/I issue this pre disclaimer, as I don't have references to back all my
following statements up. Bill B. is very right about references./
My "Understanding" is that if a UL listed wire, cable, or device has a
voltage rating and that volt rating just says 600 V, then it applies to
both AC and DC. Some switches for instance have a volt rating that says
"120 v - AC only" so its very clear not to use those for DC.
Meanwhile, a pull chain light fixture is rated at 250 V, with no mention
of AC and DC. Us old off gridies know that these are indeed DC rated.
Another device I found says "125 VAC" , so I would assume it couldn't be
used on DC.
Some UL listed relays I have are rated differently for ampacity,
depending on whether it is 120 VAC, 240 VAC, or 30 VDC.
Now, I'll actually use a reference: NEC 2011, Article 100 definition of
"/Voltage- the greatest root mean square (RMS) difference of potential
between any two conductors of the circuit concerned"/
It also has this informational note: /Some systems , such as 3 phased 4
wire, single phase 3 wire, and 3 wire *D*irect *C*urrent, may have
various circuits of various voltages./ ( I bolded the DC part, :-) )
IF you take the RMS of a DC system, it just equals the DC voltage. Its
interesting, because this definition covers it all: AC, DC, and what is
actually often the case AC + DC.
I have always used the NEC voltage tables and code for DC, unless it
specifically says other wise. Table 310.15B7 for instance, can't be
used for DC, because it specifically says "120/240-Volt, 3 Wire, Single
Phase"
My take is that if a wire is labeled "600 V", then it is fine for both
AC and DC. If this isn't true, we're all in big trouble, as none of the
wire I have is listed specifically for DC.
I respect John Wiles, so I think this 3rd hand info lost something in
translation.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 10/21/2013 2:47 PM, Jason Szrom wrote:
Howie,
I've looked at a few different MC cable manufacturer data sheets and I can't
find a distinction between AC or DC. In general I wouldn't expect there to be a
difference since AC RMS voltage and current ratings are sort of a 'DC
equivalent'. But the only way to be sure is contact a cable manufacturer and
ask a specialist. There could be something buried in the listing. And we all
know it is expensive to get a hold of UL standards to look the damn thing up
ourselves.
Example MC cable data sheet: http://www.afcweb.com/pdf_2011/p12_mc_s_lt.pdf
I am guessing that John Wiles was referring to 690.31(E) which has to do with
running inside a building with DC conductors before hitting a DC disconnect.
This section was changed in 2011 NEC to include MC cables specifically. Maybe
from this it got lost in translation back to you?
Good luck,
Jason M. Szrom, PE
Engineering Supervisor
Solar Energy Systems, LLC
718-389-1545 x13
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Howie Michaelson
Sent: Monday, October 21, 2013 2:15 PM
To: RE-wrenches
Subject: [RE-wrenches] MC Cable for DC runs
Hi All,
Just had a conversation with an electrician friend who took a John Wiles course
for inspectors a couple of weeks ago, and he thought (but wasn't
sure) that John said in order for it to be code to use MC cable for PV homeruns
inside the building envelope, the cable needed to be rated for DC. This was
new information to me, particularly since I've never heard of MC cable being
rated specifically for DC. Is this accurate information, and does anyone know
what the wire spec would be for this?
Thanks,
Howie
--
Howie Michaelson
NABCEP Certified Solar PV Installer(tm)
Catamount Solar, LLC
Renewable Energy Systems Sales and Service VT Solar Electric & Hot Water
Incentive Partner http://www.CatamountSolar.com
802-272-0004
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