# Hidden Dangers of Multiwire Branch Circuits

## Dangers of Multiwire Branch Circuits

**Dangers of Multiwire Circuits**

One of the dangers of multi-wire circuits is the effect of an open neutral. If the neutral becomes open, the circuit becomes a series circuit and the voltages will divide between the two loads based on the size of the loads. If the neutral is open and the loads are balanced, the circuit will continue to work. The circuit continues to work as long as both loads are turned on. If one load is turned off, the other load will turn off as well.

An open neutral on a multiwire circuit may cause a fire or damage the loads. The load that will stop working first will always be the load with the smallest wattage. This is because the load with the smallest wattage has a higher resistance internally. Most multiwire circuit questions on the exam will include a single-phase, three-wire circuit with two ungrounded conductors and one neutral. To determine the voltage on each load of a multiwire circuit with an open neutral, apply the following procedure:

- Find the current of each load. To find the current of each load, divide the load’s rated wattage by its rated voltage.

**Load 1:** 150 Watts / 120 Volts = 0.68 Amps

**Load 2:** 1500 Watts / 120 Volts = 12.5 Amps

- Find the resistance of each load. To find the resistance of each load, divide the rated voltage by the current found in Step 1.

**Load 1:** 120 Volts / 0.68 Amps = 177 Ohms

**Load 2:** 120 Volts / 12.5 Amps = 9.6 Ohms

- Find the total circuit resistance. To find the total circuit resistance, add the resistance of the two loads. Since the circuit is now a series, the circuit resistance is added together.

**Load 1** (177 Ohms) + Load 2 (9.6 Ohms) = 186.6 Ohms

- Find the series circuit current. To Find the series circuit current, divide the line-to-line voltage (usually 240 V) by the total resistance as found in Step 3.

240 Volts / 186.6 = 1.286

- Find the new voltage on each load. To find the new voltage on each load, multiply the current of the circuit by the resistance of the load as found in Step 2.

**Load 1:** 1.286 x 177 = 227.622 Volts

**Load 2:** 1.286 x 9.6 = 12.3456 Volts

Total: 239.9676 (Rounds to 240 Volts)

**Summary** – As you can see, when things go BAD on a multiwire branch circuit, such as losing the shared neutral connection, the LOAD with the lowest wattage ends up being the load with the greater voltage, which will exceed the voltage ratings of the equipment that portion of the circuit is connected to.

CEO and Founder of Electrical Code Academy, Inc. A Texas Corporation located in McKinney, Texas.