You've correctly identified the core concept and the potential confusion! Let's break this down to clarify how it works in practice.

Understanding the Zero Neutral Current in a Balanced 3-Phase System Phasor Addition

In a perfectly balanced 3-phase system, the currents in each phase are 120 degrees apart. When you add these currents as phasors (vectors with magnitude and angle), they cancel each other out in the neutral conductor. Think of it like three people pulling equally on a rope at 120-degree angles – the net force at the center is zero.

Why it Doesn't "Teleport" Away

The cancellation doesn't happen magically at some distant point. It happens continuously along the neutral conductor back to the source. The currents from each phase are constantly interacting and canceling each other out.

Addressing Your Scenario: A-N, B-N, C-N Loads at 100 Amps Each

Let's clarify the circuit. You have a 3-phase, 4-wire system. After the switch, you have: A-N: 100 Amps B-N: 100 Amps C-N: 100 AmpsHere's how it works in practice


Upstream of the Switch

The neutral should carry zero amps, as you correctly pointed out if the loads are equal.2.

Downstream of the Switch (Individual Connections)

This is where your thinking is spot on. Each individual A-N, B-N, and C-N circuit does have 100 Amps flowing in its neutral wire, near the load! The key is that these aren't three independent 100A neutrals, but instead the same neutral, and you're only moving further up the neutral conductor.3.

The Neutral Conductor Back to the Source

All of these neutral conductors are connected to the same neutral wire. The current in that common neutral wire decreases along its length as you move away from the loads and towards the source. By the time the neutral reaches the switch it carries 0 amps. The current starts at 100 amps at each individual load, then the three individual neutrals carry 100 amps. The current adds up to 0 amps as the current travels up the wire.

Conductor Sizing and Termination Conductor Sizing

This is crucial. Here's the breakdown of what you would do in this case:

Phase Conductors (A, B, C)

Each phase conductor needs to be rated to carry at least 100 Amps (and likely larger, for safety and potential future expansion).

Neutral Conductor

The neutral conductor, for a balanced system, must be sized for the maximum unbalanced load. In a perfectly balanced system, the minimum allowable is dictated by code, and is usually sufficient to carry the small amount of current that actually flows due to slight imbalances and harmonic currents. The code typically says it should be no smaller than the size of the ground conductor and can be smaller than the phase conductors.

Termination (How it's Wired)

You are correct. The three loads must share the same neutral conductor. It should be one continuous run from the electrical panel all the way to the furthest load. You don't need three completely separate 100A-rated neutrals. The connection could be made using a buss bar as well.

Why Your Thinking Was Confusing

Your confusion likely stems from trying to isolate each A-N, B-N, and C-N circuit as completely independent up to the service panel. The neutral is a shared conductor.

Analogy

Imagine three water pumps (the loads) draining water into a single pipe (the neutral). Each pump draws 100 liters per minute (LPM). If they operate at equal intervals, the net flow in the pipe is theoretically zero (although in practice, there would be some flow due to imperfections). The single pipe acts to take all three currents into account, in a way which cancels everything out.

Key Takeaways In a perfectly balanced 3-phase, 4-wire system, the neutral current is ideally zero. Each individual A-N, B-N, C-N circuit does have current flowing in its neutral wire near the load. The neutral conductor is shared, and the currents cancel continuously along its length. Size the neutral conductor according to electrical codes, considering the potential for unbalanced loads and harmonic currents.I hope this detailed explanation helps! Let me know if you have more questions. Flag for review