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Www Ecmweb Com Images Archive 205ecm18fig1
Www Ecmweb Com Images Archive 205ecm18fig1
Www Ecmweb Com Images Archive 205ecm18fig1
Www Ecmweb Com Images Archive 205ecm18fig1

Multiwire Branch Circuits Can Be Dangerous

May 1, 2002
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

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:
R=E2÷P
a. Hair dryer rated 1,275W at 120V
R1=(120V)2÷1,275W=11.3 ohms
b. Television rated 600W at 120V
R=(120V)2÷600W=24 ohms

Step 2. Calculate circuit resistance:
RT=R1+R2
RT=11.3 ohms+24 ohms=RT=35.3 ohms

Step 3. Calculate circuit current:
IT=ESource÷RT
IT=240V÷35.3 ohms=6.8A

Step 4. Calculate the voltage for each appliance:
E=IT×Rx
a. Hair dryer: 6.8A×11.3 ohms=76.8V
b. Television: 6.8A×24 ohms=163.2V

Step 5. Calculate power consumed:
P=E2÷R
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.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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