Sizing Up

The grounded conductor is defined in Article 100 in the National Electrical Code (NEC), but this definition can cause confusion when it’s used in an electrical system. For example, the grounded conductor, as previously defined, could be a corner-grounded phase conductor per 250.26(4) and 250.20, Informational Note. It could also be a common grounded-neutral conductor in a three-phase, 4-wire, wye-connected system per 250.26(3). To help Code users, new definitions of the terms “neutral point” and “neutral conductor” were added to the 2008 NEC. Article 100 effectively covers these definitions, and other sections are used when dealing with these defined terms.

Neutral point
Article 100 defines the term “neutral point” as the common point on a wye-connection in a polyphase system or the midpoint of a single-phase, 3-wire system; of a single-phase portion of 3-phase delta system; or of a 3-wire, direct-current system. It seems that the XO of the source, such as at a transformer or the LO connection at a generator, is the neutral point.

Neutral conductor
The term “neutral conductor” is defined as the conductor connected to the neutral point of a system that is intended to carry current under normal operating conditions.

Grounded conductor
The term “grounded conductor” is defined as a system or circuit conductor that is intentionally grounded.

Note that a grounded-phase conductor must be sized differently than the neutral conductor.

Sizing the grounded conductor
Section 250.24(C)(3) specifies that a grounded-phase conductor of a three-phase, 3-wire delta-connected service has an ampacity not less than that of the ungrounded conductors. Remember that the grounded conductor must be identified using white or gray as required in Section 200.6 of the NEC.

Sizing the neutral
The feeder or service-neutral load (basic calculation) is the maximum unbalance of the load determined by Article 220. The wording in this section makes it clear that the maximum unbalance load must be the maximum net calculated load between the neutral conductor and any ungrounded-phase conductor.

Under certain conditions of use, the neutral may be reduced in size. For example, a feeder or service supplying household electric cooking equipment and dryers may have a demand factor of 70 percent applied to its total load, based on the number of appliances (dryers) present. Review 220.61(B)(1) to get familiar with this permitted rule. By following the requirements of 220.61(B)(2), the neutral may have a demand factor of 70 percent applied when deriving the load to size the neutral conductor.

The neutral in the branch-circuit loads
The 2011 NEC has deleted Exception 2 to 210.19(A)(1). The reason is that 240.4(D) requires the neutral conductor to be the same size as the ungrounded-phase conductors that are listed in sizes 18 AWG through 10 AWG copper. Also, this provision is permissive and is not mandatory.

The neutral in feeder loads
The Exception in 215.2(A)(1) has been retained and does not require the neutral to be calculated at 125 percent for the continuous load plus the noncontinuous load, including loads where demand factors are applied per Article 220.

The neutral and service loads
The 2011 NEC has a new exception to 230.42(A)(1), which permits the neutral to be sized at 100 percent of the continuous load plus 100 percent of the noncontinuous load, which also permits application of demand factors per Article 220.

An Informational Note 2 in 220.61 addresses three-phase, 4-wire, wye-connected systems, where the neutral might be carrying a high amount of nonlinear (harmonic currents). When such a situation exists, the neutral is usually increased in size to account for the added heat that these types of loads can produce. Note that 220.61(C)(2) prohibits the neutral to be reduced in size if the major portion of the load on such a neutral consists of nonlinear loads. Reduction is also prohibited if any portion of a 3-wire circuit consisting of 2-wire ungrounded conductors and the neutral conductor of a three-phase, 4-wire, wye-connected system is installed as outlined in 220.61(C)(1).

When is the neutral current-carrying?
Contractors and electricians use 310.15(B)(5)(c) to determine whether the neutral is current-carrying or not. This section states that on a 4-wire, three-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; therefore, the neutral is considered a current--carrying conductor. “Major portion” is described in 400.5(A) as being more than 50 percent, and if calculated in this manner, the neutral is considered current-carrying.

STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.