Understanding the Dangers of Multiwire Branch Circuits

By Mike Holt for EC&M Magazine

As in life, there are no benefits without risk. Yes, multiwire circuits offer fewer conductors, reduced raceway size and reduced circuit voltage drop; however, improper wiring or mishandling of multiwire circuits can cause overloading of the grounded (neutral) conductor and/or destruction of equipment.

According to NEC Article 100, a multiwire branch circuit consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

Overloading Of The Grounded (Neutral) Conductor. Failure to properly terminate the ungrounded (hot) conductors to separate phases could cause the grounded (neutral) conductor to become overloaded from excessive neutral current, and the insulation could be damaged or destroyed. Conductor overheating is known to decrease insulating material service life, potentially resulting in a fire from arcing faults in hidden locations. We do not know just how long conductor insulation will last, but heat does decrease its life span.

Destruction of Equipment. Never remove the grounded (neutral) conductor from the grounded terminal bar in the panelboard if the phase conductors are energized. The grounded (neutral) conductor you remove could be part of a multiwire branch circuit, so this could result in destruction of electrical equipment. More important, even if the return conductor is not part of a multiwire circuit, removing a conductor from the grounded terminal bar when the circuit is energized could result in injury due to shock or arcing.

A typical 3-wire circuit is actually two otherwise-separate parallel circuits with a common conductor. If the grounded (neutral) conductor is accidentally opened, the circuit changes from two separate parallel 120 V circuits to one 240 V series circuit. This can result in fires and the total destruction of electrical equipment.

For example: A single-phase, 3-wire, 120/240 V circuit supplies a 1,275 W, 120 V hair dryer and a 600 W, 120V television. If the grounded (neutral) conductor is interrupted, it will cause the 120 V television to operate at 163 V and consume 1,110 W of power (instead of 600 W) for only a few seconds before it burns up. Figure

Step 1. Determine the resistance of each appliance, R = E2/P.

  • Hair dryer rated 1275 watts at 120 volts.
  • R = E2/P, R = 1202/1275 = 11.3 ohms
  • Television rated 600 watts at 120 volts.
  • R = E2/P, R = 1202/600 = 24 ohms

Step 2. Determine the circuit resistance: RT = R1 + R2

  • RT = 11.3 ohms + 24 ohms = RT = 35.3 ohms

Step 3. Determine the current of the circuit: IT = ES/RT

  • IT = 240 V/35.3 ohms = 6.8 A

Step 4. Determine the voltage for each appliance: E = IT x Rx

  • Hair dryer: 6.8 A x 11.3 ohms = 76.84 volts
  • Television: 6.8 A x 24 ohms = 163.2 volts

The 120 V rated TV in the split second before it burns up or explodes is operating at 163.2 volts.

Step 5. Determine the power consumed by each appliance: P = E2/R

  • Hair Dryer: P = 76.82/11.3 = 522 watts
  • Television: P = 163.22/24 = 1100 watts

The 600 W, 120 V rated TV will operate at 163 volts and consume 1110 watts. You can kiss this TV goodbye!

A new Code section 408.21 was added to the 2002 NEC specifying that each grounded (neutral) conductor must terminate within the panelboard in an individual terminal.

Mike Comment: You can order EC&M magazine at $30 per year, by calling 1-800-441-0294. They are closed Monday, February 18, 2002 for President’s Day.


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