A clamp meter measures current by sensing the magnetic field around a conductor, so nothing has to be disconnected. To measure current with a standard multimeter you must break the circuit and put the meter in series — impractical and often unsafe on a live installation. The clamp simply opens around the cable.
How it works, and the one rule that trips people
Current flowing through a conductor creates a magnetic field around it. The jaws concentrate that field onto a sensor, and the meter converts it to a current reading.
The rule that catches everyone: clamp one conductor only. Clamp a two-core flex containing live and neutral and you will read approximately zero, because the two currents are equal and opposite and their fields cancel. This is not a fault — it is the physics — and it is also the basis of how earth-leakage detection works.
AC-only or AC/DC
Cheaper clamps read AC only, using a current transformer. That covers most mains electrical work.
DC measurement requires a Hall-effect sensor, which costs more. You need it for solar installations, battery systems, vehicles and anything with a DC bus. Given how much solar is going in across Egypt, an AC/DC clamp is usually the better buy even at higher cost.
Specifications worth checking
- True RMS — essential with variable-speed drives, LED lighting and switching supplies, all of which draw non-sinusoidal current. An averaging meter can read substantially low on these.
- Jaw opening — must fit around the largest cable you work with. Main incomers on commercial boards can be surprisingly thick.
- Inrush measurement — captures the starting surge of a motor. This is the specification most often needed and most often missing.
- Low current resolution — for measuring standby draw, resolution below one amp matters.
Why inrush matters in practice
A motor draws several times its running current at start, for a fraction of a second. That surge is what nuisance-trips breakers and what damages undersized contactors and timers.
Without an inrush function the meter simply averages through the event and shows a normal figure. With it, you get the actual peak — which is the number you need when sizing protection.
Safety category
CAT ratings describe where a meter may safely be used. CAT III is appropriate for distribution boards; CAT IV for the supply side. This is not marketing: the rating reflects how much transient energy the instrument can survive without failing dangerously in your hand. Match it to the work.
See Electrical Test & Clamp Meters and Power Meters & Battery Testers.
Frequently asked questions
Why does my clamp read zero on a power cable?
You are clamping both conductors and their fields are cancelling. Separate a single core and clamp that.
Can I measure solar DC current?
Only with an AC/DC clamp using a Hall-effect sensor. An AC-only clamp will read nothing.
Is True RMS worth the extra?
Yes. Most modern loads are non-sinusoidal, and an averaging meter can be significantly wrong on them.