The savings from multiwire branch circuits can come at a high cost.
It's true that multiwire branch circuits reduce raceway size, voltage drop, and the number of conductors. However, mishandling or improper wiring of multiwire branch circuits can cause overloading of the grounded (neutral) conductor and/or the destruction of equipment.
Failure to properly terminate the ungrounded (hot) conductors of a multiwire branch circuit to separate phases could cause the grounded (neutral) conductor to become overloaded with excessive neutral current. Conductor overheating decreases insulating material service life, creating the potential for arc-fault fires in hidden locations.
Never remove the grounded (neutral) conductor from the grounded terminal bar in the panelboard if the phase conductors are energized — it could be part of a multiwire branch circuit, and removing it could ruin electrical equipment. A typical 3-wire circuit is actually two separate parallel circuits with a common conductor. If the grounded (neutral) conductor is opened, the circuit changes from two 120V parallel circuits to one 240V series circuit.
Consider the following example. A single-phase, 3-wire, 120/240V circuit supplies a 1,275W, 120V hair dryer and a 600W, 120V television. If the grounded (neutral) conductor is interrupted, at what voltage will the television operate, and how much power will it consume before burning up (Figure above)?
Step 1. Calculate the resistance of each appliance:
a. Hair dryer rated 1,275W at 120V
b. Television rated 600W at 120V
Step 2. Calculate circuit resistance:
RT=11.3 ohms+24 ohms=RT=35.3 ohms
Step 3. Calculate circuit current:
Step 4. Calculate the voltage for each appliance:
a. Hair dryer: 6.8A×11.3 ohms=76.8V
b. Television: 6.8A×24 ohms=163.2V
Step 5. Calculate power consumed:
a. Hair dryer: P=(76.8)2÷11.3=522W
b. Television: P=(163.2)2÷24=1,110W
The TV will temporarily operate at 163V and consume 1,110W!
The removal of a wiring device, such as a receptacle, must not cause an interruption of continuity for the grounded (neutral) conductor in a multiwire branch circuit [300.13(B)]. Therefore, the grounded (neutral) conductors must be spliced together, and a pigtail must be provided for device terminations. The opening of the ungrounded (hot) or grounded (neutral) conductor of a 2-wire circuit during the replacement of a device doesn't create a safety hazard, so the Code doesn't require pigtailing of the grounded (neutral) conductor.
A new section in the 2002 NEC (408.21) requires each grounded (neutral) conductor to terminate within the panelboard in a terminal.