A multiwire branch circuit can supply:
I. 120/240 volts to only one piece of utilization equipment
II. 120/240 volts where all ungrounded conductors are opened simultaneously
a) I only
b) II only
c) either I or II
d) neither I nor II
The Correct Answer and Explanation is:
The correct answer is:
b) II only
Explanation:
A multiwire branch circuit (MWBC) is a wiring method used in electrical systems where two or more ungrounded (hot) conductors share a common neutral conductor. These circuits typically provide 120/240 volts in residential and commercial settings.
Let’s analyze the two statements:
I. “120/240 volts to only one piece of utilization equipment”
- This statement is incorrect for a multiwire branch circuit.
- An MWBC typically serves multiple loads or utilization equipment, not just one.
- The circuit contains two or more hot wires (ungrounded conductors) and a shared neutral, allowing simultaneous supply of multiple devices or loads at 120 volts from each hot conductor to neutral, or 240 volts across the two hots.
- Although it could theoretically supply one large 240V load, MWBCs are generally designed to supply more than one utilization equipment (e.g., multiple outlets or appliances on different branches).
II. “120/240 volts where all ungrounded conductors are opened simultaneously”
- This statement is correct and reflects a critical safety feature of MWBCs.
- The National Electrical Code (NEC) requires that all ungrounded conductors of a multiwire branch circuit be opened simultaneously by a common disconnect or a two-pole circuit breaker.
- This ensures that when the circuit is turned off, all hot conductors are de-energized, preventing potential shock hazards or damage when servicing or working on the circuit.
- Because the two or more ungrounded conductors share a neutral, if only one conductor were opened while the others remained live, the neutral could carry unexpected current, creating dangerous conditions.
- By opening all ungrounded conductors simultaneously, the circuit ensures complete disconnection of power for safety.
Additional details:
- A multiwire branch circuit generally consists of two hot wires (often on different phases of a 240V supply) and one neutral wire.
- Each hot wire supplies 120 volts to its own load, but the loads share a neutral.
- When loads on both hots operate simultaneously, the neutral current is the difference of the currents on the two hot wires, rather than their sum, reducing the neutral conductor size requirement.
- The shared neutral concept and the simultaneous disconnection are key safety features unique to MWBCs.
Summary:
- Statement I is false because MWBCs supply multiple loads, not just one piece of equipment.
- Statement II is true and critical for safe operation.
- Therefore, the answer is b) II only.