Extech Instruments CT80 User Manual page 9

Test 3: Voltage Drop Measurements
To determine voltage drop, the CT80 measures line voltage, factors in the load, measures the
loaded voltage, and then calculates the voltage drop. Results for 12A, 15A, and 20A loads are
provided. For nominal efficiency, a voltage drop of 5% is the maximum recommended by the
National Electrical Code (NEC) board. When a voltage drop measurement of less than 5% is made,
the meter's display backlight turns blue in color. If the voltage drop is higher than 5%, the meter
display appears in red.
An efficient branch circuit should have less than 5% voltage drop at the furthest receptacle from the
breaker panel at the termination of the cable run. A steady decrease in the voltage drop should then
be measured for each receptacle tested in sequence towards the breaker panel.
If the voltage drop is higher than 5% and does not noticeably decrease as the testing moves closer
to the first device on the circuit, then the problem lies between the first device and the breaker
panel. Visually check the terminations at the first device, the wiring between the device and the
panel, and the circuit breaker connections.
High resistance points can be identified as hot spots using an infrared (IR) thermometer or by
measuring the voltage across the breaker. If a voltage drop measurement exceeds 5% but
noticeably decreases as the testing moves closer to the panel, then the circuit may have an
undersized wire, too long of a cable run, or excessive current on the circuit. Check the wires to
ensure that they are sized per code and measure the current on the branch circuit. If a voltage drop
reading changes significantly from one receptacle to the next, then the problem could be a high
impedance point at or between two the receptacles. It is usually located at a termination point, such
as a bad splice or loose wire connection, but could also be a faulty receptacle.
Problems
Voltage drop > 5%
Test 4: ASCC Measurements
The CT80 calculates the ASCC (Available Short Circuit Current) that a branch circuit can deliver
through a breaker in a dead short circuit condition.
The ASCC is calculated by dividing the line voltage by the circuit's line impedance. See equation
below:
Use the ► button to simulate a situation where all three conductors (hot, neutral, and ground) are
shorted together. Note that this second test will trip a GFCI.
Shop for Extech products at:
Voltage Drop Measurement Troubleshooting Suggestions
Likely Causes
Overloaded circuit
Wrong wire gauge size for
length of cable run
High resistance connection in
the circuit or at the panel
ASCC = Line Voltage / Hot impedance + Neutral impedance
ShopExtech
9
Possible Solutions
Redistribute loads
Check code and rewire if necessary
Locate bad connection and rewire or
replace
1.877.766.5412
.ca
CT80-EN V1.1 08/12