Let's break down your questions regarding NEC 310.15(E) and neutral conductor considerations

Why NEC Doesn't Need Point 3 (Nonlinear Loads in 3-Phase, 4-Wire Wye Systems)

You are correct that at first glance, it might seem redundant. However, the NEC adds point 3 to emphasize the importance of counting the neutral conductor when dealing with nonlinear loads in 3-phase, 4-wire wye systems. While the general rule (point 2) would apply, the Code makes it explicitly clear because of the potential for harmonic currents.Here's the breakdown: Harmonics and the Neutral

Nonlinear loads (like electronic ballasts, variable frequency drives, some LED drivers, and computers) draw current in a non-sinusoidal waveform. This waveform contains harmonics – multiples of the fundamental frequency (60 Hz in the US). In a balanced 3-phase system with linear loads, these harmonic currents tend to cancel out in the neutral. However, in a 3-phase, 4-wire wye system with significant nonlinear loads, the

third harmonic (and odd multiples like the 9th, 15th, etc.) do not cancel out. Instead, they add in the neutral conductor. Overheating and Damage

This can lead to the neutral carrying a current that is significantly higher than the phase currents. If the neutral is not sized and treated as a current-carrying conductor, it can overheat, damage the insulation, and even create a fire hazard.

Code Emphasis

The Code specifically calls this out to ensure that designers and installers are aware of the potential for harmonic currents and properly size and protect the neutral conductor in these situations. The emphasis on this point serves as a reminder to conduct due diligence regarding the harmonic nature of loads on the system.

Why Single-Phase/Split-Phase Systems Aren't Explicitly Included (Regarding Receptacles and Neutral Conductor Counts)

This is a crucial point about how the NEC addresses ampacity adjustments. Here's why the situation you described isn't covered by the 310.15(E) rules:1. The Focus of 310.15(E) is Ampacity Adjustment Due to Bundling

The core purpose of 310.15(E) (and its associated tables) is to provide derating factors when multiple current-carrying conductors are bundled together in a raceway, cable, or similar enclosure. The heat generated by these bundled conductors affects their ampacity. The NEC's concern in your scenario is with multiple conductors in the same raceway, causing a heat build-up that affects the ampacity rating of the conductors.2.

Standard Single-Phase Circuits Aren't "Bundled" Enough

In a typical installation of two 120V circuits (one hot and one neutral per circuit) for two receptacles, the NEC generally assumes they aren't so closely bundled that significant heat buildup will occur. You are right about each wire carrying equal current. BUT, the question is are those circuits in close proximity that their heat will affect each others ampacity rating?3.

Individual Circuit Capacity

Each individual circuit of the two receptacles, if installed as a single circuit (1 hot, 1 neutral, 1 ground), will have the same ampacity rating as if it were a dedicated circuit with no other conductors bundled with it (unless the total number of current-carrying conductors in the raceway exceeds three).4.

What Would Make the Neutral Count? Multi-Wire Branch Circuit (MWBC)

As you correctly excluded, an MWBC is the key exception. In an MWBC, you have two or more ungrounded (hot) conductors sharing a single neutral conductor. If these ungrounded conductors are on different phases, the neutral will carry the unbalanced current. In this case, the neutral must be counted as a current-carrying conductor when applying derating factors according to 310.15(E). This is because an MWBC is considered a significant bundling of conductors.

Long Runs with High Load

If the runs are excessively long with extremely high loads on each circuit, it might* be argued that the combined heat of the two circuits significantly reduces the ampacity. However, this is less common and would require careful engineering calculations to justify.

In Summary:

The NEC's focus in 310.15(E) is primarily on addressing situations where multiple current-carrying conductors are closely bundled, leading to significant heat buildup and necessitating ampacity derating. Standard single-phase circuits for a few receptacles, while having current in the neutral, typically don't fall under this bundling scenario, and the code accounts for their individual current-carrying capacity without the need for additional derating.If you install multiple circuits in the same conduit, you would need to calculate the adjustment factors based on the number of current-carrying conductors. The neutral would be counted as current-carrying for any MWBC, and for the third harmonic issues. Flag for review