In the UK, if that makes any difference, I was wiring a motor and then I wondered (and kinda needed to know), how does a plug socket know how much power to put out? because all the input ampages on all the plugs sad different things but can all be plugged into the same socket.

How does this madness work?!? XD

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    \$\begingroup\$ No, it doesn't. \$\endgroup\$ – user3528438 Mar 2 '17 at 18:37
  • \$\begingroup\$ Are you stating that the equipment you're using states an amp rating and you're wondering how the outlet "knows" to give it that current? \$\endgroup\$ – Envidia Mar 2 '17 at 18:39
  • \$\begingroup\$ Essentially, the load you plug in tells it. The load has a certain resistance, which accepts a certain current from the voltage supplied by the socket.Lower the resistance : more current, more power. Infinite resistance : no current, no power. That's what happens when you switch off. \$\endgroup\$ – Brian Drummond Mar 2 '17 at 18:39
  • \$\begingroup\$ In the first few microseconds you'd be surprised at the complexity of how the current decides on a final stable value. I presume you are talking about the stable value? \$\endgroup\$ – Andy aka Mar 2 '17 at 18:52
  • \$\begingroup\$ Yeah I am @Envidia \$\endgroup\$ – Torin Clark Mar 2 '17 at 19:08

The rating of a plug is the maximum current it can provide. The rating of equipment is the current it will actually demand. The plug provides a fixed voltage, and the equipment (the load) decides how much current to draw at that voltage.

If the load tries to draw more current than the plug can handle, then in theory a breaker should trip. In a properly designed system, the trip point of the breakers are set to a bit less than the current capacity of other parts of the system.

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  • \$\begingroup\$ Okay, so I have a small AC motor with a voltage of 230-240 and 2W. If i wire this up with just a normal plug lead with nothing else, then plug it in and turn it on, will it draw 2w? \$\endgroup\$ – Torin Clark Mar 2 '17 at 19:16
  • \$\begingroup\$ yes, it does. They grid just provides an (for anything a single person is ever going to do) infinite amount of Amperage, and the motor just takes what it needs. \$\endgroup\$ – Christian Mar 2 '17 at 19:33
  • \$\begingroup\$ Yes. ---------- \$\endgroup\$ – Olin Lathrop Mar 2 '17 at 19:33

As Olin mentioned the socket does not technically put out power, it supplies amps (current) at a fixed voltage. The load determines how much current, which translates to power, is needed and used.

If your load is too high for the fuse in the plug, or ultimately the breaker or fuse in the fuse-box, you will blow a fuse.

As for wiring your motor YOU need to figure out a few of things.

  1. What is the load of the motor, or more specifically, what is the maximum current you need to start and run it. The start current will be a lot higher than the run current.

  2. Can the circuit supplying power to the outlet safely carry that much current. When you calculate that you need to bare in mind, and add in, anything else that could be attached to this circuit. e.g. If it's on the same circuit as say the air conditioning unit you are probably out of luck.

  3. Is the main fuse-box fuse or breaker for that circuit big enough to handle that much current. DO NOT INCREASE IT IF IT IS NOT. The current breaker will probably be rated for the wire gauge of the circuit and should not be messed with.

  4. Finally, is the outlet and plug fuse rated for that current.

If the answers to any of questions 2,3, and the outlet fuse in 4 is NO.. Call an electrician to install a new circuit for your motor.

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  • \$\begingroup\$ So my motor is 2W and 240V so about 0.0083 amps. If i just use a plug and connect the wires, then turn it on, will it work or does something specify the load. Sorry I'm not familiar with the term... :/ \$\endgroup\$ – Torin Clark Mar 2 '17 at 19:35
  • \$\begingroup\$ The motor is the load, it already has the appropiate reactance to draw 0.0083A. Also you have obviously no idea what you're doing and shouldn't play with mains. You might not be able to come back and ask what went wrong! \$\endgroup\$ – Christian Mar 2 '17 at 19:36
  • \$\begingroup\$ Like I said motors have a start current that will be a lot higher than the run current, but that current does not last long unless the motor is stalled. In your case that is a small motor so just a normal plug will do, maybe with a 1A fuse in it. \$\endgroup\$ – Trevor_G Mar 2 '17 at 19:38
  • \$\begingroup\$ LOL.. @Christian. I always have this picture in my head of a queue at the pearly gates and a bunch of guys going.. "Wiring an outlet...You?... "Fixing my gas stove!" \$\endgroup\$ – Trevor_G Mar 2 '17 at 19:40

It is really very simple:

When you close the switch the electric field between the wires travels almost at the speed of light towards the load. When it reaches the load a current starts to flow.

This current travels back as a magnetic field between the wires. When the current reaches the load the voltage drops. This voltage drop travels towards the the load. When the voltage drop reaches the load the current drops. This current drop travels back to the source.

This process repeat, almost at the speed of light, until it becomes stable.

In this process some of electromagnetic field 'leaks' into the wires, heating up the wires, and some of the electromagnetic field 'leaks' into the isolation, heating up the isolation.

...and this is the simple explanation.

James Clerk Maxwell described this in exact mathematical details in 1865.

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