How Many Amps Will Flow in a Short Circuit Powered by a 120V, 20A Breaker?

In the Case of a Short Circuit Occurring in a 120V Circuit Powered by a 20-Amp Breaker, How Many Amps will Flow Before the Breaker Trips?

There is no “one-size-fits-all” answer to this question since multiple factors affect the amount of fault current flowing in a short circuit path. The amount of current in amps that will flow in a short circuit depends on the resistance of the circuit. In an ideal short circuit, where the resistance is zero, the current flow would theoretically be infinite according to Ohm’s Law (I = V ÷ R). However, in reality, no circuit has zero resistance.

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How to Find the Value of Short Circuit Current in a 120/240V Circuit?

When you short a circuit, you create a low-resistance path that allows a very high current to flow. The amount of current that flows during a short circuit is determined by the voltage of the circuit and the resistance of the short circuit path.

Ohm’s Law can be used to calculate the current (I) in a circuit:

I = V ÷ R​

where:

• $I$ is the Current in Amperes,
• $V$ is the Voltage in Volts, and
• $R$ is the resistance in Ohms (Ω).

In the case of a short circuit, the resistance is very low, approaching zero. Therefore, the current becomes extremely high.

Theoretically,

• I = V ÷ R​
• I = 120V ÷ 0Ω
• I ≈ ∞

For practical purposes, when a short circuit occurs, the current can be limited by the internal resistance of the power source, the wiring, and the source impedance of the power supply. These factors may prevent the current from reaching an infinite value, but it can still be very high.

The purpose of a circuit breaker is to protect the circuit and the connected devices from excessive current. A 20-amp breaker is designed to trip when the current exceeds 20 amps. That’s why a 20 amp circuit breaker should be used having a safety factor for 16 amp of load circuit (16A x 1.25 = 20A i.e. only 80% of load should be connected to the rated ampacity of breaker). In the case of a short circuit, the breaker should trip almost immediately to prevent damage to the circuit and reduce the risk of fire. The precise time it takes for the breaker to trip depends on its design and the specifics of the short circuit.

A 20 amp breaker is designed to trip when the current through it exceeds 20 amps. So, in a short circuit situation, the breaker should trip when the current reaches slightly above 20 amps. This is to protect the wiring and prevent overheating or fire.

In our scenario, if you have a 120-volt circuit powered by a 20-amp breaker, and you short the circuit, the current can be very high. The exact value depends on the resistance of the short circuit path, which is often very low.

Now, let’s see the amount of current produced by short circuit reaching before the breaker to trip the circuit.

Generally, US homes are supplied by a 7200V/240V and 120V transformer, rated at 10 to 167kVA. In this setup, the power supplied by the source is almost constant, meaning it has a defined value.

Now, in the case of a 120V AC circuit connected to a 75kVA transformer, let’s determine the short circuit current that occurs on a branch circuit before reaching the attached circuit breaker.

I = P ÷ V

Where I in Amps, P in Watts and V in Volts.

In case of a resistive circuit, the kVA is equal to the kW. Now, the value of short circuit current in amperes:

• I = P ÷ V
• I = 75 kVA ÷ 120V
• I = 625 A.

This means that a high current of 625 amperes will flow to the 20-amp breaker in the case of a 120V circuit connected from the main panel box via a 75kVA transformer.

If the rating of the utility pole-mounted transformer is 167kVA, the short circuit current will be substantial, approximately 1391 amps. This represents the maximum instantaneous current in amperes before the circuit trips.

However, that’s not enough. Even if we disregard the internal impedance (about 2%), inductance, and the wire’s length, which causes voltage drop and affects the ampacity, the voltage drop will be significant. In ideal scenarios, the voltage approaches zero, and all the power source is essentially equivalent to the current.

In the event of a short circuit, when the voltage becomes nearly zero (assume it’s 1V to counter the infinite value):

• I = P ÷ V
• I = 75 kVA ÷ 1V
• I = 75 kA

Theoretically, this means that in the event of a short circuit, approximately 75,000 amps of fault current may be generated for an instant. If the size of the transformer is larger (from 100kVA to 167kVA), even higher amperage of fault current may be generated in the case of a short circuit.

In case of faulty breaker, it may cause a serious damage to the circuit and connected appliances. Additionally, it may lead to the entire system burning due to arcing and blasting the unit, with the additional risk of dangerous electric shock.

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