I’m seeing negative P1 values from my clamp on power meter.

I’m seeing negative P1 values from my clamp on power meter.

Question

I’m seeing negative P1 values from my clamp on power meter.

Answer

When you view data from a 3P3W2M measurement setup, you’ll see P1 and P2 values. One of those values may be negative, regardless of whether the meters have been connected to the circuit under measurement correctly. 

For example, the P1 measured value may be negative when measuring power at a motor under light load.

In a 3P3W2M connection, measurement is performed using two power meters. Since 3-phase power is calculated by adding together the P1 and P2 values (according to Blondel’s theorem), it is not problematic for either the P1 or P2 value to be negative as long as the meters have been connected properly.