Licensed Electrical Contractor Branch Circuit Calculations Questions and Answers 1 — Questions and Answers
Question 1: A 120V branch circuit in a commercial office supplies a continuous lighting load of 13 amperes. What is the minimum required rating for the overcurrent protective device (OCPD) for this circuit?
- 15A
- 20A (Correct answer)
- 25A
- 30A
Correct answer: 20A
According to NEC 210.20(A), a branch circuit supplying continuous loads must have an overcurrent device rated for not less than 125% of the continuous load. The calculation is 13A * 1.25 = 16.25A. Since 16.25A is not a standard OCPD size listed in NEC 240.6(A), you must use the next higher standard size, which is 20A.
Question 2: What is the calculated load, in volt-amperes (VA), for a general-purpose branch circuit in a commercial building that supplies 12 duplex receptacles?
- 1,800 VA
- 1,440 VA
- 2,160 VA (Correct answer)
- 1,500 VA
Correct answer: 2,160 VA
NEC 220.14(I) specifies that for non-dwelling occupancies, each single or multiple receptacle on one yoke (strap) is calculated at a minimum of 180 VA. A duplex receptacle is on a single yoke. Therefore, the calculation is 12 receptacles * 180 VA/receptacle = 2,160 VA.
Question 3: A new single-family dwelling is being wired. Which of the following is a requirement for the small-appliance branch circuits serving the kitchen countertops?
- They must be rated at 15 amperes.
- They are permitted to supply lighting fixtures within the kitchen.
- A minimum of two 20-ampere circuits are required. (Correct answer)
- They can also serve receptacles in the attached garage.
Correct answer: A minimum of two 20-ampere circuits are required.
NEC 210.11(C)(1) and 210.52(B)(3) require that at least two 20-ampere small-appliance branch circuits be provided to supply the receptacle outlets serving the kitchen countertop surfaces in a dwelling unit. These circuits are not permitted to serve lighting or outlets in other areas like a garage.
Question 4: A conduit is installed containing eight current-carrying THHN/THWN copper conductors. According to NEC Table 310.15(C)(1), what adjustment factor must be applied to the ampacity of these conductors?
- 80%
- 70% (Correct answer)
- 50%
- 45%
Correct answer: 70%
NEC Table 310.15(C)(1) provides adjustment factors when there are more than three current-carrying conductors in a raceway or cable. For 7 to 9 current-carrying conductors, the table specifies that the allowable ampacity must be multiplied by an adjustment factor of 70%.
Question 5: What is the calculated branch-circuit demand load for a single 14 kW household electric range installed in a dwelling unit, using the standard calculation method?
- 14 kW
- 8.8 kW (Correct answer)
- 8.0 kW
- 11.2 kW
Correct answer: 8.8 kW
Per NEC Table 220.55, Column C, the demand load for one range rated not over 12 kW is 8 kW. Note 1 to the table states that for ranges over 12 kW through 27 kW, the demand load in Column C (8 kW) shall be increased by 5% for each additional kW over 12 kW. The range is 2 kW over 12 kW (14 kW - 12 kW = 2 kW). The increase is 2 * 5% = 10%. Therefore, the calculated load is 8 kW * 1.10 = 8.8 kW.
Question 6: A 20-ampere, 120-volt branch circuit supplies a fixed electric heater with a rating of 1,600 VA (a continuous load) and two general-purpose receptacles. What is the maximum permissible non-continuous load that can be connected to the receptacles?
- 800 VA
- 2,400 VA
- 400 VA (Correct answer)
- 1,600 VA
Correct answer: 400 VA
First, calculate the circuit's total capacity: 20A * 120V = 2,400 VA. According to NEC 210.20(A), the continuous load must be calculated at 125%: 1,600 VA * 1.25 = 2,000 VA. This is the portion of the circuit's capacity reserved for the continuous load. The remaining capacity available for the non-continuous load is the total capacity minus the calculated continuous load: 2,400 VA - 2,000 VA = 400 VA.
Licensed Electrical Contractor Branch Circuit Calculations Questions and Answers 1
A 120V branch circuit in a commercial office supplies a continuous lighting load of 13 amperes.
What is the minimum required rating for the overcurrent protective device (OCPD) for this circuit?