Thread: Transition from Type NM to NM-B wiring

I am looking for a date range. I had a 1987 condo today where the sheathing on the wiring was concealed in walls or conduit.

I could not verify by looking in the panel if the wiring was Type NM or Type NM-B. Approximately what years would we start having NM-B in place of NM?

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The 1984 NEC required the temperature rating of type NM be raised from 60C to 90C. So very close in that time frame you would start to see NM-B. The major reason for this was recessed lighting fixtures requiring wiring connection ratings greater than 60C.

Originally Posted by Roger Frazee

The 1984 NEC required the temperature rating of type NM be raised from 60C to 90C. So very close in that time frame you would start to see NM-B. The major reason for this was recessed lighting fixtures requiring wiring connection ratings greater than 60C.

I've been told the major reason for changing to NM-B and the 90C insulation was that when people started paying attention to derating for ambient temperature, and then bundling and lack of maintaining spacing, that NM with its 60C insulation basically had to be upsized just to be allowed to use NM as it had been used in the past. This was because the rating from which you would derate was so low to start with, NM-B starts with 20 amp for #14 and 30 amp for #12 (as I recall).

An often useful, but not foolproof, way to check is to carry a sample of each type of the wire around with you. The old 60 deg. wire often has thicker insulation on it than the 90 deg variety does. The black ( and if you look closely, the white) of 90 deg. insulation tend to be a bit shiny, the 60 deg. a dull color. Something to compare may help.

But, only way to be sure is find the marking on the jacket. The NEC changed the manufacturing requirements for NM to the 90 deg. insulation ( and designated as NM-B) but didn't prohibit using up old stock (and still doesn't, which is a problem). So, if a contractor had an old roll of NM laying around it could have been used years after the 90 deg. requirement was placed in the code - so no definite cut off date. And, if a jurisdiction delayed implementing the code with the 90 deg. requirement chances are pretty good stocks of the old stuff would have been shipped to that area to get used up.

Originally Posted by Hank Spinnler

I am looking for a date range. I had a 1987 condo today where the sheathing on the wiring was concealed in walls or conduit.

I could not verify by looking in the panel if the wiring was Type NM or Type NM-B. Approximately what years would we start having NM-B in place of NM?

There have been several "generations" of NM cable. NM-B was developed and referenced in the 1984 NEC. Nothing required the adoption of that edition at that time, nor prevented older inventory from being used if not prohibited by locally adopted editions with ammendments for wiring practices.

Here is an excerpt from a wonderful NFPA hosted document from UL (you can find here - clickable link: http://www.nfpa.org/assets/files//PD...ial_Wiring.pdf ) entitled ".Some History of Residential Wiring Practices in the U.S" by David Dini, P.E. of Underwriters Laboratories (C: 2006):

Although nonmetallic-sheathed cable, or NM for short, was first listed and described in the NEC in 1926, it was actually invented a few years earlier by General Cable at their Rome Wire Division in Rome, NY, and marketed under the trade name "Romex". Today many installers still refer generically to NM cable as Romex. Early NM cable had their individual conductors jacket wrapped in a cotton braid that was impregnated with either a varnish or tar-like substance for moisture protection.

Around 1950, synthetic spun rayon was being permitted to replace the cotton thread in the jacket braid. Then in the early 1960's, thermoplastic began replacing the braided jacket altogether, and by about 1970, most all NM cable had a PVC outer jacket, even through a braid was still permitted until 1984. Also in 1984, NM-B cable was developed and required to have 90 degrees C rated individual conductors, and a 75 degree C outer jacket.

Until the early 1960's, most NM cable for residential use did not have a grounding conductor. However, changes in the 1962 Code that mandated equipment grounding for all branch circuits popularized the use of NM cable with ground. Earlier versions of NM cable with ground permitted the grounding conductor to be No. 16 AWG for 14 and 12 gauge copper NM, and No. 14 AWG ground for 10 gauge copper NM. In 1969, new requirements no longer permitted an undersized grounding conductor for 14, 12 and 10 gauge NM cable.

The entire article is a GOOD summary resource/review for home inspectors. It addresses wiring types, materials, and changes in specificaitons recommend it be d/l & saved for your perusal and personal library.

I've attached it (the article) as well for your convenience.




HTH.

Madison Wisconsin presently uses the 2011 NEC code in which I see 334.112 "Conductor insulation shall be rated at 90°C" in NM cable. I see above references to permission to use up old stock of NM labeled cable. Does the 2011 code still permit old stock of NM cable to be used or was that permission deleted in a previous code update? The 2008 code uses the same language as the 2011 code though I see posts here dated in 2008.

I unknowingly reused some old NM labeled cable and my building inspector ordered me to remove it. Until this I was aware that new Romex was labeled NM-B and I didn't know codes had changed for Romex-style wire. Now I will recycle my old scraps of NM cable but am not eager to rip out the NM cable I just now carefully installed and won't if the code still permits it to be used.

Ironically the same inspector tried to prevent me from removing knob tube wiring with cloth insulation and insisted instead that I extend my knob & tube wires with more new knob & tube wiring.