NEC 2005 Ungrounded PV systems - opinion survey [RE-wrenches]

Nick Nicholas APS at SBCGLOBAL.NET
Fri May 7 01:12:54 PDT 2004


 

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Hi John:
    That it is a double-edged sword.  This may not be a deal killer, but it
is a obstacle!  Installation times are certainly quicker when MC cables can
be utilized with a top-down mounting system on sloped roofs.  I am assuming
we are talking about for 300-500 VDC series string configurations.
    The obvious alternatives are, like in the old days, pre-racking and
wiring sub-arrays on the ground, which means we add a lot of labor, combiner
boxes, and cost to the job.  I don't want to go back to that method.
    The better option is to use a top-down mounting system and wire each
module just before it is installed.  This takes time, and may not be
practical on a thirty degree roof pitch.  The only time I did this in a
higher-voltage system was at your house, (J. Berdner) and it seemed somewhat
dangerous, because you have to either wire the array hot or cover the
modules.  However, now the positive will be floating, so it is safer, as
long as you don't short out on the positive and negative at the same time.
This method would be awkward using LTFlex, but seems easy with tray cable.
This method would be fastest if a single USE-2/RHW wire could be used as the
series connector, and could be brought in and out of the j-box with a
two-hole cord connector.  It seemed clear that USE-2 would not be
acceptable, though.
    The labor may be increased by 15 or 20 minutes per module.  However,
every six to twenty four modules, you would save the cost of the MC cord
connector and it's attachment labor.  APS sells the first foot of the MC
cable for $15, so removing those would offset costs.  For a nine module
string, $30 would be saved.  However, if using tray cable, you would add 2-
Cord Connectors for each module.  We sell the cord connector for $2.25, so
this would add $40.50 ($4.50 x 9).
    In conclusion, if evaluating a 3 KW system with 18- 24 volt, 150 watt to
170 watt modules, the $10.50 for each series string would add $21 in
materials, and 8 hours of labor at $50 per hour. (18 modules x 20
minutes/module  + 1 hour extra driving time + 1 hour increased warranty
time.)  To convince me to use the floating positive design, the 2500 watt
inverter would have to cost $421 less (retail) to be comparable.
If LTFlex where used, the materials and labor would be higher.
    It really is too bad that these strings are attached (no pun intended).

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: "John Berdner" <jberdner at sma-america.com>
To: <RE-wrenches at topica.com>
Sent: Thursday, May 06, 2004 11:02 AM
Subject: NEC 2005 Ungrounded PV systems - opinion survey [RE-wrenches]




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Wrenches:

I need to ask for a little info on an issue.
While I think this is actually a relevant topic for the List (in the
medium to longer term) I do not want to clog the List so I request you
respond to me offline.
If the group feels it is appropriate then I am sure it will happen
online but I do not want to presume use of the bandwidth.

As many of you know the 2005 NEC looks like it is a go for ungrounded
PV arrays.
IMHO we should see this as great news since it means safer systems,
lower inverter costs, and more efficient inverter designs can finally be
used in the US.
The 05 Code is a a double edged sword, however, since they also slipped
in a new requirement for all wiring in conduit or jacketed cable (tray
cable).
In Europe the modules all use a double jacketed "PV Wire" and I
actually think it is a better way (tougher/safer) to go for pigtails
with connectors.
Unfortunately, in the 2005 Code as written this new requirement will
apply only to ungrounded systems.

My survey questions are:

- Is the requirement for conduit or tray cable instead of pigtails and
Multi-Contact style connectors a deal killer for ungrounded arrays ?
- What do you estimate the additonal labor costs ($/kWac) are for using
conduit  / tray cable versus pigtails and connectors ?

Thanks in advance.

Best Regards,

John Berdner


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