Welcome back to the continuing in-depth discussion on National Electrical Code requirements related to branch circuits and feeders. Last month’s article looked at overcurrent protection and the concept of the feeder tap rules that allow conductors to be protected from overcurrent by splitting the purpose of the overcurrent protective device (OCPD) using carefully selected upstream and downstream devices. This month, we will look at another allowance to have a circuit conductor with an ampacity less than the rating of the OCPD: feeder grounded (neutral) conductors.

When it comes to the feeder grounded conductor, if present, one thing to understand is its purpose. Unlike the service grounded conductor, the grounded conductor typically doesn’t serve as the only or main return path for fault current. This means that if the system uses a neutral conductor, which must be the grounded conductor in a grounded system, it must be brought to the service equipment. 

But feeders are different. In a feeder, the main reason a neutral conductor is needed is whether the feeder supplies any line-to-neutral loads. During normal circuit operations, this conductor will carry the imbalance between the ungrounded conductors. Therefore, there are many requirements that affect the size and ampacity of this circuit conductor.

Determining current

First and foremost, there are situations where the imbalance between ungrounded conductors will result in essentially the same amount of neutral current as the ungrounded conductors. Probably the most common example of this is a feeder on a three-phase wye system that uses two of the three ungrounded conductors and the neutral, such as a feeder to an individual unit in an apartment building. The service might be 208Y/120V and the feeder supplies a single-phase panelboard in the unit. In this situation, the neutral must be the same size conductor as the ungrounded conductors and will be protected from overcurrent by the feeder OCPD.

However, what if the system is 120/240V single-phase? In this situation, the neutral conductor in the feeder to the unit is only carrying the imbalance current of the ungrounded conductors. So, if leg A has a current of 65A and leg B has a current of 60A, the neutral will only carry 5A. 

If the ungrounded conductors are protected by a 100A OCPD and sized appropriately, they would likely be 3 AWG conductors, if copper. But since there will only be 5A of current on the neutral conductor, a much smaller conductor could be used. However, before we start thinking that a 16 AWG copper conductor with an ampacity of 10A would suffice, the NEC has a few tricks up its sleeve.

Section 120.61 discusses how to account for the load that will fall on the feeder neutral conductor. The basic rule is that the calculated neutral load used for sizing the conductor must be the maximum unbalanced load. This can be determined by going through the total feeder load calculation and using all line-to-neutral loads expected on the feeder. 

Although it would be poor practice to put all line-to-neutral loads on the same phase, it could happen. There is an example of how the feeder neutral load is determined in Annex D, Example D4(a). 

Load diversity

However, feeders often benefit from the concept of “load diversity.” This means that for all the loads supplied by the feeder, it is unlikely that all loads will be running at maximum current draw all at once. The NEC allows users to reduce the load on the feeder neutral because of this, but there are some restrictions.

For feeders supplying household cooking equipment and electric clothes dryers, the neutral load portion of the feeder calculation is determined using a demand factor of 70% of the maximum unbalanced load as determined in Tables 120.54 and 120.55. This is sort of a double reduction, as these tables in the NEC also factor in load diversity, especially for household cooking equipment where it is unlikely that all heating elements will be used at maximum capacity at the same time. Table 120.54 also makes it clear that the more electric clothes dryers are supplied by the feeder, the larger the reduction of the total load on the feeder.

The second allowance for reduction of the neutral load applies only to the portion of neutral load that exceeds 200A. If the neutral load on the feeder is 200A or less, then there is no reduction. But if the neutral load exceeds 200A, the portion that exceeds this value is allowed to have a demand factor of 70% applied. For example, if the maximum line-to-neutral load on a feeder is 1,200A, the load ultimately used for determining the size of the neutral conductor is only 900A. However, this only applies to feeders that include all system conductors, such as a 4-wire, three-phase system or a 3-wire, single-­phase system. 

There are also restrictions on certain feeders where the reduction in calculated load is not allowed. As mentioned previously, when feeders consist of two ungrounded conductors and the neutral conductor of a 4-wire, three-phase, wye-connected system, the neutral conductor will carry approximately the same amount of current as the ungrounded conductors. A reduction in the size of the feeder’s neutral would not make sense as the smaller conductor would likely overheat and cause damage to the circuit, or worse.

There is also a prohibition on reducing the neutral load for the portion of the feeder neutral that supplies nonlinear loads. Nonlinear loads present an issue with harmonic currents on the circuit. This leads to overheating issues and, ultimately, damage to the conductors and equipment if the neutral conductor is not sized properly. An informational note in Section 120.61(C) mentions that high harmonic neutral currents might need special attention during the design stage to account for these currents. There are several types of loads classified as nonlinear, but the most common type is fluorescent or LED lighting.

Finally, remember that the neutral conductor is part of the fault current path in a short circuit. Therefore, it must be sized large enough to facilitate the operation of the feeder OCPD. This is identical to the approach for the equipment grounding conductor that forms the effective ground-fault current path. Because of this function of a feeder neutral conductor, it must never be sized smaller than the feeder equipment grounding conductor sized from Table 250.122, which is based on the size of the OCPD and ensures the fault current path is properly sized to facilitate automatic operation of the OCPD. 

Where’s the difference?

There is one major difference in sizing the grounded/neutral conductor such as the EGC. Since the neutral will carry short-circuit current and the EGC will carry ground-fault current, the neutral is not subject to the requirement that the full-sized conductor be installed in each parallel run. Parallel neutral conductors will be connected and short-circuit current will be divided among the parallel conductors, whereas parallel EGCs could need to handle the full fault current for longer.

To summarize the feeder grounded/neutral conductor conversation, there are two major concerns: 

• Ensure the conductor can handle the amount of current likely to be imposed.
• Ensure the conductor can safely facilitate opening the OCPD in a fault.

When the system is properly designed and the load is balanced between phases, the neutral conductor will rarely be an issue. But deviate from proper design, even a little, and the results can be catastrophic! 

Next month’s article will wrap up this deep dive into branch circuits and feeders and preview the next area of the NEC we will cover.

Until next time, remember to stay safe and always test before you touch!

Derek Vigstol