[RE-wrenches] MC Cable for DC runs

[Ray Walters]

/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: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] 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
>
>
>
>
>
> _______________________________________________
> List sponsored by Home Power magazine
>
> List Address: RE-wrenches at lists.re-wrenches.org
>
> Change email address & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>
> List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
>
> Check out participant bios:
> www.members.re-wrenches.org
>
> _______________________________________________
> List sponsored by Home Power magazine
>
> List Address: RE-wrenches at lists.re-wrenches.org
>
> Change email address & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>
> List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
>
> Check out participant bios:
> www.members.re-wrenches.org
>
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20131021/6f948133/attachment-0004.html>