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Ask The Engineer - Should Neutral Conductors Be Oversized?

Posted by Robert Faulkner on Dec 16, 2009 12:47:00 PM

Question from Jay <omitted>: I am presently evaluating a CDU model #CS-27VY-L30M and have noticed that the power input cordage does not involve an oversized neutral conductor.  All five conductors are 10-gauge AWG.  Isn't an oversized neutral required to avoid the possibility of overheating the conductor?

Engineer Reply: The CS-27VY-L30M is designed for a 30A 120/208V 3-Phase Wye service and provides 21 x 208V outlets w/ IEC 60320 C13 receptacles and 6 x 120V outlets w/ NEMA 5-20R receptacles.  With respect to the 120V outlets, two are wired phase-X to Neutral, two are wired phase-Y to Neutral, two are wired phase-Z to Neutral and current is only carried on the common neutral conductor when some or all of these 120V outlets are used and the loads are unbalanced.  

Harmonic currents aside, the common neutral conductor carries only approximately the same current as the line-to-neutral load currents of the other conductors and therefore cannot be overloaded as a direct result of the loads.  Here are some examples:

Balanced:
If X-to-neutral supports a 30A load, Y-to-neutral supports a 30A load and Z-to-neutral supports a 30A load, current on the common neutral will be zero (0). 

Unbalanced-A:
If X-to-neutral supports a 30A load, Y-to-neutral supports a 30A load and Z-to-neutral has no load attached, current on the common neutral will be 30A. 

Unbalanced-B:
If X-to-neutral supports a 30A load, Y-to-neutral has no load attached and Z-to-neutral also has no load attached, current on the common neutral will be 30A.

Demonstrating the principle, here is a simplified equation for determining resultant neutral current with 3-Phase Wye systems containing phases that are mutually 120° apart:  N²=X²+Y²+Z²-X*Y-Y*Z-X*Z

If all line-to-neutral loads are balanced, the common neutral carries no current.  Out-of-balance, there is no combination of loads that can result in the common neutral carrying more current than the highest supported through any one of the line-to-neutral branches.  Hence, with a 30A 3-Phase Wye service, and because each phase is limited to 30A by an upstream triple-pole circuit breaker, the neutral current will typically never exceed 30A and the size of the neutral conductor need only be 100% of the conductor size present for supporting the line-to-neutral loads.

This is fully recognized in the latest (2008) edition of National Electrical Code.  Specifically, Article 310.15(B)(4) addresses the question "Should Neutral Conductors Be Oversized?".  Noted is that "for NEC editions up to and including the 2005 Code, the neutral conductor ampacity was usually 125% of the maximum continuous current allowed by the overcurrent device."  This is immediately followed by: "Revised for the 2008 Code, both 210.19(A)(1) and 215.2(A)(1) include a new exception that permits a branch circuit or a feeder neutral conductor to be sized differently.  The sizing is now permitted to be 100% of the non-continuous load plus the continuous load, thus permitting a reduction in a neutral conductor size (calculated from previous editions) by as much as 25%."

Regarding additive harmonic currents caused by portions of the load being nonlinear, the concern is relatively minor due to limitations on allowable harmonic distortion and the NEC directive that branch circuits be continuously used at only 80% of maximum rating.  Continuous current rating for a 30A service is 24A and, because harmonic distortion typically does not exceed 5%, overheating the common neutral is not a generic concern.  Furthermore, with the Smart CDU model #CS-27VY-L30M, each of the three branches of 120V outlets is actually limited to 20A maximum by UL489 Listed circuit breakers.  Typical neutral current will therefore rarely exceed 20A, which is far less than the ampacity of the 10-gauge neutral conductor in the CS-27VY-L30M cordset.

Topics: PDU

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