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Thread: Single Phase Neutral Sizing

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    Single Phase Neutral Sizing

    What should be the size of grounded (neutral) conductor in a 120/240V 3-wire single phase system as per NEC?

    The 3-wire single phase system has one grounded (neutral) conductor and two ungrounded conductors. The one grounded (neutral) conductor carries current from both ungrounded conductors. So apparently the size of grounded (neutral) conductor should be twice as big as each ungrounded conductor; is it correct?

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    Quote Originally Posted by timm333 View Post
    What should be the size of grounded (neutral) conductor in a 120/240V 3-wire single phase system as per NEC?
    See 220.61

    Quote Originally Posted by timm333 View Post
    The 3-wire single phase system has one grounded (neutral) conductor and two ungrounded conductors. The one grounded (neutral) conductor carries current from both ungrounded conductors. So apparently the size of grounded (neutral) conductor should be twice as big as each ungrounded conductor; is it correct?
    Barring additive harmonics, (not a problem in single phase) the current in the ungrounded conductors would cancel not add.

    Roger
    Last edited by roger; 10-12-12 at 08:31 AM.
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    I thought current in ungrounded conductor cancelled out only in three phase systems. Does current also cancel out in single phase 3-wire system?

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    Quote Originally Posted by timm333 View Post
    I thought current in ungrounded conductor cancelled out only in three phase systems. Does current also cancel out in single phase 3-wire system?
    Yes it cancles in a single phase 3 wire system. It does not cancel in a 3 wire system that is derived from two ungrounded conductors and a grounded conductor from a 3 phase wye system.
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    So should the size of neutral/grounded conductor be the same as the size of each of the two ungrounded conductors in single phase 3-wire system? Or should the size of neutral/grounded conductor be bigger than the size of each of the two ungrounded conductors?

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    Quote Originally Posted by timm333 View Post
    So should the size of neutral/grounded conductor be the same as the size of each of the two ungrounded conductors in single phase 3-wire system? Or should the size of neutral/grounded conductor be bigger than the size of each of the two ungrounded conductors?
    Did you look at 220.61?


    Roger
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    Yes I checked NEC 220.61. It says that size neutral conductor equal to one of the phase conductors. But for 2-phase 3-wire system, size neutral equal to 140% of one of the phase conductors. This means that for our 120/240V 1-phse system, neutral conductor will be sized equal to one of the phase conductors.

    The only thing I want to be sure is that 120/240V system is really a 1-phase system (and is not a 2-phase system.) 120/240V 1-phase system has two phase conductors and one neutral, and the 2-phase system also has two phase conductors and one neutral. So what is the difference between these two systems? Thanks for help!
    Last edited by timm333; 10-13-12 at 02:04 PM.

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    Quote Originally Posted by timm333 View Post
    Yes I checked NEC 220.61. It says that size neutral conductor equal to one of the phase conductors. But for 2-phase 3-wire system, size neutral equal to 140% of one of the phase conductors. This means that for our 120/240V 1-phse system, neutral conductor will be sized equal to one of the phase conductors.

    The only thing I want to be sure is that 120/240V system is really a 1-phase system (and is not a 2-phase system.) 120/240V 1-phase system has two phase conductors and one neutral, and the 2-phase system also has two phase conductors and one neutral. So what is the difference between these two systems? Thanks for help!
    Yes 120/240 is a 1Ø system. A 2Ø system is a nearly obsolete, completely different animal.
    Rob

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    All responses based on the 2011 NEC unless otherwise noted

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    Quote Originally Posted by timm333 View Post
    ...

    The only thing I want to be sure is that 120/240V system is really a 1-phase system (and is not a 2-phase system.) 120/240V 1-phase system has two phase conductors and one neutral, and the 2-phase system also has two phase conductors and one neutral. So what is the difference between these two systems? Thanks for help!
    For a 120/240V 1Ø 3W system, using the neutral as a common reference, the ungrounded conductor's voltages are 180° out of phase.

    For a 120V 2Ø 3W system, using the grounded conductor as a common reference, the ungrounded conductor's voltages are 90° out of phase (could also be 270°, depending on which is used as the reference voltage). The grounded conductor currents partially cancel out, which is the reason for the 140%.
    I'll never get there. No matter where I go, I'm always here.

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    Quote Originally Posted by timm333 View Post
    Yes I checked NEC 220.61. It says that size neutral conductor equal to one of the phase conductors. ... This means that for our 120/240V 1-phse system, neutral conductor will be sized equal to one of the phase conductors.

    ...
    That's not really what it says... but you can size it the same as the ungrounded. What it does say is that it can be smaller, depending on the loads. On a 120/240 1Ø 3W system, there may be line-to-loads loads which will not contribute any neutral current. As such, the neutral size can be reduced by this amount per calculation (effectively the compliment of sizing to maximum unbalanced neutral current).
    I'll never get there. No matter where I go, I'm always here.

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