OPCOM

There is the alternative of a shunt type meter for measuring high currents. The shunt takes the place of the meter movement and is mounted in a location convenient to the high-amperage wiring, and then a pair of relatively small wires runs to the meter itself which is commonly designed to read full scale at 50 millivolts. The shunt has very large bolts for the electrical contacts, so it has less chance of causing a problem down the road. Makes it easy to have a 300 amp meter in a location where you could actually read it.

On your voltmeter, once you have it removed, just connect 24V to the center pin and the negative side of the case, it should indicate the voltage.