When I saw several electricians commenting on not knowing of a tool which tests peak current draw from an outlet in this question: "Is the Electrician trying to scam me?", it made me wonder: Is there is a safe and low-liability way to determine whether a circuit allows pulling more current than the circuit is rated for by code without inspecting all of the wires on the circuit?

Note: This question is for situations where someone may have illegally tapped off of a high-amperage circuit to power a lower-amperage circuit and you cannot tell whether the circuit was wired correctly simply by looking at the wire gauge at the breaker panel/box. E.g. If someone illegally splices a wire for a series of kitchen outlets (20A max) to the circuit for an electric range (e.g. 30A).

  • Do you mean commercial, or one to build yourself? You can surely just quickly connect a fat ptc
    – PlasmaHH
    Commented Sep 16, 2015 at 15:13
  • 1
    @PlasmaHH Personally, commercial solutions would be preferred so long as they aren't overly expensive for the job of performing home inspections. Could you elaborate on a "Fat PTC"? A quick google search on this term turns up hits related to genetic research on fats. Commented Sep 16, 2015 at 15:21
  • See also the question: How do I test circuit breakers?. Commented Sep 16, 2015 at 15:35

3 Answers 3


TL;DR: There is not a 100% safe or 0-liability option to intentionally pull more current through a circuit than allowed by applicable codes.

The cheapest design I can think of is a non-contact clamp ammeter that graphs current clamped to a piece of thick gauge wire. Turn off power to the circuit and then short the thick gauge wire between the outlet terminals. Flip the breaker back on to let it trip and then look at the max current pulled. However, this seems like a huge liability should the circuit pull significantly more current than its rated capacity because it could overheat and either immediately start a fire or pose a potential fire hazard in the future. I personally would never do this and never recommend it since it seems like the test poses too much of the hazard you would be trying to prevent.

The next design I can think of creating also uses a clamp ammeter that graphs current clamped around a wire to a test load. You would create/buy a series of test loads sized to pull just a little more current than the circuit's wire gauge should allow if the circuit is wired correctly and operating normally (e.g. the circuit breaker isn't fault), but not so much as to pose a significant fire hazard over a short period of time. As an example: Use a 5 ohm power resistor inside of a high-heat insulated enclosure to limit current to 24 amps through a 120V / 20A circuit. Similar to the first design, you would turn off power to the circuit, plug the test load into the outlet, restore power, and then remove power if the over-current protection device does not turn off power to the circuit within a second. However, I don't have the data to support that this is 100% safe and it still seems like you could potentially be opening yourself up to future liability as an electrician by performing a test this way. I therefore could not recommend either of these methods or any method that intentionally pulls more current through a circuit than allowed by the applicable codes because there is always some room for liability if you accidentally damage the wiring by pulling more current through the circuit than allowed.


There's no safe way I'm aware of to test if the wires installed are able of supporting the safety equipment that's connected to them, short of physically verifying the gauge of the wire. In fact, many fixture wires can't support 15-20 amps even though they are plugged into a receptacle or attached to an outlet that can. These wires rely on the fixture or appliance not pulling more than the wire safely provide. E.g. you don't wire the inside of a 5 watt night light with enough copper to support 20 amps because the night light itself shouldn't pull more than 0.05 amps. From the perspective of the circuit breaker, there's no way to know the difference between the night light and the wiring inside the wall.

If you are worried about improper taps between circuits, you can try shutting off all the breakers, and then one by one turning on a breaker and checking for voltage on the hot wire of all the other powered off breakers. You can also shutoff power to all but the high amp breakers and check for any lights or receptacles that are receiving power. Both of these test should be performed with a non-contact tester if possible.

The clamp on amp meters are good for determining why a breaker is tripping, or to verify that it is tripping when current exceeds the safe minimum, but less useful to tell if there's an illegal tap since current needs to travel through the wire to be tested. It could be placed on an outlet with a high usage to see if that usage exceeds the safe minimum. But if you measure at the breaker and a separate circuit has been tapped, it wouldn't tell you that power is traveling through that other circuit.


I would either selectively shut off every circuit and label each outlet affected until all outlets and fixtures were accounted for or I would use a circuit tracer to positively locate every circuit. It is simply a matter of locating all the connected loads for every circuit breaker in the panel and that is very doable. Also, inexpensive circuit tracers and voltage testers suitable for this task are in stock at most hardware stores.

I once found the exact situation you describe in a house that I owned. Some yahoo had remodeled the bathroom and spliced the bathroom sink receptacle to one leg of a 50amp range circuit within the middle of a wall. It was a seriously hazardous condition.

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