CT70/CT80 How does the Voltage Drop test work without tripping the circuit breaker?

The CT tester checks the voltage drop on the power line by applying a short circuit to the power line at a low voltage point on the rising voltage cycle.  The meter measures the Peak current drawn by that short circuit test.  This measured peak current is compared to the Expected max current of the circuit.  The resulting percentage of current is displayed.  A higher than expected percentage value indicates that there is a higher than expected impedance somewhere in the circuit.

Note: The total power consumed during the Voltage drop test is kept very low so as not to put stress on any of the branch circuit splices and connections. 

Start your testing on the furthest receptacle from the electrical panel.  If you make measurements on this branch circuit that are out of specification, measure receptacles closer and closer to the panel. 
If you see a gradual change in percentage as you get closer to the Electrical panel the wire size is most likely undersized for this branch circuit.  If there is a big change in percentage  from the last measured receptacle to the next one closer to the panel, there is most likely a high resistance splice or connection between these 2 receptacles.


Examples – The branch circuit is designed as a 15 amp circuit

The CT meter is set to the 15 amp range and the button is pressed. The meter waits for the zero crossing of the rising Voltage cycle and when the voltage reaches its trigger point, the meter applies a short circuit to the line.  The Peak current pulse is measured and a percentage is calculated.

15 amp line
CT measured -  13.5 Amps
The Voltage drop percentage is    13.5 / 15 = .90   x 100 = 90.0 %
The voltage drop percentage = 10.0%   (100% – 90.0% = 10.0%)
This branch circuit does NOT meet acceptable limits and appears to have a higher than expected impedance somewhere in the circuit.

15 amp line
CT measured 14.5 Amps
The Voltage drop percentage is    14.5 / 15 = .966   x 100 = 96.6 %
The voltage drop percentage = 3.4%   (100% – 96.6% = 3.4%)
This branch circuit meets acceptable limits.

Steve Hyde
Extech – Tech Support


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