Calculations are the part of the electrician's licensing exam that trips up the most people — motor circuits, voltage drop, conductor ampacity, and load calcs. Every question below is a real practice problem with a full step-by-step worked solution, not just an answer key.
Tap an answer to check it — the worked solution appears once you choose.
1. 10 copper THW conductors are to be installed in a 20-foot length of intermediate metal conduit (IMC), 5 side 1 AWG and 5 size 3 AWG. What is the minimum allowable trade size IMC required to contain these conductors?
Answer: 2 1/2 inches.
#1 THW: .1901 sq. in. X 5 = .9505 #3 THW: .1134 sq. in. X 5 = .5670 total = 1.5175 sq. in. 2 1/2" IMC @ 40% fill of 2.054 sq. in.
NEC reference: Chapter 9 Tables 4 & 5
2. A 150 kVA, single phase transformer having a secondary voltage of 120/240 is to be installed at a multifamily dwelling. The current available at the transformer secondary is:
Answer: 625 amperes.
150,000 / 240 volts = 625 amperes.
NEC reference: I = P/E
3. A 2 gang device box will contain 2 size 12/2 with ground NM cables connected to a duplex receptacle and 2 size 14/2 with ground NM cables connected to a single pole switch. The 2 gang device box also will contain 4 cable clamps. The device box is required to have a volume of at least:
Answer: 30 cubic inches.
14 AWG: 2.00 cu. in. * 4 = 8 cu. in. 12 AWG: 2.25 cu. in. * 4 = 9 cu. in. ground: 2.25 cu. in. * 1 = 2.25 cu. in. clamps: 2.25 cu. in. * 1 = 2.25 cu. in. receptacle: 2.25 cu. in. * 2 = 4.5 cu. in. switch: 2.00 cu. in. * 2 = 4 cu. in. total: 30.00 cu. in.
NEC reference: 314.16(B)(1), (2), (4) & (5) Table 314.16(B)
4. A 20 unit apartment building is to constructed where each unit has a habitable space of 930 square feet. Each unit will have a 6 kW electric range and a laundry area that is common to all tenants. What is the minimum connected load, in VA, required for the general lighting and small appliance circuits for each of the apartments?
Answer: 5,700 VA.
930 sq. ft. * 3VA = 2790 VA 2 small appliance = 3000 VA Connected load = 5,700 VA
NEC reference: Table 220.12 210.11(C)(1) 220.52(A)
5. A 240 volt, single phase, 17kW rated electric range is to be installed in the kitchen of a one family dwelling unit. Determine the minimum size copper NM cable the NEC requires to supply this electric range.
Answer: 6 AWG.
17kW - 12kW = 5kW *5% (increase in Coumn C) 8kW (one appliance, Column C) X 125% = 10kW demand. I = P/V I = 10,000/240 = 41.66 amperes 6 AWG NM cable with an ampacity of 55 amperes.
NEC reference: Table 220.55 & Note 1 & 4, 334.80, table 310.15(B)(16)
6. A 240 volt, single phase, 30 ampere rated, electric clothes dryer has a volt ampere rating of:
Answer: 7200.
30 X 240 = 7,200 VA
NEC reference: P = IV
7. A 3 phase, 25 kVA rated transformer with a 480 volt primary and a 208Y/120 volt secondary is to be installed. Where primary and secondary protection is required to be provided, what is the maximum standard ampere rating of secondary overcurrent protection permitted by the NEC?
Answer: 90 amperes.
25000/(208 X 1.732) 69.3 X 125%
NEC reference: 3 phase current formula Table 450.3(B)
8. A 30 ampere residential branch circuit may supply:
Answer: Single cord and plug connected equipment rated 24 amperes or less.
30 amperes * 80%
NEC reference: 210.23(b)
9. A 480 volt single phase circuit supplies loads of 40 ohms resistance, 100 ohms resistance, and 200 ohms resistance connected in parallel. What is the total load, in amperes?
Answer: 19.2 amperes.
Total Resistance of resistors in parallel = .025(1/40) + .01(1/100) + .005(1/200)=04= 25 ohms Ohms law: 480 volts / 25 ohms
NEC reference: Ohms law
10. A 50 kVA transformer has a 480 volt, three phase primary, and a 208Y/120-volt, three phase secondary. Overcurrent protection is required on both the primary and secondary side of the transformer. Determine the maximum standard size overcurrent protection device permitted on the primary side.
Answer: 150 amperes.
50,000VA / (480 X 1.732) = 60.2 amperes 60.2 amperes X 250%
NEC reference: 450.3(B) Table 450.3(B) 240.6(A)
11. A 6 unit condo has a total connected load of 15 kW per unit. What is the demand load, in VA, on the ungrounded service entrance conductors, where applying the optional method of calculation?
Answer: 39600 VA
6 units * 15kW = 90 kW * 44% = 39,600
NEC reference: 220.84(A) Table 220.84
12. A 90 ampere, 240 volt, single phase load is located 225 feet from a panelboard and is supplied with size 3 AWG THWN copper conductors. What is the approximate voltage drop on the circuit? (K = 12.9)
Answer: 10 volts
VD = 2KID/CM (2 X 12.9 X 90 X 225) / 52620
NEC reference: Chapter 9, Table 8
Full practice exams, every topic, progress tracking, and worked solutions — in the app.
