This article was posted 10/20/2006 and is most likely outdated.

Article 210 Branch Circuits 210.4
 

 
Subject - Article 210 Branch Circuits 210.4

October 20, 2006  

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Branch Circuits

210.4 Multiwire Branch Circuits

Part I. General Provisions

New FPN alerts the Code user to ensure that the continuity of the grounded neutral conductor of a multiwire circuit isn’t interrupted (open), because doing so can result in a fire and/or destruction of electrical equipment because of under- or over-voltage.
(C) Line-to-Neutral Loads. Multiwire branch circuits must supply only line-to-neutral loads.
Ex 1: A multiwire branch circuit is permitted to supply line-to-line utilization equipment, such as a range or dryer.

Ex 2: A multiwire branch circuit is permitted to supply both line-to-line and line-to-neutral loads if the circuit protection device opens all ungrounded conductors of the multiwire branch circuit simultaneously under a fault condition (multipole circuit breaker with common internal trip). Figure 210-1

FPN: See 300.13(B) for the requirements relating to the continuity of grounded neutral conductor on multiwire circuits.

CAUTION: If the continuity of the grounded neutral conductor of a multiwire circuit is interrupted (open), the resultant over- or undervoltage could cause a fire and/or destruction to electrical equipment. Figure 210-2



Figure 210-1
(Click on image to enlarge)


Figure 210-2
(Click on image to enlarge)

     

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Comments
  • Resistive and capacitive loads are in relation to whether the current or voltage is leading or lagging in the phase angle. Tnis can affect the power factor and play a part in harmonic distortion. The nuetral does not care what type of load you have when it is being shared as long as the ungrounded conductors are on opposing phases.

    Benjamin Brown
    Reply to this comment

  • While I fully agree with the over / under voltage conditions well illustrated when there exists an open circuit neutral conductor, the problem remains in that this situation is not cleared by common trip breakers. In the illustration, neither branch will draw suffient current to trip either breaker. Actually we then have a single series circuit connected to a 240 volt supply, with both devices carrying the same current. The voltage drops remain as illustrated until something burns out! This could not be resolved by a pair of GFCI breakers, since they would need neutral conductors that are serarate for each branch circuit. They require balanced current in their line and neutral conductors.

    Robert Gibby
    Reply to this comment

  • Not strictly related, but a question about multiwire circuits. On a normal three-wire circuit, say 12-3 NMB on a two-pole 20A breaker, each leg could be drawing 20A line-to-neutral without opening the breaker. That would mean 40A on the neutral leg, way too much for 12-gauge, except that the currents, being 180° out-of-phase, cancel on the neutral.

    That's really true only if the loads have the same reactance -- zero, for example, if they're both only resistive. What if I run a capacitor-start motor on one leg and a transformer on the other? Couldn't I then exceed 20A on the neutral? And wouldn't that be hazardous? What am I missing here?

    Thanks.

    Craig Appel
    Reply to this comment

  • Thanks to to your great color pictures, I use them at the Adult Edu. center for the LAUSD Home Repair classes.

    kip
    Reply to this comment

  • Not strictly related, but a question about multiwire circuits. On a normal three-wire circuit, say 12-3 NMB on a two-pole 20A breaker, each leg could be drawing 20A line-to-neutral without opening the breaker. That would mean 40A on the neutral leg, way too much for 12-gauge, except that the currents, being 180° out-of-phase, cancel on the neutral.

    That's really true only if the loads have the same reactance -- zero, for example, if they're both only resistive. What if I run a capacitor-start motor on one leg and a transformer on the other? Couldn't I then exceed 20A on the neutral? And wouldn't that be hazardous? What am I missing here?

    Thanks.

    Craig Appel
    Reply to this comment


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