The insulation on copper winding wire currently used for pickups is a type of polyurethane whose melting point is the same as that of solder. This is no coincidence but is the product of several years of research.
Rowe Industries first began manufacturing pickups, that wire was
covered with an enamel varnish, as was standard for the time. The
melting temperature of that varnish was much higher than the melting
point of solder, so that the varnish could only be removed safely by
the use of a fine emery paper. This method was time-consuming and
resulted in a high wire breakage rate which affected costs.
Copper winding wire manufactured in America is produced in a range of diameters known as AWG – American Wire Gauge. As the diameter of the wire decreases, the AWG number increases, as can be seen in the table below. Britain uses a similar system called the Standard Wire Gauge or SWG which is slightly different dimensionally to SWG. Continental Europe bypasses the confusion of a Gauge system by using dimensions.
A range of typical wire gauges as used in guitar pickups is shown below:
Note that as the AWG number increases, the diameter of the wire decreases. Also, the AWG number refers to a bare wire so that, as seen in the table above, a bare wire of AWG 42 has a diameter of 0.0025 Inches (0.064 mm.) but if the insulation thickness is included, the overall diameter becomes 0.003 Inches (0.076 mm.)
The thickness of insulation depends upon the type used, with the older enamel varnishes being in some cases thicker than the polyurethane types. Also, insulation can be applied to the copper wire in more than one layer, which results in a greater overall diameter for the same copper conductor size.
The availability of a choice of insulation thicknesses may not seem important for a guitar pickup but if a vintage pickup is to be rewound accurately, the correct AWG copper wire, thickness of insulation and number of turns all must be used.
Take for example, a 1954 Fender Stratocater pickup. According to an analysis of one of these pickups by Seymour Duncan'a Research Laboratory, the wire type was 42 AWG with Formvar Heavy Build Insulation as opposed to Single Build Insulation. This resulted in an overall diameter of 0.003 Inches for the wire. This diameter is coincidentally the same as that shown in the Essex Wire table above.
vintage pickup fans maintain that only Formvar insulation will ensure
an authentic rewind for this pickup, but this is simply not true. Formvar, in common with all other types of winding wire insulation, has no effect on the pickup's magnetic field or it's sound. The thickness of the insulation is the only consideration.
However, If a pickup is rewound with a wire of the same gauge and the same number of turns as before but with thinner insulation, it's sound will not be as before. This is because the coil will be thinner than before, resulting in a smaller magnetic 'footprint' under the string. If the coil is wound to the same outer dimension as before, it's DC resistance and impedance will be increased, resulting in yet another change in it's sound.