- Q. After my electrician finished rough wiring the last house I built, I noticed that the Romex (NM) wire he used seemed to have a thinner profile, and the individual wires seemed smaller in diameter than what I was used to. Is the copper wire (or the insulation) getting smaller, or is it just my imagination?
A.Eric Lewis responds: No, your imagination is not getting the best of you — the nonmetallic sheathed cable (NM) used in most residential construction has become smaller.
NM wire is made up of four basic components: the copper conductor, the conductor insulation, the cable filler, and the outside jacket. The diameters of the different-gauge conductors has remained the same for years, and is unlikely to change.
But recent improvements in the plastics used to produce the three remaining components have resulted in a more compact NM cable with better insulating characteristics, and a higher abrasive resistance.
The most common conductor insulation today is THHN, which measures 15 mils on a #14 conductor, whereas the older THW conductor insulation measures 30 mils. The cable filler separating the conductor insulation from the outside casing is now made from plastic and is thinner than the traditional paper filler found in older NM wire. The thickness of the outside jacket has also been reduced. The result is a NM cable with a smaller circumference, a tougher outside casing, and a smaller conductor area that requires less filler.
How much smaller is the new NM cable? It used to be that only two 14/3 NM cables would fit through a 1-inch-diameter hole — now you can fit three.
Eric Lewis owns and operates Spectrum Electrical Services in Montrose, Pa.