Application Note

2 Fluke Corporation Proper use of a clamp meter in commercial and residential settings
Clamp meters in
residential applications
For residential electricians,
clamps are a necessity to mea-
sure loads on individual branch
circuits at the service panel.
While a spot check of current
is often sufficient, sometimes it
doesn’t provide the full picture
as loads are switching on and
off, going through cycles, etc.
Voltage should be stable in an
electrical system, but current
can be very dynamic. To check
the peak or worst-case loading
on a circuit, use a clamp with
a min/max function which is
designed to catch high cur-
rents that exist for longer than
100 ms, or about eight cycles.
These currents lead to intermit-
tent overload conditions which
can cause nuisance tripping of
circuit breakers.
Take measurements on the
load side of the circuit breaker
or fuse. The breaker will open
the circuit in the event of an
accidental short circuit. This is
especially important with any
kind of direct-contact voltage
measurement. Even though
clamp jaws are insulated and
therefore have a level of pro-
tection that doesn’t exist with
direct-contact voltage measure-
ment, it’s still a good idea to be
cautious.
A common problem in
residential electrical work is
mapping outlets to breakers. A
clamp can be useful in identify-
ing which circuit a particular
outlet is on. First take a base-
line reading, at the service
panel, of the existing current
on the circuit. Then put the
clamp in min/max mode. Go to
the outlet in question, plug in
a load—a hair dryer is ideal—
and turn it on for a second or
two. Check the clamp to see
if the max current reading has
changed. A hair dryer will
typically draw 10-13 amps, so
there should be a noticeable
difference. If the reading is the
same, you’ve got the wrong
breaker.
Clamp meters in
commercial environments
Clamp meters are used at the
panelboard to measure circuit
loading on feeders as well as
on branch circuits. Measure-
ments on branch circuits should
always be made at the load
side of the breaker or fuse.
• Feeder cables should be
checked for balance as well
as loading: current on all
three phases should be more
or less the same, to minimize
the return current on the
neutral.
• The neutral should also be
checked for overloading.
With harmonic loads, it’s
possible for the neutral to be
carrying more current than
a feeder—even if the feeders
are balanced.
• Each branch circuit should
also be checked for possible
overloading.
• Finally, the grounding circuit
should be checked. There
should be minimal current on
the ground.
Testing for leakage
currents
To check if there is leakage
current on a branch circuit,
put both the hot and neutral
wires in the jaws of the clamp.
Any current that is measured
is leakage current, i.e., current
returning on the ground circuit.
The supply (black wire) and
return (white wire) currents
generate opposing magnetic
fields. The currents should be
equal (and opposite) and the
opposing fields should cancel
each other out. If they don’t,
that means that some cur-
rent, called leakage current, is
returning on another path, and
the only other available path is
the ground.
If you do detect a net cur-
rent between the supply and
return, consider the nature
of the load and the circuit. A
mis-wired circuit can have up
to half of the total load current
straying through the ground
system. If the measured cur-
rent is very high, you probably
have a wiring problem. Leakage
current may also be caused by
leaky loads or poor insulation.
Motors with worn windings or
moisture in fixtures are common
culprits. If you suspect exces-
sive leakage, a de-energized
test using a megohmmeter will
help evaluate the integrity of
the circuit’s insulation and help
identity if and where a problem
exists.
Measuring individual
loads
To measure individual loads,
you can use a break-out cord
at the receptacle. This is simply
an extension-type cable where
the outer insulation is stripped
so that the black, white and
green wires are exposed. It’s
a lot easier than taking the
receptacle out to get at a wire.
Plug the load into the cable and
the cable into the outlet. To
measure load current, clamp the
black wire. Make the ground
current check directly on the
green cable or on the black and
white wire together.
Motors and motor
control circuits
One of the most challeng-
ing places to make current
measurements is in a control
circuit cabinet, especially if it
uses IEC-style components.
European-originated IEC-style
components are much more
compact than the equivalent
NEMA parts, and the wiring
can be packed pretty tight. The
tapered jaw and “backlight”
function of the Fluke 370 Series
clamp meters are well suited to
this measurement task.
Three-phase induction
motors are commonly used in
commercial buildings to drive
fan and pump loads. Motors
can either be controlled by
electromechanical starters or by
electronic variable speed drives.