Key Takeaways
 Losing a neutral affects circuit balance.
 Identical loads maintain current flow even if neutral is lost.
 Unbalanced loads cause voltage discrepancies and potential hazards.
Basics of MultiWire Branch Circuits
A multiwire branch circuit involves more than one hot wire sharing a single neutral wire. This is common when you're running a circuit with a setup like a black and red wire, in a 12/3 configuration, to power separate loads. When a neutral wire is lost, the consequence can lead to equipment malfunction.
Equal Loads:
In the ideal scenario of equal loads, such as two fans with identical resistance and power ratings, the absence of the neutral doesn't affect function. For example, each load might be 5 ohms and consume 2880 watts. With these equal loads, the circuit effectively becomes a single 240volt circuit. The current flow across the loads will be canceled out, resulting in no current on the neutral wire.
With equal loads, calculations show that 24 amps flow through the loads whether using a 120volt or 240volt configuration due to the relationship (I = \frac{E}{R}). The resistance in the unified 240volt setup doubles to 10 ohms, matching the voltage increase, so current flow remains constant.
Unequal Loads:
The situation changes when dealing with unequal loads. If loads, like an 1800watt toaster and a 600watt TV, are powered on a multiwire branch circuit, the neutral wire must carry the imbalance in current. For circuits with differing resistances, the neutral compensates for this difference. For instance, a 15amp circuit opposed by a 5amp one leads to a neutral carrying 10 amps.
When the neutral is removed, Kirchhoff's Current Law dictates that the same current flows throughout the circuit, but voltage distribution changes due to Kirchhoff's Voltage Law. In an unbalanced scenario, you could see voltages like 180 volts on one load and 60 volts on the other, disrupting the normal operation and potentially causing damage.
Consequences of Losing a Neutral
Losing a neutral in a multiwire branch circuit can introduce several challenges and dangers due to the imbalance it creates between the loads. When functioning correctly, these circuits use a neutral to balance the current between different loads. However, if the neutral is lost, the current flow can become uneven, leading to different voltages being applied to each load.
Consider a scenario where you have two unbalanced loads, such as an 1800watt toaster and a 600watt TV. In normal conditions, the neutral would carry the imbalance between these two loads. If the neutral is removed or fails, this imbalance causes voltage shifts across the loads; one load could receive significantly more voltage, while the other receives less. For instance, an unbalanced circuit might result in about 180 volts to one device and only 60 volts to another, instead of the stable 120 volts they should each receive.
This situation can damage equipment, causing premature failure or unsafe operation. To avoid these consequences, it's crucial to ensure all connections are secure and that the neutral is always intact in multiwire branch circuits. Regular inspection and maintenance can help identify potential issues before they lead to equipment damage.
 Key Points:
 Current Imbalance: Without a neutral, current cannot balance itself between different loads.
 Voltage Fluctuations: Some devices may get higher voltages which can lead to damage.
 Safety Concerns: Risks include equipment damage or electrical fires.
Ensuring safety and functionality in electrical circuits requires careful attention to how neutrals are maintained and how circuits are balanced.
Ideal Conditions with Equal Loads
Current Flow with Identical Loads
In a scenario where you have two identical loads, such as fans with the same power and resistance values, the flow of current behaves predictably. Each load, when perfectly matched in resistance and power rating, contributes to the overall balance within the circuit. For instance, consider two loads each with a resistance of 5 ohms and power ratings of 2880 watts operating in harmony.
The advantage of equal loads is that there is no current flowing through the neutral conductor. This results from the cancellation effect, where the current flowing through each load nullifies the need for a neutral current path. With two identical loads, the system essentially functions as a 240volt circuit rather than two separate 120volt circuits. The result is a stable environment without fluctuations or extra stress on any single component within the system.
Ohm's Law in Equal Load Scenarios
Applying Ohm's Law in these setups highlights the consistency of current flow. The law, ( I = \frac{E}{R} ), confirms that even if the system is perceived differently as a 240volt setup, the current flow remains unchanged. For a single load with 120 volts and a resistance of 5 ohms, the current is 24 amps. When the loads are reconfigured in series as part of a 240volt system, the resistance doubles to 10 ohms. The voltage also doubles to 240 volts, yet the current remains at 24 amps.
This consistent current flow underscores the stability and reliability of equal load scenarios. The unchanged current demonstrates that even without a neutral wire, the loads function in unison, maintaining operational integrity. Such configurations benefit from efficient energy distribution and balanced performance across all components involved.
RealWorld Applications
Implications for Equipment
In a multiwire branch circuit where multiple loads share a neutral wire, losing the neutral can lead to potential damage to equipment. If loads are balanced, losing the neutral won't affect circuit performance, but unbalanced loads pose a risk of unequal voltage distribution.

Balanced Loads: Two identical loads, such as fans with equal resistance and power, continue functioning without current through the neutral in a 240volt configuration. This forms a larger resistance but maintains the same current flow, reducing risks.

Unbalanced Loads: Different loads, like a toaster and a TV, create an imbalance. The neutral carries the difference in current, and losing it can lead to significant voltage differences across the loads—potentially 180 volts on one and 60 on another—causing equipment issues.
Equipment Example 
Potential Impact 
Balanced Load (e.g., Two Fans) 
No impact due to neutral loss 
Unbalanced Load (e.g., Toaster + TV) 
Risk of incorrect voltage, possible damage 
Understanding these effects is crucial to maintaining equipment integrity in scenarios of neutral wire loss.
The Mathematics of Circuit Behavior
Calculating Resistance and Current
When dealing with a multiwire branch circuit, consider the implications of losing a neutral wire. In a scenario with equal loads, where both are identical and share a neutral, the cancellation of the neutral does not disrupt the circuit operation. This results in a shift from two 120volt circuits to one 240volt circuit, maintaining the same current flow due to the unchanged resistance.
For example, with each load having a resistance of 5 ohms and a power output of 2880 watts, the circuit will behave as a single 10ohm resistor when the neutral is cut. Using Ohm’s Law, the current remains constant at 24 amps for both the 120volt and 240volt setups. The relationship ( I = \frac{E}{R} ) illustrates this constancy.
Understanding the Formula Wheel
The formula wheel is essential when facing circuits with unequal loads sharing a neutral. In such cases, an imbalance occurs, causing voltage differences across the loads. Kirchhoff’s laws help explain these changes: the current in a series circuit remains constant, while the sum of voltage drops across resistances equals the total applied voltage.
Using combined laws, Ohm’s Law and power equations, you can determine the voltage drop across each load. In our example, with a 1800watt toaster and a 600watt TV, the circuit's imbalance impacts voltage distribution, potentially leading to unequal voltages, such as 180 volts and 60 volts, which can cause operational issues. The formula wheel integrates these values to aid you in understanding and calculating necessary adjustments.
Unbalanced Loads and Risks
Unbalance in Current Flow
In a multiwire branch circuit, when the neutral is lost, you often encounter an imbalance in current flow, especially with differing loads. Consider a circuit with a 1800watt toaster and a 600watt TV. These two appliances have different resistances, creating an imbalance when they share a neutral wire.
If both appliances are running, you'll usually measure the imbalance using a clampon ammeter. Suppose the toaster draws 15 amps and the TV draws 5 amps. The neutral wire will carry the difference, 10 amps, reflecting the imbalance.
Important: Should the neutral fail, the current will redistribute across the loads due to Kirchhoff's Laws, leading to unanticipated voltage distributions.
Kirchhoff's Laws in Series Circuits
Kirchhoff's Current and Voltage Laws dictate the behavior of series circuits. In these circuits, the current remains consistent across all components. If the neutral is lost, the total resistance of the circuit rises, and the current is forced to remain the same.
Voltage distribution between two unequal resistances becomes tricky. Kirchhoff’s Voltage Law requires the voltage drops across each component to sum up to the total voltage. Thus, while the toaster operates, it might receive most of the voltage, such as 180 volts, with the TV receiving only 60 volts. This disproportional voltage can damage appliances, not designed for such conditions.
Understanding these fundamentals can help identify and mitigate risks associated with electrical imbalances.
Voltage Discrepancies in Unbalanced Loads
When dealing with unbalanced loads in a multiwire branch circuit, voltage discrepancies can occur if the neutral is lost. Imagine a scenario where you have two different loads: an 1800watt toaster and a 600watt TV. In balanced conditions, the neutral carries the difference in amperage between these loads, maintaining stability. For example, if the toaster draws 15 amps and the TV draws 5 amps, the neutral would carry 10 amps.
Without the neutral, Kirchoff's Voltage Law requires that the voltage drops across each resistance sum up to the total circuit voltage. This scenario pushes an uneven distribution of voltage across the loads. Instead of each device receiving 120 volts, you might see 180 volts across one device and 60 volts across the other. This imbalance poses a significant risk to the devices, potentially leading to equipment damage.
To manage these challenges:
 Regularly inspect your neutral connections.
 Use measurement tools like a clampon ammeter to detect imbalances.
 Consider rebalancing load distribution across circuits when necessary.
Understanding these factors can help you maintain safe and effective electrical systems in these complex scenarios.