DC Wiring in Building (Was 600VDC switches) [RE-wrenches]

[William Miller]

Interesting points, some raised before.  Here are some thoughts:

I recently added modules to an existing system.  Charge control was via an
APT control center (remember them?).  The PV feed is controlled by mercury
wetted relays.  Even if the PV is disconnected at the roof, the leads are
still energized.  I believe the relays will still be closed, trying to
charge the batteries.  I am not certain that all semi-conductor based
charge controllers can be counted on to isolate batteries from PV circuits
in normal situations.  I am less certain of this in abnormal situations
(equipment failures, for example).  Therefore, rooftop disconnects are not
a panacea.

The entire PV code section is written to ensure that even if PV leads
become directly shorted, all wiring and ancillary equipment can carry that
current indefinitely without fear of over heating the conductors.

There are two remaining concerns, at least in my mind:  Ground fault, which
is addressed in the code, and arcing.  PV leads can create substantial
arcing, as we have all seen.  This is why putting leads in conduit still
seems like a good idea to me, and why tray cable is not desirable.
Furthermore, PV leads should not be in proximity to combustible materials.
I have not seen any scenario in any of my installs where this proximity
would occur.

In summary, I believe the careful installation of PV circuits present no
measurable hazards, even with conduited wiring inside of the building
envelope and without roof top disconnects.


William Miller



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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
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