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

[Robert Nuese]

Dear Wrenches
    For months I've been wrestling with a problem with which I'd really like
some help. From my searches of the Wrenches archives, I see that others have
been wrestling too, but it does not seem to have been decisively wooped. I'd
like to offer it up again, presented in a slightly different light, and with
a bit more documentation in one spot, and see if there is a any consensus.
The issue has been mostly dealt with in the thread  "RE: 600VDC switches".
This thread has also covered a number of other issues, but the one I'm
interested in was last addressed, I think, on 9/24/02 by John Berdner and
Bill Brooks. As they address the issue, it is a matter of fire marshals
requiring DC disconnects on roofs. I think that the more common issue for
wrenches is the simpler one of code compliance. As I read it, the code
doesn't so much require a disconnect on the roof, as it requires wiring to
run outside of the building envelope.

By my reading of the NEC, both 2002 and 1996 editions, wiring from solar
systems is not normally allowed inside of a building envelope, but should,
with few and specific exceptions, be run outside of the building, usually in
conduit.
In NEC 2002, section III of the Solar Photovoltaic Systems Article -
690.14(C)(1) states:  "The photovoltaic disconnecting means shall be
installed at a readily accessible location either outside of a building or
structure or inside nearest the point of entrance of the system conductors."
The NEC Handbook further elaborates:  "These requirements will generally
prohibit long runs of PV source and output circuits inside a building before
reaching the required PV disconnect. A short conductor run through a wall at
the point of first penetration to reach a disconnect mounted inside the
building is allowed."
The reason for this is that otherwise there are live wires inside the
building, often running near combustible surfaces, sometimes in very hidden
areas, possibly where fire could rapidly spread, without the means of
disconnection that are normally required, as they would be for say, all
branch circuits spreading through the house from a main breaker panel.
I asked John Wiles about all this. He said "readily accessible" means that
it must not be in a crawl space, and must not require a ladder to reach.
Further, I gather that the disconnect must be in the form of a small number
of switches (6?). Removable fuses would probably not be adequate, and
anything inside of a box that would require screws to be removed before
disconnecting would also not be adequate.
I've heard several installers claim that this is a new requirement of NEC
2002, and that in the many jurisdictions where NEC 1996 is the rule (my own
locale included) one can freely wire inside the building. This does not seem
to be borne out by reading the older code. NEC 1996 690-14 tells us "The
provisions of Art 230, Part F shall applyŠ"  These include 230-70(a): "The
service disconnecting means shall be installed at a readily accessible
location either outside of a building or structure, or inside nearest the
point of entrance of the service conductors." This is essentially identical
to the photovoltaic-specific language in the 2002 code.
In the past, it is possible that this requirement has not always been
applicable because photovoltaic systems were generally considered to be
"low-voltage" wiring, i.e. below 50 volts. This is not the case with
nominally 48vdc systems which typically operate at 65 vdc and above, and is
certainly far from the case with the newer high-voltage Sunny Boy systems.
When I first discussed this issue with Glen Minyard, he said it was a matter
of interpretation, and he thought it was probably OK to wire in attics.
After I showed him the NEC passages and the explanation that John Wiles had
provided, he said something to the effect of - hmm, this does seem pretty
clear, I'm starting to think the correct interpretation does rule out inside
wiring.

My own current thinking is that, until a new code comes out (and is
adopted!?), the correct way to build is:
Normally all DC wires (above 36v nominal) stay outside of building, inside
conduit, with the following exceptions:
Can go inside, in metal conduit, for relatively short, straight runs, going
to DC Disconnect.
Can go inside if there is an accessible DC Disconnect on the roof - but only
if the roof is flat enough to easily and safely walk on, and only if there
is a deck or stairway that allows one to readily get on the roof with no
ladder.
Can go inside, if it is in conduit and completely encased in 2" of masonry,
as would be allowed for a service entrance conductor as per NEC 230.6

For the future, it looks like John Berdner's idea for using metal conduit
may well fly, since this is essentially the recommended change for NEC 2005
being submitted by John Wiles. This was mentioned in Michael Welch's post of
11/4/02, "RE: Fwd: Proposed NEC Changes for 2005". [Check it out at
www.nmsu.edu/~tdi/NEC2005.pdf. The pertinent parts are: 690- 14(C)(1)
(Revised), and 690-31 (F) (New)]. John Wiles has also suggested a promising
variant that solves the problem with inverters on the roof
[(690.14(D)(New)].
But for now, is my reading correct, or is there more to it?

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