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

How Many Amps Flow in a Short Circuit Powered by a 120V 20A Breaker? . When dealing with electrical systems, understanding short circuits is critical for ensuring safety and proper system function. A short circuit occurs when there’s an unintended path for current flow, bypassing the normal circuit resistance. This can lead to potentially dangerous surges in current. The question of how many amps will flow in a short circuit powered by a 120V, 20A breaker is crucial for homeowners, electricians, and anyone handling electrical systems.

This article will explain the basics of short circuits, how to calculate the current, factors influencing the flow, and the safety considerations for 120V, 20A breakers.

Understanding Short Circuits

A short circuit happens when electrical current takes a shortcut, avoiding the intended load resistance. This typically occurs due to:

1. Faulty wiring
2. Damaged insulation
3. Loose connections
4. Direct contact between conductors

In such scenarios, the current spikes significantly, potentially causing breaker trips, fires, or damage to connected devices.

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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.

The Role of Breakers in Short Circuits

Circuit breakers are designed to protect electrical systems by cutting off current when it exceeds a set threshold. For a 120V, 20A breaker:

• Voltage Rating: The breaker handles circuits with a voltage of 120 volts.
• Current Rating: The breaker trips if the current exceeds 20 amps under normal conditions.

However, in a short circuit, the current can far exceed 20 amps before the breaker reacts.

Read More : Can We Use AC Circuit Breaker for DC Circuit and Vice Versa?

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.

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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.

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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.

Read More : How to Wire an AFCI Breaker?

Tips :

• Practically, the maximum fault current at the terminals of a 50kVA, 120V/240V transformer with a 2% impedance would be 10.4 kA (10,400 amperes). This value may be lower in residential applications due to primary and service impedance.
• A 7200/240V, 150kVA transformer located 1500 feet from the substation (assuming a 5% impedance on the transformer and infinite impedance on the primary side) feeds a residential distribution ped. From there, the power flows to a meter and subsequently to a QO panel with a 20-amp circuit, using 25 feet of #12 wire. The estimated current is 1469 amps, and the expected trip time is 0.016 seconds (one cycle).
• Circuit breaker has current rating of 20 Amps, voltage rating of 120 VAC and interrupt rating of 10 kilo-Amps. The circuit breaker current will not exceed the interrupt rating of the breaker.
• The fault current in a distribution transformer can surpass 10 kA and potentially reach up to 30 kA at its maximum.
• A 20-amp circuit breaker should be used for 16-amp of load circuit. This is because of the safety factor i.e. (16A x 1.25 = 20A) e.g. only 80% of the load should be connected to the rated ampacity of breaker. Same is the case for 15-amp breaker which should be used for 12-amp of load point.

Factors Affecting the Current in a Short Circuit

1. Circuit Impedance:
   • The total resistance and reactance (combined as impedance) of the circuit determine how much current flows.
   • A lower impedance results in higher current during a short circuit.
2. Wiring Material and Size:
   • Copper and aluminum wires have different resistances.
   • Thicker wires reduce resistance, allowing more current flow.
3. Source Impedance:
   • The internal resistance of the power source (e.g., the transformer feeding the circuit) also impacts the current.
4. Breaker Response Time:
   • Modern breakers are designed to trip almost instantaneously in short circuit conditions, but a surge can occur before the trip.

Read More : Difference Between Relay and Circuit Breaker

FAQs

Q1: Can a 20A breaker handle short circuit currents without damage?
A: A 20A breaker is designed to interrupt short circuit currents quickly to prevent damage. However, the breaker itself might need replacement after a significant short circuit event.

Q2: How do I determine my circuit’s impedance?
A: Measuring impedance requires specialized equipment and expertise, usually performed by electricians or engineers.

Q3: Is it safe to reset a breaker after a short circuit?
A: Yes, but only after identifying and resolving the cause of the short circuit to avoid recurring issues.

Q4: What happens if my breaker doesn’t trip during a short circuit?
A: This is a serious hazard. It may indicate a faulty breaker or improper installation. Immediately consult a professional.

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