Radio Shack 22-218 Owner's Manual page 30

To determine the line-to-line voltage, multiply the rated line-to-
ground voltage by 1.732. For example, if the rated voltage is
600 volts, the line-to-line voltage would be:
600 Volts x 1.732 = 1039.2 Volts
In this example, the voltage exceeds the meter's range and
should not be measured.
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