A clamp meter lets you measure current flowing through a conductor without touching it or breaking the circuit — simply by clamping its jaw around the wire. This makes it the fastest and safest tool for checking load current in live panels, motor control centres, and distribution boards.
How Does a Clamp Meter Work? The Physics
AC current creates a magnetic field
Alternating current in the conductor generates a changing magnetic field (B-field) around the wire — described by Faraday's Law.
The jaw closes the magnetic circuit
The ferrite iron-core jaw forms a complete toroidal transformer around the conductor, concentrating the B-field.
Secondary current is induced and measured
A secondary winding in the jaw produces a current proportional to the primary (conductor) current. The meter reads this and displays the result.
AC only vs True DC clamp meters
Traditional clamp meters only work on AC because they rely on a changing magnetic field. True DC clamp meters use a Hall Effect sensor in the jaw gap, which can detect the static magnetic field produced by DC current. If you need to measure DC current, check that your meter has this feature explicitly stated.
Parts of a Clamp Meter — Explained
Jaws / Clamp
The split ferrite core that encircles the conductor. Must close completely for accurate readings.
Jaw Trigger
Press to open the jaws — release to clamp around the conductor. Handle carefully on live conductors.
LCD Display
Shows the measured value with unit prefix (A, mA, V, Ω). Backlit on most modern meters.
Function Selector
Switches between AC current, DC current, AC/DC voltage, resistance, continuity, Hz, and more.
HOLD Button
Freezes the display reading — invaluable in tight or overhead panel work where you can't see the meter.
INRUSH / PEAK
Captures motor start-up inrush current spikes that last only milliseconds — invisible to normal mode.
COM Terminal
Black probe connection — the common/negative reference for all probe-based measurements.
VΩ Terminal
Red probe for voltage and resistance — plug in only when using the meter in multimeter mode.
Step-by-Step: Measuring AC Current
Select AC Current (A~)
Turn the dial to AC current mode. Choose a range higher than expected, or use auto-range.
Open the jaws fully
Press the trigger and open the clamp to its maximum — wide enough to fit around the conductor.
Clamp around ONE conductor only
This is the most common mistake — clamping both live and neutral cancels the magnetic fields and gives a zero reading. One wire at a time.
Centre the conductor in the jaw
Position the wire in the middle of the jaw opening, not at the edge. The magnetic circuit is most linear at the centre.
Close the jaws completely
Any gap in the jaw introduces air into the magnetic circuit, significantly reducing accuracy. Listen for the click.
Read and hold
Press HOLD to freeze the reading if the cable is in a confined or overhead location.
Measuring DC Current (Hall Effect Meters)
Select DC Current (A⎓)
Ensure your meter has Hall Effect DC measurement — not all clamp meters do.
Zero the reading
With jaws open and away from conductors, press ZERO / REL to null out residual magnetic field offset. This step is critical for DC accuracy.
Clamp around one conductor
Clamp around the positive or negative conductor only.
Check the sign
A negative reading means current direction is opposite. Flip the jaw orientation to get a positive reading.
Using Probes for Voltage & Resistance
Most clamp meters also include probe terminals (COM and VΩ) for conventional multimeter-style measurements:
Voltage
ACV or DCV
Touch probes across supply terminals, just like a multimeter. No need to open the jaw.
Resistance
Ω mode
Circuit must be de-energised. Touch probes across the component. OL = open circuit.
Continuity
Beep mode
Beeps when resistance is below ~30–50 Ω. Eyes-free confirmation of a complete path.
Measuring Motor Inrush Current
When a motor starts, it draws a brief but large inrush current — typically 6–10× the normal running current — lasting only milliseconds. Standard mode misses this entirely. Use INRUSH or PEAK HOLD:
Inrush Measurement Procedure
Clamp the meter around the motor supply conductor.
Press and hold the INRUSH or PEAK HOLD button.
Start the motor.
Release the button — the peak reading is captured and held on the display.
Compare to nameplate inrush rating. Unusually high inrush may indicate a fault.
Common Mistakes — and How to Avoid Them
Clamping around two conductors
The magnetic fields of live and neutral cancel out — the meter reads zero or near-zero regardless of load. Always clamp ONE conductor.
Not zeroing before DC measurement
Residual magnetism in the jaw core gives a false offset on DC readings. Always press ZERO with jaws open before clamping for DC.
Partially open jaws
The air gap dramatically increases the magnetic circuit reluctance. Close until you hear/feel the click.
Conductor at the jaw edge
The magnetic circuit is non-uniform at the edges. Centre the wire in the jaw opening for the most linear response.
Using an AC-only clamp for DC
An AC clamp transformer gives zero output on DC. Check the spec sheet — you need Hall Effect for DC current.
CIE has manufactured clamp meters for industrial and utility applications since 1963. Browse our clamp meter range or contact us for a product recommendation matched to your application.