How many amps will flow through a 20-amp circuit breaker?

In this example, we have a 200 amp main service, with a #12 Romex cable connected to a 20 amp branch circuit breaker in the main panel, which runs across the basement to a washing machine receptacle. Not realizing that you have turned off the wrong breaker, you begin to replace the worn-out receptacle, and as you are poking around with your screwdriver, you cause a dead short. POOF! And the breaker trips off.

How many amps flowed through the branch breaker before it tripped?

A. 20 amps.

B. 200 amps.

C. The actual fault (short-circuit) current available from the power company (1000's of amps).

D. A percentage of the actual fault current available, limited by the resistance in the wiring.


So far, 3 out of the 4 replies are correct. Which one is wrong? (I don't have voting powers yet.)

This is a hypothetical case, so I'm not expecting exact figures, but one of the choices given is much closer than the others. I am aware of the "interrupting" ratings of circuit breakers, and I am also aware that in answer D the word "impedance" would be more proper than "resistance", but I am intentionally trying to keep this simple.

Update 2:

Jacko, while it is true that the breaker will open if you exceed 20 amps, under a short-circuit condition much more than 20 amps will flow UNTIL the breaker opens.

The other 3 replies are correct. I chose BobK as best answer because he used one of the answer choices I provided, and mentioned the important fact that it takes some time for the breaker to open, during which currents much higher than the breaker's trip rating will flow.

5 Answers

  • 9 years ago
    Favorite Answer

    If I'm reading your problem correctly, I say the answer is D.

    At the time of the short, the current will be I=V/R according to ohm's law. The circuit breaker will not instantly trip so the current will go much higher than the breaker's 20 amp rating. After the breaker senses the massive current, it will trip to shut off the current. The time between short and trip should be a fraction of a second, but the current will get very high during that short time.

  • Anonymous
    5 years ago

    The amp rating on the breaker identifies the trip point or the amount of load that the breaker can handle. Up grading a breaker can be a very complex issue. Usually the breaker is matched with a Gage of wire and other components in the circuit and changing just the breaker could put something else in danger and that item may not be able to "Trip" like the breaker. The safest solution is to reduce the load on the existing breaker or have a qualified electrician come in and evaluate the situation, make recommendations and then you decide what you want to do or can afford to do. NOTE: It is not safe to knowingly use a breaker that is overloaded and tripping. Eventually the breaker will fail and could result in a serious fire.

  • Anonymous
    9 years ago

    Short circuit current is only limited by what is known as the "fault loop impedance" which is the "resistance" of the wiring from the supply transformer on the street, all the way to your receptacle and back to the transformer.

    If the fault loop impedance was zero, then the short circuit current would be whatever the power station could supply!

    once you have measured the fault loop impedance with a special meter, you can work out the prospective short circuit current, I=V/R

    Typically this will be from hundreds of amps to thousands.

    It is important to note that circuit breakers are rated in more than one way. Not only is there a fusing current (in your case 20A) but also a maximum fault current rating, typically 2 or 4 thousand amps for a household circuit breaker. This is the amount of short circuit current the breaker can safely interrupt. Any more current than this and the breaker may not function correctly.

    I have installed very large (loaf of bread size) circuit breakers, only rated at 60A, but with a fault current rating of 50,000A because the fault loop impedance was so low.

    In your situation, As an absolute guess I would say somwhere between 800A and 2000A would be typical from what you have explained above. just a guess though!

    Hope this helps.

  • Jacko
    Lv 6
    9 years ago

    A 20 Amp circuit breaker will carry 20 amps without tripping. Any more than that will cause it to trip.

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  • 9 years ago

    D. The actual fault (short circuit) current limited by resistance in the wiring

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