[Nathan Slingerland]

As Nick3092 mentioned, the insulation material and thickness would have more to do with the voltage rating. The conductor gauge and material would affect how much current the wire can safely carry without heating up too much.

As a means for making some quantitative comparisons of the insulation material I took some measurements with my handy digital calipers:

Code:

               Philco Original      Rhode Island Wire      Radio Daze W6ST-18-25      

 Diameter      2.27mm               2.90mm                 3.41mm

    Gauge      22ga                 18ga                   18ga

Insulation      0.52mm (rubber?)     0.81mm (plastic)       0.96mm (plastic)

   Jacket      0.35mm (cloth)       0.37mm (cloth)         0.36mm (cloth)

The Radio Daze wire serves as the reference wire since it is rated at 600V and is the same wire gauge as the Rhode Island wire.

Of course the physical dimensions don't tell the whole story. Here are listed dielectric strengths extracted from the CRC Handbook of Chemistry:
thermoplastic polymers (polypropylene, polystyrene): 15.0 - 23.6 kV/mm
extrusion compounds (polyethylene, polyvinyl chloride): 11.8 - 18.9 kV/mm
natural rubber: 100-215 kV/mm
butyl rubber: 23.6 kV/mm
neoprene: 15.7-27.6 kV/mm
silicone rubber: 26-36 kV/mm
mica: 118 kV/mm

So the insulating material makes a huge difference. If the original wire really is rubber coated internally that probably explains why it is sufficient for high voltages even though it is small by comparison. Even for the new wires, the specific plastic used makes a difference.

Even if we assume that the new wire's insulation has a strength of 11.8 kV/mm (the lowest of the listed plastic insulators) then it would be expected to break down around 11.8 kV/mm * 0.81mm = 9.558kV = 956V. This also ignores the insulating effect of the cotton jacket.

Based on these back of the envelope calculations I think I feel OK using them for my own restoration projects.