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I want to know how much power is still available on a circuit. I went to the electric supply store but they could not locate the device I described... I want to plug a device into an outlet and slowly increase the draw until the breaker pops so I know how many more things I can plug into the outlet.

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    Hello, and welcome to Home Improvement. As others have said, what you describe would be dangerous, as breakers aren't perfect when you're running near (or a bit above) the limit. And, you should probably take our tour so you'll know how best to participate here. Nov 11 '19 at 1:49
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Intentionally overloading breakers isn't a good thing to do and I don't know of any device that does that. What you can do is add up the wattage on the circuit and divide by the circuit voltage. A 20 Amp breaker will handle 2400 watts at 120 volts; a 15 Amp breaker will handle 1800 watts at 120 Volts.

You can also buy an AMPROBE which is a meter to measure current. You clamp it on to a single wire in the circuit and it measures the current being drawn.

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It's called a Kill-A-Watt

A testing instrument that tells you what your appliances are currently drawing.

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But you don't load up the circuit to breaker trip, because that would be reckless and unsafe. You use it (or them) measure the actual current draw of the other loads and add them up.

You must also multiply any continuous loads by 125% when factoring.

Example: You have a 20A circuit.

  • 1 amp of floor lamp
  • 5 amps (a 4 amp PC, on continuously so rated 125%)
  • 8 amps of air conditioner

That leaves 6 amps usable for a non-continuous load, or 4.8A for a continuous load.

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I think the safer way to go about this is to determine what else is connected to that breaker. Then total up the amps of everything. (Don't forget about the potential unused outlets that could consume current someday). I believe Harper's derating (125%) is important as well. Many things will tell you the maximum current they consume (printed somewhere). The meter Harper mentions will tell you what it is consuming at a point in time (similar for Jack's amp meter/probe). Use a meter if a device's current consumption is unknown, but be aware it's possible it could consume more at times (for example, have current peaks like a motor has when starting up).

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