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PEC: branch circuits, ampacity and protection: board exam questions with solutions

On the REE exam under Electrical Engineering Professional Subjects and the RME exam under Philippine Electrical Code, Parts 1 and 2. Try each question before you open its solution, then practise with fresh numbers below: 1 computed question type on this topic.

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Build it in the lab

Outlet circuit: three outlets on a 20 A breaker · Aircon branch circuit: 1.5 HP on 3.5 mm² and a 20 A breaker, on its own · Water heater circuit: 3.5 kW on 5.5 mm² and a 30 A breaker: the same circuits on a live bench, with meters you can move.

Questions

1. A branch circuit supplies both continuous and noncontinuous loads. The overcurrent device must be rated at least:

  1. The noncontinuous load plus 125% of the continuous load
  2. 100% of the total load
  3. 125% of the total load including noncontinuous load
  4. 80% of the continuous load plus the noncontinuous load
Show the answer and solution

Answer: A. The noncontinuous load plus 125% of the continuous load

Unless the device and assembly are listed for 100% continuous operation, the rating must be at least the noncontinuous load plus 125% of the continuous load.

Reference: PEC 2017 Part 1, Art. 2.10 (Branch Circuits: ratings)

2. A lighting branch circuit supplies a continuous load of 16 A and no other load. Using a standard (not 100%-rated) circuit breaker, what is the minimum breaker rating?

  1. 16 A
  2. 20 A
  3. 15 A
  4. 30 A
Show the answer and solution

Answer: B. 20 A

Minimum rating = 125% × 16 A = 20 A. A 20 A breaker satisfies this.

Reference: PEC 2017 Part 1, Art. 2.10 (Branch Circuits: ratings)

3. A feeder supplies 40 A of continuous load and 20 A of noncontinuous load. What is the minimum feeder overcurrent device rating (standard-rated device)?

  1. 60 A
  2. 75 A
  3. 70 A
  4. 87.5 A
Show the answer and solution

Answer: C. 70 A

Minimum rating = 1.25 × 40 + 20 = 50 + 20 = 70 A.

Reference: PEC 2017 Part 1, Art. 2.15 (Feeders)

4. A single continuous-duty motor has a full-load current of 28 A (from the code table). What minimum ampacity must its branch-circuit conductors have?

  1. 28 A
  2. 42 A
  3. 56 A
  4. 35 A
Show the answer and solution

Answer: D. 35 A

Branch-circuit conductors supplying a single motor must have an ampacity of at least 125% of the motor full-load current: 1.25 × 28 = 35 A.

Reference: PEC 2017 Part 1, Art. 4.30 (Motors: conductor sizing)

5. A feeder supplies three continuous-duty motors with full-load currents of 28 A, 16 A and 10 A, and no other load. What minimum ampacity must the feeder conductors have?

  1. 61 A
  2. 54 A
  3. 67.5 A
  4. 70 A
Show the answer and solution

Answer: A. 61 A

Feeder ampacity = 125% of the largest motor FLC + sum of the others = 1.25 × 28 + 16 + 10 = 35 + 26 = 61 A.

Reference: PEC 2017 Part 1, Art. 4.30 (Motors: conductors supplying several motors)

6. A motor has a nameplate full-load current of 20 A and a marked service factor of 1.15. What is the maximum rating of a separate overload device under the basic rule?

  1. 23 A
  2. 25 A
  3. 20 A
  4. 28 A
Show the answer and solution

Answer: B. 25 A

For motors with a service factor of 1.15 or more, the overload device may be rated up to 125% of nameplate FLC: 1.25 × 20 = 25 A. A 115% limit applies to other motors.

Reference: PEC 2017 Part 1, Art. 4.30 (Motors: overload protection)

7. When a conductor's ampacity does not match a standard overcurrent device rating, the next higher standard rating may be used only if, among other conditions, that rating does not exceed:

  1. 100 A
  2. 400 A
  3. 800 A
  4. 1200 A
Show the answer and solution

Answer: C. 800 A

The next-higher-standard-rating allowance applies only where the selected device rating does not exceed 800 A (and the conductors are not part of a branch circuit supplying multiple receptacles for cord-and-plug loads).

Reference: PEC 2017 Part 1, Art. 2.40 (Overcurrent Protection)

8. In general, in which conductor of a circuit must an overcurrent device be connected?

  1. In the grounded (neutral) conductor only
  2. In the equipment grounding conductor
  3. In both the grounded and equipment grounding conductors
  4. In each ungrounded conductor
Show the answer and solution

Answer: D. In each ungrounded conductor

Overcurrent protection is placed in each ungrounded conductor. A device may not be placed in series with the grounded conductor unless it opens all circuit conductors simultaneously.

Reference: PEC 2017 Part 1, Art. 2.40 (Overcurrent Protection)

Worked examples with computed numbers

These numbers are fixed on this page; in practice they change every time.

9. A continuous load draws 24 A. What minimum branch-circuit rating does the 125% rule require?

  1. 36 A
  2. 19.2 A
  3. 30 A
  4. 24 A
Show the answer and solution

Answer: C. 30 A

A continuous load (3 hours or more) is sized at 125%: 24 × 1.25 = 30 A minimum.

Other topics in the same subjects

DC circuits · AC circuits · Three-phase systems · Transients (RC, RL, RLC) · Semiconductors and diodes · Power electronics · Instrumentation and measurement · Control systems · Transformers · DC machines · Induction motors · Synchronous machines · Power systems and transmission · Protection: fuses, breakers, relays · Illumination · Power plant engineering · PEC Part 1: general requirements · PEC: wiring methods and materials · PEC: grounding and bonding · PEC: special occupancies and equipment

An independent, free reviewer: not affiliated with the PRC or the Boards. Subject weights and passing rules are from the PRC examination programs and the laws (RA 9292, RA 7920); check the current PRC program before you file. Code questions follow the Philippine Electrical Code 2017 Part 1; check them against your own copy of the Code.