CT70 V7.0 09/10
7
Test 3: Voltage Drop Measurements
To determine voltage drop, the CT70 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.
Voltage Drop Measurement Troubleshooting Suggestions
Problems Likely
Causes
Possible Solutions
Voltage drop > 5%
Overloaded circuit
Redistribute loads
Wrong wire gauge size for
length of cable run
Check code and rewire if
necessary
High resistance connection
in the circuit or at the panel
Locate bad connection and
rewire or replace
Test 4: ASCC Measurements
The CT70 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:
ASCC = Line Voltage / Hot imp Neutral impedance
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.
