PGT-61-164 SureTest Circuit Analyzer
7
Voltage Drop (V
D
) Measurements
The SureTest measures the line voltage, applies a load on the circuit, measures the loaded
voltage, then calculates the voltage drop. Results are displayed for 12A, 15A, and 20A loads.
The National Electrical Code recommends 5% as the maximum voltage drop for branch circuits
for reasonable efficiency (NEC article 210-19, FPN 4). And, the voltage under load (V
L
) should
not drop below 108VAC for reliable equipment operation.
A good branch circuit should start out with less than 5% voltage drop at the furthest receptacle
from the panel at the end of the cable run. Then, each receptacle tested in sequence towards
the panel should show a steady decrease in voltage drop. If the voltage drop is above 5% and
does not noticeably decrease, as you get closer to the first device on the circuit, then the
problem is between the device and the panel, and the circuit breaker connections. High
resistance points can usually be identified as hot spots using an infrared thermometer or by
measuring the voltage across the breaker. If the voltage drop exceeds 5% but noticeably
decreases as you nearer the panel, the circuit may have undersized wire, too long of a cable
run, or too much current on the circuit. Check the wire to ensure that it is sized per sode and
measure the current on the branch circuit. If a voltage drop reading changes significantly from
one receptacle to the next, then the problem is a high impedance point at or between two
receptacles. It is usually located at a termination point, such as a bad splice or loose wire
connection, but it might also be a bad receptacle.
Troubleshooting Tips for Voltage Drop
Measurement
Expected
Result
Problem
Possible
Causes
Possible
Solutions
Too much load
on the circuit
Redistribute the
load on the
circuit
Undersized wire
for length of run
Check code
requirement and
re-wire if
necessary
Voltage Drop
<5%
High Voltage
Drop
High resistance
connection within
the circuit or al
the panel
Locate high
resistance
connection/
device and
repair/replace
ASCC Measurement
The SureTest calculates the Available Short-Circuit Current (ASCC) that the branch circuit can
deliver through the breaker during a bolted fault (dead-short) condition.
The ASCC is calculated by dividing the line voltage by the circuit’s line impedance (hot +
neutral). Depressing the side arrow (
→
) displays the worst-case scenario where all three
conductors (hot, neutral, ground) are shorted together – the neutral and ground provide a lower
impedance via a parallel return path. Note that this second test will trip a GFCI. See the
following equations for clarification.