Okay, let's break down why your plan to derate the neutral to #2 might be problematic in this scenario. While there isn't a single NEC section explicitly forbidding you from using a #2 neutral in every situation, the interaction of your specific loads with the Code's requirements makes it highly unlikely to be compliant.Here's why

1. Non-Linear Loads from VFDs:
Harmonics

Variable Frequency Drives (VFDs) are significant generators of harmonic currents, especially odd-order harmonics like the 3rd, 5th, 7th, etc. A crucial aspect is that

triplen harmonics (3rd, 9th, 15th, etc.) are additive in the neutral conductor in a three-phase, four-wire system. This means these harmonics don't cancel out; instead, they sum up in the neutral. Excessive Neutral Current

The neutral current can easily exceed the phase (line) current due to these harmonic currents. This is the core reason why the neutral conductor needs to be sized carefully when dealing with VFDs.

2. NEC Requirements and Considerations



NEC 220.61 (Neutral Conductor Size)

While this section allows for demand factors and reduced neutral sizing for certain resistive loads (like electric ranges, dryers), it specifically excludes loads where harmonics are present.

NEC 220.20 (Unusual Loads)

This section is a catch-all for loads that aren't specifically covered elsewhere. VFDs, with their harmonic content, definitely fall into this category. You'd likely need an engineering study or load calculation that specifically accounts for the harmonic currents to justify a reduced neutral size.

NEC 215.2(A)(1) (Feeder Conductor Ampacity)

This section requires the feeder conductors to have an ampacity sufficient to carry the calculated load after applying any applicable demand factors. If the neutral carries more current than the phase conductors due to harmonics, the feeder neutral must be sized accordingly.

NEC 310.15 (Ampacity of Conductors)

This section covers the ampacity of conductors in general and includes provisions for derating due to ambient temperature, the number of conductors in a raceway, etc. You'd need to ensure your #2 neutral has sufficient ampacity after these deratings, considering the likely increased current from the VFDs.

3. Why a #2 Neutral is Almost Certainly Too Small



200 Amp Panel

A 200 amp panel is typically fed with conductors sized at a minimum of #3/0 copper or 250 kcmil aluminum (based on 75°C termination rating). Derating the neutral that significantly (to a #2) is highly unusual, especially with VFDs involved.

VFD Load Dominance

Since you're feeding two VFDs, they are likely the dominant load on this panel. Even if the nameplate current of the VFDs doesn't seem that high, the harmonic currents they produce will greatly increase the neutral current.

7 Amps of Lighting and 2 x 20A Receptacles

While the lights and receptacles contribute to the overall load, they are likely secondary compared to the VFDs in terms of neutral current impact. The harmonic current from the VFDs will swamp out the relatively small contribution from these linear loads.

How to PROVE it's Wrong

To prove that a #2 neutral is sufficient, you would need to do the following (and convince the Authority Having Jurisdiction - AHJ):1.

Comprehensive Load Calculation

A detailed load calculation that includes all loads on the panel, including the nameplate currents of the VFDs, the lighting load, and the receptacle load. This is the starting point, not the ending point.2.

Harmonic Current Analysis

This is the critical part. You need to obtain harmonic current data from the VFD manufacturers (typically as a percentage of the fundamental current at different load levels). If you cannot obtain this data, you MUST assume the worst-case scenario.3.

Neutral Current Calculation

Calculate the expected neutral current. This calculation

must account for: The additive effects of the triplen harmonics from the VFDs. Any imbalances in the single-phase loads.4. Ampacity Verification

Verify that the #2 neutral conductor (after any applicable derating factors for ambient temperature, number of conductors in the raceway, etc.) has sufficient ampacity to carry the calculated neutral current with a substantial margin of safety. If you cannot find manufacturers harmonic data you MUST account for 100% of all harmonic loading.5.

Professional Stamped Engineering Report (Highly Recommended)

Present all of this information in a professional, well-documented engineering report stamped by a licensed electrical engineer. This significantly increases the chances of AHJ approval.

Bottom Line

While it's theoretically possible that a #2 neutral could be sufficient under very* specific and unusual circumstances, it's extremely unlikely to be the case with two VFDs. Attempting to drastically undersize the neutral conductor poses a significant risk of overheating, insulation damage, and potentially fire. A full-size neutral is necessary!

I strongly advise against trying to use a #2 neutral in this situation without a thorough harmonic analysis and the approval of a licensed electrical engineer and the local AHJ. It's simply not worth the risk. Flag for review