When using ohms law on a residential A/C unit I got 7.3, But when using the multimeter it dropped to 5.5. What can cause this?
Several possible reasons:
1) The resistance is different when more current flows. The ohmmeter will put a small current, measure the voltage, and calculate the resistance. The actual circuit may be using a much larger current that changes the resistance (heating element)
2) The impedance is inductive (motor) or capacitive.
3) There is a residual voltage from a capacitor. The ohmmeter requires a completely powered off circuit.