Electrical engineers, inspectors and contractors, and the National Electrical Code (NEC), have dealt with nonlinear loads for many years. The issue arose in the late 1970s and early 1980s with the increased use of solid-state devices in the mainstream of the electrical industry. (Electronics, such as computers, electronic and magnetic-ballasted lighting and adjustable-speed drive units, are responsible for most nonlinear loads.) However, the effect of nonlinear loads on the electrical system was not recognized until the latter part of the 1980s when several technical articles warned that nonlinear loads were affecting the amount of current on the neutral conductor. Studies were conducted by interested parties, and proposals and comments were submitted to the 1993 NEC. However, it wasn’t until after the 1993 NEC was adopted that the NEC Correlating Committee assigned an Ad Hoc Committee on Nonlinear Loads to determine what changes were needed in the Code to deal with the effects of nonlinear loads.
At the time, I was a nonvoting member of the committee, serving as the NFPA staff liaison with the assignment of developing necessary proposals for the 1996 NEC to address nonlinear load problems. The Ad Hoc Committee was staffed with some of the most brilliant, leading experts in the electrical industry.
The first mission was to establish a definition for nonlinear loads and resulted in this: “A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.” This definition has not changed since its inclusion in the 1996 NEC. We then studied data on approximately 2,100 office buildings supplied by data and design consultants, electrical equipment manufacturers, the Environmental Protection Agency and testing laboratories.
We found one neutral-related failure in the 2,100 buildings and, upon further investigation, determined the failure was due to a bad cable splice at the time of cable manufacture. We investigated the effects of harmonics caused by nonlinear loads on the neutral conductors for feeders and branch circuits, transformers, the electrical equipment, such as panelboards, motors and adjustable speed drive units, and the possible increased sizing of the neutral conductor for both linear and nonlinear loads. We inserted various Fine Print Notes (FPNs) dealing with nonmandatory warnings about nonlinear loads throughout the NEC, and two new mandatory or permissive sections were inserted into the NEC itself. Let’s look at the FPNs, now called Informational Notes, and the two mandatory changes inserted into the NEC, and how these changes still apply to our present installations.
Section 210.4(A), Informational Note No. 1 (was in 210-4, FPN in the 1996 NEC), covering multiwire branch circuit high harmonic currents, still states as follows: “A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic currents on the neutral conductor.” The same FPN was inserted into 220-22 in the 1996 NEC and was relocated to 220.61(C) in the 2005 NEC where this section covers neutral conductor sizing and applying prohibited reductions in neutral conductor size. This warning about high harmonic currents in the neutral is still necessary for anyone sizing a neutral conductor. Finally, an FPN was added to 450.3 in 1996 that is still in the same location and states, “Informational Note No. 2: Nonlinear loads can increase heat in a transformer without operating its overcurrent protective device.” The mandatory or permissive requirements were inserted into 310.15(B)(5)(c) that states, “On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.” An exception with FPN also was added to 310-4 dealing with parallel conductors in 1996. It is now located in 310.10(H) as an exception and Informational Note as follows: “Exception No. 2: Under engineering supervision, 2 AWG and 1 AWG grounded neutral conductors shall be permitted to be installed in parallel for existing installations. Informational Note to Exception No. 2: Exception No. 2 can be used to alleviate overheating of neutral conductors in existing installations due to high content of triplen harmonic currents.”
The electrical industry is still dealing with nonlinear loads, harmonics and the effect on the electrical system. Though there have been few changes between the 1996 and 2017 NEC editions, these issues are still as important as they ever were.