Search

20 Simple Questions to Check Your Overcurrent Protection Knowledge

Overcurrent protective devices

As you already know, reliable overcurrent protective devices, such as fuses and circuit breakers, prevent or minimize costly damage to transformers, conductors, motors, equipment, and the other many components and loads that make up a complete electrical system.

20 Simple Questions to Check Your Overcurrent Protection Knowledge
20 Simple Questions to Check Your Overcurrent Protection Knowledge

As an electrician, you should understand all facts and NEC codes related to overcurrent protection.

Let’s take a 20-question quiz to test your electrician’s knowledge! Answers and following NEC explanations are listed at the bottom. Don’t peak :)


Question #1

When a circuit breaker trips, its operating handle will be in which of the following positions?

  1. Closed (energized) position
  2. Open position
  3. Half-way between the ON and OFF positions
  4. No change

Question #2

Which of the following conductors need overcurrent protection on a residential electric service?

  1. Grounded conductor
  2. Bonding conductor
  3. Ungrounded conductors
  4. Equipment grounding conductor

Question #3

A circuit feeding three single-phase motors with full-load current ratings of 40, 20 and 20 amperes, respectively, requires the following dual-element time-delay fuse size:

  1. 110 amperes
  2. 150 amperes
  3. 200 amperes
  4. 250 amperes

Question #4

A nontime-delay fuse protects a circuit with all motor loads with the largest motor drawing a full-load current of 40 amperes, and two other motors drawing a full-load current of 15 amperes each.

What fuse size should be used?

  1. 100 amperes
  2. 150 amperes
  3. 175 amperes
  4. 250 amperes

Question #5

Conductors may be tapped, without overcurrent protection at the tap, to a feeder or transformer secondary if certain conditions are met. If the ampacity of the tap conductor is not less than the combined computed loads on the circuits supplied by the tap conductor, the length of the tap conductor cannot exceed:

  1. 5 feet
  2. 10 feet
  3. 15 feet
  4. 25 feet

Question #6

Where must an overcurrent device be located in a circuit?

  1. At the point where the conductor receives its supply
  2. At the last outlet on the circuit
  3. On the line side of the electric meter
  4. At the first fitting or connector in the circuit

Question #7

If a multiwire branch circuit supplies line-to-line loads, the branch-circuit protective device must open each:

  1. All ungrounded conductors simultaneously
  2. Grounded conductor
  3. Ungrounded conductor individually
  4. Bonding conductor

Question #8

To meet NEC regulations, fuses or circuit breakers connected in parallel must be:

  1. Assembled by the electrician or technician on the job
  2. Encased in a PVC housing
  3. Factory assembled in parallel
  4. Rated over 200 amperes

Question #9

When a fuse or circuit breaker is used for circuit protection, the fuse or overcurrent trip unit shall be connected:

  1. In series
  2. In series and parallel
  3. In parallel
  4. In tandem

Question #10

At what point must a branch-circuit overcurrent protection device be placed in an electrical system?

  1. At the equipment ground location
  2. Where the conductor being protected receives its supply
  3. At a tap on the conductor being protected
  4. At the outlet where power is consumed

Question #11

A feeder tap less than 25 feet long does not require overcurrent protection at the tap if the ampacity of the tap conductor is at least:

  1. 50% of the feeder conductor
  2. 40% of the feeder conductor
  3. 33 1/3% of the feeder conductor
  4. 20% of the feeder conductor

Question #12

When designing or installing circuits for diagnostic equipment in health care facilities, what is the minimum percentage allowed for the momentary ampacity of the equipment when sizing branch-circuit conductors and the overcurrent protection?

  1. 25%
  2. 50%
  3. 75%
  4. 100%

Question #13

If an autotransformer has a full-load input current of 40 amperes, what is the maximum overcurrent protection device rating allowed?

  1. 45 amperes
  2. 50 amperes
  3. 60 amperes
  4. 70 amperes

Question #14

In an autotransformer, overcurrent devices shall not be installed:

  1. In parallel with the output conductors
  2. In parallel with the shunt winding
  3. In series with the output conductors
  4. In series with the shunt winding

Question #15

A grounding autotransformer is used to create a three-phase, 4-wire distribution system from a three-phase, 3-wire ungrounded system.

At what percent must the overcurrent device be set to trip in case of an overload?

  1. 100% of the transformer’s continuous current per phase or neutral rating
  2. 110% of the transformer’s continuous current per phase or neutral rating
  3. 125% of the transformer’s continuous current per phase or neutral rating
  4. 150% of the transformer’s continuous current per phase or neutral rating

Question #16

What must the continuous neutral current rating be for an autotransformer used to create a three-phase, 4-wire distribution system from a three-phase, 3-wire ungrounded system?

  1. It must be sufficient to handle the minimum neutral unbalanced load
  2. It must be sufficient to handle the maximum possible neutral unbalanced load
  3. It must be sufficient to handle 75% of the neutral unbalanced load
  4. It must be sufficient to handle 50% of the unbalanced load

Question #17

What is the minimum branch circuit and maximum overcurrent protection rating for a circuit supplying a household electric range rated at 8 ¾ kW or more?

  1. 20 amperes
  2. 30 amperes
  3. 40 amperes
  4. 50 amperes

Question #18

If the allowable current rating of a busway is 46 amperes, what is the maximum allowable rating of the overcurrent device?

  1. 40 amperes
  2. 50 amperes
  3. 60 amperes
  4. 70 amperes

Question #19

A circuit breaker with a slash rating of 120/240 volts is rated for what voltage to ground?

  1. 120 volts to ground
  2. 240 volts to ground
  3. Any voltage up to 240
  4. None of these

Question #20

What overcurrent requirements are specified for grounded conductors?

  1. An overcurrent device must be provided in parallel for every conductor that is intentionally grounded
  2. An overcurrent device must be provided in series for every conductor that is intentionally grounded
  3. No overcurrent device is permitted in series with any conductor that is intentionally grounded
  4. An overcurrent device must be provided both in series and in parallel for every conductor that is intentionally grounded

Answers //

To check your overcurrent protection knowledge , results should be the same as provided answers. NEC articles mentioned in answers you can observe in a free draft version of NEC 2014 //

NEC 2014 free draft

  1. C – The conventional circuit-breaker handle has four possible positions: on, off, trip, and reset. When tripped, the handle jumps to the middle position.
  2. C A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor. NEC Section 240.15.
  3. A – Size the fuse at 175% of the full-load current of the largest motor plus the full-load current of all other motors. NEC Table 430.52.
  4. B – Size the fuse at 300% of the full-load current of the largest motor plus the full-load current of all other motors. NEC Table 430.52.
  5. B – The length of the tap conductor must not exceed 10 feet. NEC Section 240.21(B)(1).

    Summary of the NEC “10-foot” tap rule
    Summary of the NEC “10-foot” tap rule


  6. A – An overcurrent device shall be connected at the point where the conductor to be protected receives its supply. NEC Section 240.21.
  7. A – For example, if a 3-wire circuit is connected to a 3-pole circuit breaker, and one of the three wires should develop a fault, the circuit breaker must open all three wires simultaneously and not just the one that developed the fault. NEC Section 240.15(B).
  8. C – Fuses, circuit breakers, or combinations thereof shall not be connected in parallel unless they are factory-assembled and listed as a unit. NEC Section 240.8.
  9. A – Fuses or overcurrent trip units (circuit breakers) must be connected in series with each ungrounded conductor so they will open or trip when an overload or ground fault occurs. NEC Section 240.15(A).
  10. B – For example, branch circuits must be protected at the panelboard where they receive their supply. NEC Section 240.21.
  11. C – NEC Section 240.21(B)(2) requires that feeder tap conductors must not be over 25 feet in length and the ampacity of the tap conductor must be not less than one-third that of the feeder conductor. Therefore, answer C (331/3%) is the correct answer. See Figure 1.

    Summary of NEC 25-foot tap rule
    Summary of NEC 25-foot tap rule


  12. B – NEC Section 517.73(A)(1) requires that the ampacity of supply branch-circuit conductors and the current rating of overcurrent protective devices must not be less than 50 percent of the momentary rating or 100 percent of the long-time rating, whichever is greater.
  13. B – 40 amperes × 1.25 = 50 amperes. NEC Section 450.4(A).
  14. D – NEC Section 450.4(A) requires an overcurrent device to be installed in series with each ungrounded input conductor. However, an overcurrent device shall not be installed in series with the shunt winding.
  15. C – NEC Section 450.5(A)(2) requires the overcurrent device to open or trip if the load on the autotransformer reaches or exceeds 125% of its continuous current per phase or neutral rating.

    Delayed tripping for temporary overcurrents sensed at the autotransformer overcurrent device is permitted for the purpose of allowing proper operation of branch or feeder protective devices on a 4-wire system.

  16. B – NEC Section 450.5(A)(4) requires the autotransformer to have a continuous neutral current rating sufficient to handle the maximum possible neutral unbalanced load current of the 4-wire system.
  17. C – NEC Section 210.19(A)(3) states that the minimum branch-circuit rating for this load should be 40 amperes. A 40-ampere circuit breaker or cartridge fuse is the maximum allowable rating for a 40-ampere circuit.
  18. B – A rating of 46 amperes is not a standard overcurrent device rating. However, 50 amperes is a standard rating and may be used in accordance with NEC Section 240.4(B).
  19. A – NEC Section 240.85 requires the voltage to ground to be the lower of the 2 values on a slash rated circuit breaker.
  20. C – NEC Section 240.22 requires that no overcurrent device be connected in series with any conductor that is intentionally grounded, unless one of two conditions listed is met.

Reference // Electrician’s Exam Study Guide (Purchase at Amazon)

SEARCH: Articles, software & guides //

Premium Membership //

Premium membership gives you an access to specialized technical articles and extra premium content (electrical guides and software).
Get Premium Now ⚡

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

Leave a Comment

Tell us what you're thinking... we care about your opinion!

Print this