The difference between SPT-1 and SPT-2 wire is the thickness of the insulation surrounding the copper wire. SPT-1 wire has a thinner insulation than SPT-2.
18/2, SPT-1 wire has an insulation thickness of .030″ and 18/2, SPT-2 has insulation that is .045″ thick.
UL recommends using SPT-2 when building table lamps. However, because the thickness of the insulation is thicker it is more difficult to to use with our 1/8ips nipples and pipe. It does fit but the fit is tight.