Journeyman Electrician Exam (NEC)General Electrical Knowledge and Plan ReadingMedium
An industrial facility has several large 3-phase motors. A technician observes that Motor A draws 40A, Motor B draws 30A, and Motor C draws 50A on each phase. If these motors are connected to a common 3-phase feeder, what is the approximate total current on one phase of the feeder, assuming balanced loads and no diversity factor applied?
- A150 A
- B70 A
- C120 A
- D40 A
Show answer & explanationAnswer & explanation
Correct answer: C. 120 A
For balanced 3-phase loads, the current drawn by each motor on a given phase simply adds up on that phase of the feeder. Total current = 40A (Motor A) + 30A (Motor B) + 50A (Motor C) = 120A. The question specifies 'on one phase' and 'balanced loads,' meaning the current sum applies to each phase equally.
Why the other options are wrong
- A. This would be the sum if there were five 30A motors, or if it was a single-phase circuit adding up to 150A.
- B. This is the sum of Motor A and Motor B only.
- D. This is the current of only Motor A, not the total.
3-Phase Balanced Load Current Summation
For balanced 3-phase loads, the total current on each phase of a feeder is the arithmetic sum of the individual phase currents of the connected loads.
- Assumes balanced loads, where each phase draws approximately the same current.
- Diversity factors are often applied in real-world scenarios but are excluded here.
- This principle applies to the current on each individual phase conductor.
Memory trick: Balanced 3-Phase: Just add 'em up, like a shopping list.