA 95945
(530) 274-0588 phone
(530) 274-0589 fax
----- Original Message -----
From: "William Miller" <wrmiller at slonet.org>
To: <RE-wrenches at topica.com>
Sent: Monday, April 21, 2003 2:22 PM
Subject: Another reason to dislike tray cable [RE-wrenches]


Friends:

I am bidding an off-gird installation.  The customer purchased his
equipment from a well known mail order outfit.  He has mounted his 8 Sharp
165 modules on two fixed, pole mount racks.  I need to connect the Sharp
modules to the Outback PV combiner.  Each module is a separate feed to the
combiner, since the system is 24 volt, nominal.  I can't just run the tray
cables to the combiner because 1, they aren't long enough and 2, there is
no hardware I know of to bring the tray cables into an enclosure.  The tray
cables with MC connectors are meant to connect to another MC connector. If
I cut the MC connector off to feed the lead into a J-box, I void the
warranty of the module.

Here's the only way I can see to connect these modules up:  I provide a
J-box on each pole with 8 tray leads hanging out of the bottom, each with
an MC connector.  To do this, I need to spend $192.80 and lots of time.

Below is an "ÄSCII" CAD drawing, if it doesn't get hacked up by line
wrapping:



-------------------------------------
Module plus -----> >----Gland nut-| box ---splice-------THHN--|----\
Module plus -----> >----Gland nut-| box ---splice-------THHN--|----\

              conduit-----combiner
Module neg  -----> >----Gland nut-| box ---splice-------THHN--|----/
Module neg  -----> >----Gland nut-| box ---splice-------THHN--|----/

-------------------------------------

For each mounting pole I need to provide a J-box.  In each J-box I need to
bring in 9 leads (4 modules with plus and minus and a ground).  I need 8
gland nuts (Arlington LPCG50, $1.10 each) on the bottom of each J box.  I
can't use a standard metallic Nema 3R box because by the time I find one
with 8 existing KOs or room to add enough KOs to total 8, it would be huge.
 So I need an 8X8X4 plastic J box (Carlon E898N, $24.00).  Then I need to
purchase 8 prefabricated tray cables with MC connectors ($14.00 each).  I
cut each one in two and install one end in each gland nut.  Once in the
J-box, I need to get my leads over to the combiner.  I run PVC conduit from
each J-box to the combiner.  If I put the combiner on one of the two poles,
the tray leads from the J-box on that pole might be long enough to reach
the combiner, but the leads from the other pole are not.  I need to splice
the tray leads (the ones in the gland nuts) to some THHN.  I could use wire
nuts, but I prefer a terminal strip ($24.00).

Here's what this will cost:

2 J-boxes 24.00 48.00
8 MC cables 14.00 112.00
16 gland nuts 1.10   17.60
1 terminal strip 24.00 24.00

Total 211.60

If instead of the infernal tray cables, I had J-boxes on the modules, I'd
run 1/2"non-metallic flex and THHN from the module terminals straight to
the combiner.  Cost:

16 1/2 NMLT connectors 1.00 16.00
6 feet NMLT flex 1.38 8.40

Total 24.40

I would transition to 1/2 or 3/4 PVC and run right into the combiner.  I
don't count the cost of the PVC in either scenario.

Plus, with tray cable, I have multiple splices in the feed from each
module, half of them outside of an enclosure.

Where's the savings or reliability with tray cables?  Is there an easier,
better way to do this?


William Miller


PS:  I am halfway through my first "top down"job using conduited wiring.
It's a bit of a pain in the ass to use flex, but I have developed a
technique that will become easier with practice, and it looks very solid.
I will post pictures as soon as I can.  Contrast this with the Sharp
modules on the neighbors house.  The prevalent view from the ground of the
Sharp installation is of the gap between the modules and the roof.  You can
see the spider web of wire hanging down, in spite of many wire ties used.
The first Frisbee or ball that hits the roof will tempt some kid to use a
pole to dislodge it-- and the MC connections.  Lets hope that pole is non
conductive!

WM





__________________________________________________________________
William Miller
Miller Power and Communications
PO Box 50, Santa Margarita, CA 93453
Voice :805-438-5600     Fax: 805-438-4607
email: wrmiller at slonet.org
http://millerpowerandcomm.com
License No. C-10-773985
_____________________________________________________________
Compatibility:
Word processor: WP10
Spreadsheet: Quatro Pro 10
CAD: Microstation 95, DXF, Visio 4.1T
_____________________________________________________________
"Every decent man is ashamed of the government he lives under."
                                    H. L. Mencken
__________________________________________________________________

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