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.