Compare the total copper cross sections in terms of current-carrying capacity for single-phase and three-phase 120 V systems with effective load resistances of 12 Ω.

Question

Accepted Answer

Single-phase system : The load current is

[latex]I_{R L}=\frac{120 V }{12 \Omega}=10 A[/latex]

The conductor to the load must carry 10 A, and the return conductor from the load must also carry 10 A.

The total copper cross section, therefore, is 2 × 10 A = 20 A.
Three-phase system : For an effective load resistance of 12 Ω, the three-phase generator feeds three load resistors of 36 Ω each. The current in each load resistor is

[latex]I_{R L}=\frac{120 V }{36 \Omega}=3.33 A[/latex]

Each of the three conductors feeding the balanced load must carry 3.33 A, and the neutral current is zero.
Therefore, the total copper cross section is 3 × 3.33 A ≅ 10 A. This is half the copper cross section of the single-phase system with an equivalent load.

Question 21.1: Compare the total copper cross sections in terms of current-...

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