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
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
Eric Lewis owns and operates Spectrum
Electrical Services in Montrose, Pa.