Measuring the Ampere of a coil

I would like to know how can I measure the ampere of a coil, if for example I know the wattage of the coil is 200 watt and the resistance of the coil is 500 ohm and it works on 24 VAC so how much current will it uses? so i can decide type of cable to use.

I know the wattage of the coil is 200 watt and the resistance of the coil is 500 ohm and it works on 24 VAC

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The numbers don't go together at all.

A "coil" (solenoid valve, relay or contactor coil) will expose an AC resistance larger than the DC value. In any case, the current for 24 VAC can't be larger than the respective DC current 24V/500 Ohm = 0.048 A.

so for example if I have a coil heater, which is 52 ohm resistance and rated 220 VAC (I apply 220 alternative voltage from the supply) and 1000 watt, this means that the maximum current from this coil is 220/52 = 4.23 AMP. in this case I should use SSR with this specifications right??? that's all

And for my case of 24VAC, the resistance is 3 ohm(i am sorry not 500 ohm i was mistaken) and power is 200 watt, so the current will be 24/3 = 8 amp???

mohamed.elsabagh said:

so for example if I have a coil heater, which is 52 ohm resistance and rated 220 VAC (I apply 220 alternative voltage from the supply) and 1000 watt, this means that the maximum current from this coil is 220/52 = 4.23 AMP. in this case I should use SSR with this specifications right??? that's all

And for my case of 24VAC, the resistance is 3 ohm(i am sorry not 500 ohm i was mistaken) and power is 200 watt, so the current will be 24/3 = 8 amp???

Click to expand...

That would be Valid for DC...
So, If you are doing with a DC aprox. why not use I=P/V?
200W/24V= 8.333A
1000W/220V = 4.545A

To easily measure the current you can use an Ammeter Clamp or simply a multimeter

mgate said:

That would be Valid for DC...
So, If you are doing with a DC aprox. why not use I=P/V?
200W/24V= 8.333A
1000W/220V = 4.545A

To easily measure the current you can use an Ammeter Clamp or simply a multimeter

Click to expand...

I already did this, In fact i am using AC not DC. What happened is always use these rules, and choose SSR accordingly, but yesterday i wanted to do an experiment i used SSR with 5 amp and wire of 16 AWG . I expected the wires to melt down or the SSR not to work but surprisingly it worked fine, so i suspected that i am doing something wrong. I was using 24 VAC coil, 200 watt, 3 ohm resistance. I think now my calculations are valid in DC only but in AC what should I do to find the right current that will pass through the coil??

You need to know the inductance of the coil as well as the resistance, then you can calculate the current... other wise you can only measure the current... the current will be a function of the Voltage frequency, voltage amplitude and, coil resistance and coil inductance .. check here how to calculate theoretical inductance for a coil: https://en.wikipedia.org/wiki/Inductor

I mentioned the DC resistance calculation in response to your initial post, to clarify that the numbers must be wrong. As I said, the AC resistance (impedance) is always larger than the DC resistance.

If the "coil" is any kind of electromagnetic actor, e.g. solenoid valve or contactor, an inductance measurement doesn't bring you far, because the inductance is changing when the magnet is engaged. Measuring the AC current will be in fact the most simple method.

Obviously geometrical inductance formulas won't help either. An AC impedance calculation however applies:

Z² = R² + X², with R ohmic (DC) resistance, X inductive resistance (reactance) and Z total impedance,
so that Iac = Uac/Z.