why you ruin my day spruce 
how can i check the wire for voltage?
Sorry.
If you have a Shurflo 100 psi pump, it will draw about 8 amps under load, ie., when its pumping. At 12volts that is virtually (slightly less) than your 2 front head light bulbs on full beam. (8 x 12 = 96 watts).
The courtesy light in your vehicle will only draw 8 watts at the most, usually less. So if you look at the wires going to your headlamps you will see that they are much thicker than the wires going to your courtesy light. Remember that the wire is thick enough to carry the current for that head light only ie., 55 watts.
The feed wire to your cigarette lighter is usually protected by a 10 amp fuse, and that is quite thick.
On the other end on the scale, the wire going to your starter motor from the battery is very thick, as it has to carry a large amperage to start the engine. If you measured the voltage at the end of each of these cables they would all read 12 volt.
The same principal applies in your house.
The cable the supplies the lights is thinner than the cables that supply your power sockets on the wall. The cable that supplies the electric cooker is much thicker again as it has to carry a higher amperage (the volts will always be the same ie., 230v.)
In each of our vans we have used 2.5mm 3 core cable (rated at 18.5amps at 230v) that you would used on an electric iron, electric heater etc. and these have been fine. Each cable is protected by a 10 amp inline fuse close to the leisure battery to protect the cable. 1 wire supplies the power to 1 pump. This maybe overkill, but it hasn't let us down yet. The cable from the van battery to the leisure battery via a split charge relay is also the same cable.
Spruce.