Overcurrent Protection

Question 1

A feeder supplies a load that does not contain general use receptacles and consists of 40
amperes of continuous load and 36 amperes of noncontinuous load. All terminations for
the feeder are rated at 75EC. The feeder conductor is copper with THHN insulation and
there are only three current-carrying conductors in raceway. The minimum standard rating overcurrent device permitted for this feeder is:

A. 60 ampere. C. 80 amperes. E. 100 amperes.
B. 70 ampere. D. 90 amperes.

Answer 1

D 90 amperes
Section 215.3 gives the rule for the minimum size of overcurrent device for a feeder.
Then look up the standard ratings of overcurrent devices in 240.6. The continuous load is
multiplied by 1.25 and added to the non-continuous load.
40 A × 1.25 = 50 A
36 A × 1.00 = 36 A

Total = 86A
240.6 - next highest rated is 90A

Question 2

A resistance type heating element in an industrial electric heater rated at not more than 48 amperes is required to be protected from overcurrent with a set of fuses or a circuit
breaker with a rating not exceeding:

A. 45 amperes. C. 60 amperes.
E. any specified value.
B. 50 amperes. D. 70 amperes.

Answer 2

C 60 amperes
Section 424.22(B) requires that any resistance type heating element is required to be
subdivided into loads not exceeding 48 amperes which are permitted to have overcurrent
protection rated at not more than 60 amperes.

Question 3

A 3-phase 75 kVA transformer is connected 480 volts on the primary and 208/120 volts on
the secondary. The full-load current of the transformer secondary is:

A. 90 amperes. C. 184 amperes. E. 361 amperes.
B. 156 amperes. D. 208 amperes.

Answer 3

D 208 amperes
This is a calculation that is required before the rules of overcurrent protection for a
transformer can be applied.

Isec = 75 kVA × 1000 / 1.73 × 208 V = 208A