advanced search

Non-linear loads and the 1996 NEC

Feb 1, 1996 12:00 PM, Andersen, William O., Jr.

How has the 1996 NEC addressed the problems created by non-linear loads and harmonics?

Non-linear loads are not new; forms of such loads have been present in our power supplies for more than a century. What's happened is that the quantity of non-linear loads has grown exponentially. We've witnessed an electronic revolution:

Efficient electronic power conversion equipment, combined with more stringent energy conservation policies, have dramatically changed traditional commercial, industrial, and, most recently, residential electrical loads.

What's been the NFPA's response to this electronic revolution? The NEC, in the 1968 code cycle, first recognized this phenomenon through a single note to the ampacity tables. As reports of overheated neutral conductors, failed transformers, and motor burnouts increased, so did concerns over fire safety problems associated with conventional electrical equipment and circuits supplying non-linear loads.

To address these problems, the NEC Correlating Committee assembled a group of 17 interested people to study the effects of electrical loads that produced substantial current distortion on electrical system distribution components including, but not limited to, the following:

* Distribution transformers, current transformers, and others.

* Switchboards and panelboards.

* Phase and neutral feeder conductors.

* Phase and neutral branch circuit conductors.

* Proximate data and communication conductors.

In addition, the group (subcommittee) was instructed to study the harmful effects of non-linear loads on system components, to recommend methods of minimizing these effects, "including compensating methods at load sources," and to prepare proposals to amend, where necessary, the NEC.

Results of investigation and research

The subcommittee examined proposals to the 1993 NEC concerning non-linear loads, researched available technical literature, and reviewed electrical theory on harmonic distortion and its sources. It concluded that "while non-linear loads can cause undesirable operational effects, including additional heating, no significant threat to persons and property has been adequately substantiated." The lack of substantiation was due to the confidential nature of fire information from industry sources as well as the lack of data on fires directly related to harmonic loads. Thus, no specific proposals to increase neutral conductor sizes were recommended.

However, the subcommittee did agree that "with 3-phase, 4-wire, wye-connected power systems, the system design must consider the possibility of high harmonic neutral currents when non-linear loads are used."

Non-linear loads, which are referenced in Note 10(c) to Ampacity Tables 310-16 through 19 and implied in Sec. 210-4(a) FPN (fine print note), Sec. 450-3 FPN No. 2, and Sec 220-22, were not explained in the 1993 NEC. Consequently, the subcommittee recommended both a definition for Article 100 and an accompanying FPN to help identify non-linear loads in the 1996 NEC.

Text revisions reflecting the new definition also were suggested for applicable Code sections, and additional FPNs were added to explain the effects of non-linear loads. Additional use of parallel neutral conductors in existing installations, with neutrals as small as No. 2 now being allowed in parallel, was also permitted "under engineering supervision" to alleviate overheated neutral conductors.

Enforcement options

Perhaps the key change is a subtle addition to Sec. 310-10(2) where the phrase "including fundamental and harmonic currents" was added. Although the NEC has been criticized for not instituting a plethora of mandatory rules to address non-linear loading, this particular section does hold the answer when an enforcement authority thinks the wiring system design isn't adequate to address the effects of non-linear loading.

Sec. 310-10 prohibits using a conductor in such a manner that its rated operating temperature is exceeded. This rule stands on its own and, with good objective substantiation, can be enforced without recourse to a specific table ampacity. The change calls attention to the fact that excessive conductor operating temperatures are contrary to Code rules even when they result from non-linear loads that otherwise calculate correctly under Art. 220. Be sure, however, that there are solid, objective criteria behind any citation made on this rule.

A listing of 1996 NEC proposals, along with Panel actions, is shown on page 102.

How to measure these loads

Because nonsinusoidal voltages and currents can't be accurately measured with traditional average responding meters, the subcommittee recommended that information regarding the use of proper measuring equipment on systems suspected of containing non-linear loads be included in the NEC Handbook. The illustration, on page 102, was provided to illustrate proper measurement techniques.

Currently, this subcommittee has not been officially discharged; it serves at the discretion of the NEC Correlating Committee, which could always reactivate it.

William O. Andersen, Jr. is Manager, Product Engineering, AFC Cable Systems, Inc., New Bedford, Mass.

Want to use this article? Click here for options!

© 2012 Penton Business Media, Inc.

---

Acceptable Use Policy blog comments powered by Disqus

Browse Back Issues Select an Issue EC&M Magazine | April 2012 March 2012 February 2012 January 2012 December 2011 November 2011 October 2011 September 2011 August 2011 July 2011 June 2011 May 2011 April 2011 March 2011 February 2011 January 2011 December 2010 November 2010 October 2010 September 2010 August 2010 July 2010 June 2010 May 2010 April 2010 March 2010 February 2010 January 2010 December 2009 November 2009 October 2009 September 2009 August 2009 July 2009 June 2009 May 2009 April 2009 March 2009 February 2009 January 1, 2009 December 2008 November 2008 October 1, 2008 September 2008 August 2008 July 2008 June 1, 2008 May 2008 April 2008 March 2008 February 2008 January 2008 December 2007 November 2007 October 2007 September 2007 August 2007 July 2007 June 2007 May 2007 April 2007 March 2007 February 2007 January 2007 December 2006 November 2006 October 2006 September 2006 August 2006 July 2006 June 1, 2006 May 1, 2006 April 2006 March 1, 2006 February 2006 January 2006 December 2005 November 2005 October 2005 September 2005 August 2005 July 2005 June 2005 May 2005 April 2005 March 2005 February 2005 January 2005 December 2004 November 2004 October 2004 September 2004 August 2004 July 2004 June 2004 May 2004 April 2004 March 2004 February 1, 2004 January 1, 2004 December 1, 2003 November 1, 2003 October 1, 2003 September 1, 2003 August 1, 2003 July 1, 2003 June 1, 2003 May 1, 2003 April 1, 2003 March 1, 2003 February 1, 2003 January 1, 2003 December 1, 2002 November 1, 2002 October 1, 2002 September 1, 2002 August 1, 2002 July 1, 2002 June 1, 2002 May 1, 2002 April 1, 2002 March 1, 2002 February 1, 2002 January 1, 2002 December 1, 2001 November 1, 2001 October 1, 2001 September 1, 2001 August 1, 2001 July 1, 2001 June 1, 2001 May 1, 2001 April 1, 2001 March 1, 2001 February 1, 2001 February 1, 1995 January 1, 2001 December 1, 2000 November 1, 2000 October 1, 2000 September 1, 2000 August 1, 2000 July 1, 2000 June 1, 2000 May 1, 2000 April 1, 2000 March 1, 2000 February 1, 2000 January 1, 2000 December 1, 1999 November 1, 1999 October 1, 1999 September 1, 1999 August 1, 1999 July 1, 1999 June 1, 1999 May 1, 1999 April 1, 1999 March 1, 1999 February 1, 1999 January 1, 1999 December 1, 1998 November 1, 1998 October 1, 1998 September 1, 1998 August 1, 1998 July 1, 1998 June 1, 1998 May 1, 1998 April 1, 1998 March 1, 1998 February 1, 1998 January 1, 1998 December 1, 1997 November 1, 1997 December 1, 1996 November 1, 1996 October 1, 1996 September 1, 1996 August 1, 1996 July 1, 1996 June 1, 1996 May 1, 1996 April 1, 1996 March 1, 1996 February 1, 1996 January 1, 1996 December 1, 1995 November 30, 1995 November 1, 1995 October 1, 1995 September 1, 1995 August 1, 1995 July 1, 1995 June 1, 1995 May 1, 1995 April 1, 1995 March 1, 1995

browse e-newsletters