DC wiring standards

Lamar Owen lowen at pari.edu
Thu Jan 26 10:04:32 CST 2012


[Digging up an older post; I let a couple of thousand NANOG posts pile up in my NANOG folder]

On Tuesday, January 03, 2012 02:40:39 PM Leigh Porter wrote:
> Does anybody know where I can find standards for DC cabling for -48v systems?

Book Resource that anyone dealing with telecom DC power systems should have on their shelf:

'DC Power System Design for Telecommunications" by Whitham D. Reeve, published by Wiley, ISBN (print) 97680471681618 and is available in the Wiley online library.

It is not an inexpensive book, but is written from the point of view of someone with 30 years of practical 'in-the-CO' experience.

The various standards for DC distribution are referenced in this volume.  If you have access to the Telcordia standards, the relevant standard is referenced in this volume (I left my copy at home, so can't quote the Telcordia standard right now). 

Saying all that, the NEC does have covering articles, and a good rule of thumb is to use black or red (or other normal AC 'HOT' color like blue, brown, orange, or yellow) for the ungrounded conductor, white or gray for the grounded conductor, and green, yellow with a green stripe, or bare for the grounding conductor (using the definitions in the NEC for those conductors).  

(In an AC circuit the grounded conductor is commonly referred to as the 'neutral' for center-tapped or wye systems, but grounded phase three-phase systems (corner-grounded) are known that have no neutral.)

In the typical 'protect the outside plant's lead sheathed buried cable' -48VDC system, the battery/rectifier positive is the grounded conductor and should be white or gray per NEC, with the negative ungrounded conductor being black, red, blue, or other approved NEC ungrounded conductor color (basically anything except an approved color for the grounded or grounding conductors) or using other site-specific and posted identifiers per the relevant NEC article(s).  I'm citing the 2008 edition of the NEC here, even though the 2011 edition is out, simply because I don't have a 2011 edition handy, and I do have a 2008 edition....and article numbers have been known to change between editions....

You can find the requirements for identifying conductors in NEC (2008) articles 250.119 (grounding conductors), 200.6 (grounded conductors), 210.5(C) (branch circuit ungrounded conductors), and 215.12 (feeder ungrounded conductors).  Examples of the colors are found in the Handbook version's exhibit 200.3, and accompanying commentary around that exhibit. (The handbook version of the NEC is worth the extra expense for the exhibits and commentary alone). 

Now, having said all that, I have seen common 'in the rack DC rectifiers with no battery' setups with black and red as negative and positive, respectively.  And, as long as neither positive nor negative are grounded, that seems to meet NEC.  As soon as you ground one conductor, and get into NEC-covered territory, you need to use white or gray (or other 200.6 approved means with the 200.7 exceptions allowed) for the grounded conductor, regardless of polarity.

Hope that helps, and doesn't overwhelm.



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