# Way to measure AC power in 2 wire cable

Is there a way to measure AC power/current delivered through 2 or 3 wire cable? I know that from Ampère's Law the total current of both wires is zero, but is there a way to measure power in cable in other methods?

My intention is to measure average power from typical home devices like washing machine, oven etc,but without entering the circuit. I know the RMS voltage of my net (220V) so I need to know the current. I know that there are pen testers which may figure out whether a current is flowing through a certain cable but it's not telling how much current flows.

• Are you wanting to measure power, current or both. If it's a 3 wire cable does this mean there are two circuits that are drawing power/current? More details required. – Andy aka Jun 18 '14 at 10:52
• Could you maybe give a more specific question. E.g. what order of magnitude of voltages and current are you going to measure? Do you have the possibility to cut wires in the cable for measurement or isolate individual wires? Do you have a particular measurement instrument in mind (e.g. multimeter, current clamp)? – user36113 Jun 18 '14 at 10:52
• Hi again and thanks for your comments/answers. My intention is to measure average power from typical home devices like washing machine, oven etc,but without entering the circuit. I know the RMS voltage of my net (220V) so I need to know the current. I know that there are pen testers which may figure out whether a current is flowing through a certain cable but it's not telling how much current flows. – user3712454 Jun 21 '14 at 14:49

Power is v(t)$\times$i(t) which is (voltage $\times$ current), so if you want to know the power you need both voltage and current. The instantaneous power generally varies with time and may flow in either or both directions (one at a time, obviously, but it may flow back and forth over an AC cycle, netting out in either direction or even at zero if the load is inductive or capacitive).

Usually people are interested in mean power, which represents energy flow per unit time.

So you need to know the voltage (across the wires) and the current (through at least one wire if you can assume there is no other path for current to flow). The best method will depend on whether it's 15kVAC at high current or microamperes at a volt, but generally direct connection to the wires (for voltage measurement), and either separation of them magnetically (to allow a current transformer to work) or insertion of a known resistance "shunt" (for current measurement) would be best.

• Interesting point. Why is the net current zero if the load is either inductive or capacitive? Is this an ideal assumption regarding the zero-power dissipation of these energy storage devices? Also, wouldn't RMS power be relevant in this situation as well? – sherrellbc Jun 18 '14 at 16:04
• The net power is zero because and ideal inductor or capacitor does not dissipate any power, so the load must return just as much as it took. There is no (useful) thing called "RMS power". – Spehro Pefhany Jun 18 '14 at 17:28
• I was thinking more along the lines of average power consumption, but peak power and instantaneous power are far more interesting and useful concepts. – sherrellbc Jun 18 '14 at 17:55

The related question linked below may shed some light on your search.

Basically you have to measure the alternating magnetic nearfield of the conductor so that you can estimate the current in the circuit.

When summed together the coming and going currents should cancel and the mgnetic field with them. The unbalanced nearfield that can be measured will be proportional to the actual current but will be smaller than the true amount as a result of the return conductor field cancelling some of the field.

Using a sophisticated sensor array of 3 or more multi axis magnetic field sensors around a 2/3 conductor cable could theoretically enable reasonably accurate measurement of the current flow based on the measured field using clever calculations. Cunning placement of field warping magnetic cores may make it easier to couple to the fields but may be better suited to marketing purposes than accuracy improvements.