You're right to question this! It sounds like there might be some confusion or oversimplification in the way the examples are presented. Here's a breakdown of why the amps should be flowing through all three lines (phases) in both cases, and why you might see variations in calculations

Understanding 3-Phase Power
Balanced Load

In a balanced 3-phase system (which is the ideal and common scenario for motors), the load is distributed evenly across all three phases. This means each phase carries approximately the same amount of current.

Wye (Y) and Delta (Δ) Connections

3-phase motors and systems can be wired in either a Wye or Delta configuration. Both will have current flowing through all lines.

Neutral Wire (Wye Only)

A Wye system can have a neutral wire. If the load is perfectly balanced, the current in the neutral wire is zero. If the load is unbalanced, the neutral wire carries current to compensate. Delta systems do not have a neutral wire.



Line vs. Phase Values



Voltage



Wye

Line Voltage = √3 Phase Voltage

Delta

Line Voltage = Phase Voltage

Current



Wye

Line Current = Phase Current

Delta

Line Current = √3 Phase Current

Why Amps Flow Through All Three Lines (Phases)

Regardless of whether you have one motor or multiple motors, each phase is supplying power. Even in a perfectly balanced system, current flows in all three lines.

Possible Explanations for the Discrepancy and What You Might Be Seeing


Simplification for Calculation

The examples might be simplifying the calculations for the sake of the practice problem. They might be focusing on calculating the current in one line and then assuming the other lines are similar. This is fine for quick estimation, but it's not a complete picture. Example: To find the required wire size, you only need to calculate the current in the one line.2.

Focus on Line-to-Line Voltage

Sometimes, calculations focus on the line-to-line voltage (208V in your example) because that's the voltage you're readily measuring. The formula for 3-phase power is: `Power (Watts) = √3 Voltage (Line-to-Line) Current (Line) Power Factor` The examples might be implicitly using this formula, and therefore only explicitly showing the line current.3.

Unbalanced Load (Less Likely)

If the load is unbalanced (e.g., one phase is drawing significantly more current than the others), then the current distribution is uneven. However, electrical systems are designed to be as balanced as possible. This is very uncommon.4.

Error in the Prep Material

It's possible there's a genuine error in the prep material. Exam prep materials, while helpful, are not always perfect.

How to Approach These Types of Problems

Assume Balanced Load

Unless explicitly stated otherwise, assume the 3-phase system is balanced.

Use the 3-Phase Power Formula

Use the formula mentioned above to calculate current if you know power, voltage, and power factor.

Consider the Context

Think about what the question is asking. Are you calculating total power, current in a specific line, or something else?

Consult NEC (National Electrical Code)

The NEC is the ultimate authority. It has tables and rules for calculating conductor ampacity, motor circuit protection, etc. If you are ever unsure, refer to the NEC.

Recommendation Review the specific examples you are referring to again, paying close attention to the exact wording of the problem and what it's asking you to calculate. If possible, cross-reference the examples with explanations in a reputable electrical textbook or online resource. If you still believe there's an error, report it to the publisher of the prep material so they can correct it.

In summary, you are correct to question this. In a standard 3-phase system, current flows through all three lines. The examples might be simplifying calculations or focusing on specific values, but it's important to understand the underlying principle of balanced 3-phase power.* Flag for review