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Clamp Meters and Apparent Power

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Would be grateful if someone would sanity check my understanding of AC current meters..

If you have an AC device that's heavily reactive and consumes a lot of apparent power, am I right in saying that your run of the mill current meter (clamp or moving iron 2 wire) will register the reactive current flowing into and out of it?

So a clamp meter on the primary of an MOT (say) with open secondary would register the 3 amps of reactive current?

How does the real power consumed on top of this show on the meter? It's just a larger value right?

Can't find anything in meter manuals on this. Lots of articles explaining the difference between apparent and real power, but nothing on meters.

And you need a 4 wire setup (voltage and current) with phase measurement for real power measurements - something that clamp meters alone will never achieve.


A clamp-meter only measures the current, so it can not calculate the active power correctly for reactive loads.

If the load is resistive, for example an incandescent light bulb, then the active power can be calculated close enough assuming the nominal mains voltage is known.  Unless the load is a big AC motor, or some heavily switched load without PFC (Power Factor Correction), then measuring only the current (i.e. with a clamp meter) can be close enough to calculate the active power.

For high precision measurements, or in any energy meters, all 3 must be known (U, I, and phase between U and I).

Hi RoGeorge,

I'm aware of the theory.

My question was; does a clamp meter (or two wire ammeter) register (measure) purely reactive current.

Or to put the question another way, if you were to put a near ideal inductor across an ac supply (theoretically), would a clamp meter  register the current in any one wire feeding it?

In the above example, no real power is expended in the inductor, the current used to magnetize the inductor is fed back to the supply (ignoring resistive losses), so the net current flow is near zero. I think this is why some older mechanical kwh house meters don't record (or bill you) for reactive power used.


--- Quote from: killingtime on March 20, 2023, 07:01:04 pm ---If you have an AC device that's heavily reactive and consumes a lot of apparent power

--- End quote ---

reactive load doesn't consume power, it just get it store for a very small period of time and then returns it back to the mains on the AC next cycle. So, when your device has reactive part of impedance, then there is active part of power which is consumed by device and reactive part of power which flows back and forth on every AC cycle from mains to device and back from device to mains. Usually home power counters doesn't count reactive part of power they should take into account just active one. But industrial power counters (for factory and high power machines) can count both - active and reactive part of power separately. This is because reactive part of power add energy loss in wires (due to wire heating), and when load consumes very high power that losses may be significant and needs to be taken into account.

Since reactive current flows back and forth through wire in both directions, you cannot see it with a clamp meter, just because clamp meter measure average current over some period of time, so when there are two currents +Ir and -Ir which flows in opposite direction during AC cycle, they will cancel each other on average result. But it depends on AC meter implementation, in some cases reactive current can affect measurement result in unpredictable way, so you can see some random error on some meters when there is reactive current is present. Reactive current can distort AC sine wave, so it also can affect measurement, because AC current measurement depends on a waveform.

As said before, in order to measure reactive power, there is need both measurement simultaneously - voltage and current and phase delay between them.

CA F205 Clamp Meter

not sure if this is "run of the mill" but it looks like it will give VA var and pf


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