Yes, you are absolutely correct! The differences in current flow between a 120/240V single-phase system and a 208/120V three-phase system, especially when loaded with single-phase loads, significantly impact conductor sizing, voltage drop, and heat generation. Here's a breakdown of why this matters and how it's handled

Understanding the Issue: Neutral Current and I²R Losses
120/240V Single-Phase (3-wire)

When equal 120V loads are connected to each leg (L1-Neutral and L2-Neutral), the neutral current ideally cancels out. This is because the current flows in opposite directions in the L1 and L2 lines. However, in practice, there might be a slight imbalance, leading to a small amount of neutral current. But ideally, neutral has zero current.

208/120V Three-Phase (4-wire, Wye)

This is where it gets trickier. With balanced 120V single-phase loads connected to each phase (A-Neutral, B-Neutral, C-Neutral), the neutral current ideally would also be zero.

However, if the loads are unbalanced (i.e., different wattages on each phase), the neutral will carry current, and this is a significant factor. Also, with no loads present, due to the three phases being 120 degrees out of phase with each other, each leg carries current. I²R Losses (Heat Generation)

The power loss due to the resistance of a conductor is calculated as I²R, where: `I` is the current flowing through the conductor. `R` is the resistance of the conductor.Higher current means significantly higher power loss (and thus, heat generation) because of the squared relationship.

Why It Matters: Implications for Wiring and Safety1. Conductor Sizing



208/120V, Unbalanced Loads

The neutral conductor must be sized to handle the maximum potential unbalanced neutral current. NEC (National Electrical Code) Article 220.61 provides guidance on calculating the neutral load for feeders and services. The code recognizes that in many installations, the unbalanced load is likely to be less than the total connected load, allowing for a demand factor. The neutral conductor in a single family dwelling is not required to be larger than the ungrounded conductors. However, the neutral shall not be smaller than the minimum size required by 250.24(C).
2.

Voltage Drop

Higher current leads to higher voltage drop along the conductors. This voltage drop can affect the performance of sensitive equipment. Voltage drop calculations must account for the actual current being carried by each conductor, including the neutral.3.

Overheating and Fire Hazard

Excessive current in conductors that are not properly sized can lead to overheating, insulation degradation, and potentially fires. This is especially critical in bundled cables where heat dissipation is reduced.4.

Harmonic Currents

Non-linear loads (like many electronic devices) can introduce harmonic currents, which can exacerbate neutral current issues in three-phase systems. These harmonics often add arithmetically in the neutral, leading to significantly higher neutral current than anticipated based on the fundamental frequency load alone.

How It's Addressed in Electrical Design and Installation Load Balancing

Designers try to balance single-phase loads across the three phases of a 208/120V system as much as possible to minimize neutral current.

Oversizing Neutrals

In some cases, particularly where significant unbalanced loads or non-linear loads are expected, the neutral conductor is intentionally oversized to handle the higher current.

Dedicated Neutrals

For circuits serving sensitive electronic equipment, dedicated neutral conductors can help isolate noise and reduce the impact of harmonic currents.

Proper Wiring Practices

Correct connections, tight terminals, and proper grounding are crucial to minimize voltage drop and ensure safe current flow.

Derating

The NEC requires ampacity derating of conductors when multiple current-carrying conductors are bundled together in a raceway or cable. This accounts for the reduced heat dissipation and prevents overheating.

Your point about the I²R loss increasing the heat in the jacket is exactly why this derating factor is applied. Neutral Overcurrent Protection (Limited)

While generally not permitted, there are limited cases where neutral overcurrent protection is allowed or required, especially in specific industrial applications.

Using Transformers

Some installations use step-down transformers to create isolated 120/240V single-phase circuits from the 208/120V three-phase source, which can help manage neutral current issues.

In summary:

Yes, the differences in current flow and associated I²R losses between 120/240V single-phase and 208/120V three-phase systems absolutely must be taken into consideration during electrical design and installation. Failure to do so can lead to safety hazards, equipment malfunctions, and code violations. Load balancing, proper conductor sizing (including the neutral), derating, and consideration of harmonic currents are all essential aspects of a safe and reliable electrical system. Flag for review