You're asking about shared neutrals in a 120/208V panel, which falls under the umbrella of multi-wire branch circuits (MWBCs). Here's a breakdown of the rules and considerations

The Basics of Multi-Wire Branch Circuits (MWBCs) in a 120/208V System
What is it? An MWBC has two or more ungrounded (hot) conductors sharing a common neutral conductor. These ungrounded conductors originate from different phases of the panel. How it works (ideally)

In a balanced system, the loads connected to each phase are relatively equal. Since the phases are 120 degrees apart (in a three-phase system), the currents returning on the neutral from each phase partially cancel each other out. Ideally, the neutral current will be significantly lower than the current on any one of the hot wires.

Key Requirement

All hot conductors of an MWBC

MUST originate from different phases. This is critical for the cancellation effect to work.

Can you share a neutral with all three phases? Yes, BUT with specific conditions and caveats



Theoretically Correct

If you are running a balanced MWBC from all three phases (A, B, and C), and each phase is loaded similarly, the neutral current should be less than the individual phase currents. In such a scenario, you could use a single neutral of the same size as the hot conductors.

Real-World Considerations - Where it gets tricky



Balance is Key

The cancellation effect only works well if the loads on each phase are relatively balanced. In reality, it's difficult to guarantee perfect balance, especially with convenience receptacles where you don't know what appliances might be plugged in.

Harmonics

Nonlinear loads (like computers, LED lighting, electronic ballasts, etc.) generate harmonic currents. These harmonics do not cancel out on the neutral; in fact, the third harmonic and its multiples (9th, 15th, etc.) add together on the neutral. This can cause the neutral conductor to carry a significantly higher current than the hot conductors, even with seemingly balanced loads.

Code Requirements (NEC) and Best Practices1. Common Trip

The NEC

requires that all ungrounded conductors of an MWBC be disconnected simultaneously by a multi-pole circuit breaker. This prevents a scenario where one phase is still energized while the neutral is disconnected, potentially causing dangerous overvoltage on the remaining circuits. Individual single-pole breakers are NOT permitted for MWBCs. The MWBC must have a 2-pole (for 2 hots sharing a neutral) or a 3-pole (for 3 hots sharing a neutral) breaker.2. Neutral Sizing

The NEC generally allows the neutral to be the same size as the hot conductors if the load is primarily linear (resistive loads like incandescent lighting, heaters). However, the Code also requires you to increase the neutral size if a significant portion of the load is nonlinear (generates harmonics). There is guidance in the NEC on determining when an increase is necessary. Consult your local electrical code enforcement or a qualified electrician for the rules in your area.3.

Neutral Integrity

The neutral conductor of an MWBC

MUST NOT be broken or opened at any point. This is why you can't use standard receptacles to "splice" or "pass through" the neutral. Pigtail the neutral connections at each receptacle.4. Marking

It's good practice (and may be required by local codes) to clearly mark all conductors associated with an MWBC (e.g., with colored tape at the panel and at each outlet box). This helps prevent accidental disconnections or miswiring.5.

Dedicated Neutral is Safer and Often Recommended

While sharing a neutral can be done correctly, the safest and often the simplest approach is to run a dedicated neutral for each branch circuit. This eliminates the risk of overloading the neutral and simplifies troubleshooting.

Recommendations


Consider Dedicated Neutrals

For general convenience receptacles, running a dedicated neutral for each circuit is strongly recommended. It simplifies the design, reduces the risk of overloading the neutral, and makes troubleshooting easier.2.

Evaluate Your Load

If you are determined to use MWBCs, carefully evaluate the types of loads that will be connected to the receptacles. If you anticipate significant nonlinear loads (computers, electronics, etc.), increase the neutral size.3.

Consult a Qualified Electrician

The best approach is to consult with a licensed electrician who can assess your specific situation, perform load calculations, and ensure that the wiring complies with all applicable codes and regulations. They can properly size the conductors, select the appropriate circuit breakers, and ensure the installation is safe and reliable. This is especially important in commercial or industrial settings.4.

Don't Guess

Electrical work is not something to guess at. Improper wiring can create serious fire and shock hazards.

In summary: While sharing a neutral on a 120/208V system is theoretically possible, the complexity of balancing loads, dealing with harmonics, and complying with the NEC makes it a more advanced wiring technique. Unless you have a very specific reason to use MWBCs for convenience receptacles and are confident in your ability to do so correctly, using dedicated neutrals is the simpler and safer option. Always consult a qualified electrician for any electrical work. Flag for review