Yes, your electrician is correct. What you're proposing, as described, is likely a code violation. Here's why, and a breakdown of NEC implications

The Problem: Shared Neutral and Multiwire Branch Circuits (MWBCs)

The core issue is the potential for creating a Multiwire Branch Circuit (MWBC) unintentionally and sharing a neutral between two circuits that could be on different phases. Even if the neutral doesn't physically connect in the fan fixture itself, it's the potential for current to flow back on the shared neutral in an unexpected way. When you connect a 120V exhaust fan/light combo to two different 120V circuits, and both circuits share a neutral back to the panel, you create a potential for a dangerous situation. Here's why:1. Unbalanced Current

If both circuits are on different phases (which is likely, unless you intentionally specify otherwise), the neutral can carry the sum of the current from both circuits if they are loaded unevenly. This can overload the neutral conductor, potentially causing overheating and fire.2.

Violation of NEC 300.3(B)(1): Conductors of Different Systems. This section generally prohibits the installation of conductors of different systems (e.g., a Class 1 power-limited circuit and a general lighting circuit) within the same raceway, cable, or enclosure, unless specifically permitted elsewhere in the code. While technically you're not talking about "different systems," the intent is similar: to prevent the accidental mixing of power sources in a way that can create hazards.3. Violation of NEC 300.3(C)(1) Single Conductor. This section states "All conductors of the circuit, including the equipment grounding conductor and, where used, the grounded conductor, shall be contained within the same raceway, cable, or cord or shall otherwise run together."Specific NEC Sections to Consider NEC 210.4 Multiwire Branch Circuits

This section outlines the requirements for MWBCs, specifically addressing the need for a common disconnect to simultaneously disconnect all ungrounded conductors. Your proposal might unintentionally create a condition that resembles an MWBC in terms of neutral loading.

NEC 300.3 Wiring Methods

This article covers the general requirements for wiring methods. The sub-sections related to conductors of different systems and circuits within the same enclosure are relevant.

NEC 404.8 Accessibility and Grouping

This might not be a direct violation, but could be argued if the switches are close together, suggesting a single circuit.

NEC 404.2(C) Switch Connections. "The grounded circuit conductor shall not be disconnected by switches or overcurrent protection." Your proposal might accidentally disconnect the neutral conductor if one circuit's switch is off.

What if both circuits originated from the SAME phase?

Even if both circuits originate from the same phase, it still isn't a good idea and might still violate code, although the immediate danger of 240V isn't present.1. Neutral Overloading (Still Possible)

Even on the same phase, if the neutral conductor is undersized for the combined load of both circuits, it can still overheat and fail. You need to perform load calculations and ensure the neutral is sized appropriately.2.

Confusion and Future Modifications

Even if it's technically compliant, it's a very unusual configuration. Future electricians might not understand the setup and could unknowingly create a hazard by reconfiguring the circuits.3.

NEC 300.3(B)(1): There might be some interpretation issues by electrical inspectorAcceptable Solutions

Here are some safer and code-compliant options:1. Single Circuit with Controls

The best option is to use a single 120V circuit for the fan/light combo. Use a smart fan and light controller that can be programmed for occupancy (motion sensor) and humidity. Many such devices are available and are specifically designed for bathrooms. This eliminates the problem of shared neutrals and potential phase conflicts.2.

Separate Fan and Light Fixtures

Install a dedicated exhaust fan on one 120V circuit (motion/humidity controlled) and a separate light fixture on another 120V circuit (key card controlled). This avoids the shared neutral issue and allows for independent control.3.

Relay Control

Use one 120V circuit to power the fan/light combo. Then use a low-voltage (e.g., 24V) relay circuit from the key card switch to control the power to the fan light combo. The relay would act as a switch, allowing the key card to energize or de-energize the 120V circuit to the fan/light combo. This keeps the 120V power separate and controlled by a single circuit.4.

Low-Voltage Lighting (LED)

Use a 12V or 24V DC LED light powered by a power supply. The power supply can be controlled by the key card switch. This completely isolates the lighting from the 120V fan circuit.

Key Takeaways Avoid sharing neutrals between circuits that could be on different phases. Follow NEC guidelines for Multiwire Branch Circuits if you intentionally create one. Consider simpler solutions like integrated fan/light controllers. Always consult with a qualified electrical engineer or master electrician for your specific project to ensure compliance with all applicable codes and regulations.

Disclaimer: Always consult with a qualified professional for your specific project. The NEC is a complex document, and interpretations can vary. Your local electrical inspector has the final authority on code compliance. Flag for review