The National Electrical Code offers some advice but contains few rules about power quality. With the exception of two rules about neutrals serving harmonic-producing loads, most of what the Code has to say about power quality is in Fine Print Notes.
The NEC is a safety standard and not a design specification. This is clearly stated in Section 90-1. Power quality issues include such things as electromagnetic Interference, distortion of voltage and current waveforms, reliability and continuity of power, voltage regulation, and so on. These are primarily performance issues. The NEC does not contain many performance requirements.
Power quality problems, although disruptive to operations, are not necessarily safety problems. For example, excessive voltage drop can be a significant power quality issue affecting the operation of equipment. The NEC contains two well known "rules" about voltage drop. Section 210-19(a) FPN No. 4 and Section 215-2(b) FPN No. 2 address the issue of voltage drop. However, neither of these references actually contains a "rule." Both references are FPNs, which, according to 90-5, are advisory only. The Code basically treats voltage drop as a design issue.
To facilitate design solutions to some power-quality problems, the NEC allows some deviations from the normal requirements if the safety objectives are met. Specifically, Section 250-74 Exception No. 4 contains the installation requirements for isolated grounding conductors. Other sections of the NEC allow isolated grounding conductors to pass through boxes and panelboards without being connected to the boxes or panelboard, thus slightly increasing the impedance of the ground-fault return path. These other sections of the Code also reference 250-74 Exception No. 4 for installation requirements. The installation requirements ensure that the safety objectives of equipment grounding conductors are preserved and allow for the reduction of electromagnetic interference.
Many devices are available to reduce harmonic distortion or otherwise improve power quality. Article 280 contains rules about surge arresters; some are useful in protecting equipment from power-quality problems. Otherwise, the Code has no special rules about power conditioners, filters, special-purpose transformers, or other mitigation devices. Manufacturer design, installation guides, and IEEE documents provide information about the proper use of these devices.
Harmonic currents that distort waveforms and can lead to overheating conductors or equipment are of particular interest. Excessive neutral currents due to the additive nature of odd harmonics in wye-connected systems have been the subject of much discussion. Many proposals were made for the 1993 NEC to address the issue of harmonic currents. These included proposals ranging from adding warnings or advice in Fine Print Notes to requiring neutrals to be doubled in size.
In response to these proposals, the NEC Correlating Committee formed a subcommittee to study the issue of harmonic currents and nonlinear loads. The Report of the NEC Correlating Committee Ad Hoc Subcommittee on Nonlinear Loads was published in the Report on Proposals for the 1996 NEC. The report included the following summary statement:
"The conclusion of the subcommittee is that while nonlinear loads can cause undesirable operational effects, including additional heating, no significant threat to persons and property has been adequately substantiated." This is not to say that there was no evidence, only that it was anecdotal and not "adequately substantiated." Although the committee stated it agreed "with the present Code text regarding nonlinear loads," it did make four recommendations, all of which were implemented in the 1996 NEC. The committee recommended adding a definition of nonlinear load, revising Code text to use the term nonlinear load, adding Fine Print Notes that describe or warn of the problems that come with nonlinear loads, and changing the rules of Section 310-4 to permit paralleling of neutrals smaller than 1/0 AWG in existing installations.
New installations can be readily designed to handle the harmonic distortion created by nonlinear loads. The expanding use of nonlinear loads in existing installations is less easily remedied. Therefore, the subcommittee also recommended that standards organizations "work with manufacturers of nonlinear load products" to reduce harmonic distortion at the sources.
Additional heating due to harmonics is most likely to occur in wye-connected four-wire systems with line-to-neutral connected loads. Other than the change in Section 310-4 mentioned above, two Code rules are most important with regard to the neutral conductors of such systems. First, Section 220-22 allows neutral conductors to be reduced in size from the ungrounded conductor size where the load on the neutral is primarily due to unbalance. A reduction is not allowed for that portion of the load that is nonlinear. The second rule is in Note 10(b) and (c) to the Allowable Ampacity Tables 310-16 through 310-19. This rule requires neutral conductors of wye-connected systems to be counted as current-carrying conductors for the purposes of Note 8 "where the major portion of the load consists of nonlinear loads." Directly or indirectly, both rules have the effect of increasing the size of neutral conductors subjected to harmonic currents.
The NEC does not have any specific rules that require neutral conductors to be increased in size above the size of the ungrounded conductors. The neutral current can theoretically be as much as 173% of the balanced ungrounded conductor current due to nonlinear loads. Nevertheless, the subcommittee found that in actual surveys relatively few installations (about 5%) had neutral currents exceeding the ampacity of the neutral conductor. The subcommittee concluded there was insufficient evidence to require neutral conductors be oversized by any standard factor.
Heating of transformers due to non-sinusoidal currents is also a concern. Special transformers, commonly called K-rated transformers, are available to help deal with this problem. The NEC does not provide any special rules for K-rated transformers. However, 450-3 FPN No. 2 calls attention to the problem, and 450-9 FPN No. 2 provides a reference to IEEE Recommended Practice for Establishing Transformer Capability When Supplying Nonsinusoidal Load Currents, ANSI/IEEE C57.110-1986. Other Fine Print Notes that address nonlinear loads can be found following the definition of nonlinear load in Article 100 and following Sections 210-4(a), 220-22, and 310-4. All of these Code references provide advice, but leave the solutions to design professionals.