Master Electrician Exam (NEC)Overcurrent ProtectionMedium
A residential dwelling unit has a 200-ampere service. A new electric range with a nameplate rating of 12 kW (120/240V) is being installed. The branch circuit conductors for the range are 6 AWG copper with THHN insulation. What is the minimum size overcurrent protective device required for the branch circuit supplying this range?
- A30 amperes
- B40 amperes
- C60 amperes
- D50 amperes
Show answer & explanationAnswer & explanation
Correct answer: D. 50 amperes
First, calculate the current: I = P/E = 12,000 W / 240 V = 50 A. According to NEC 210.19(A)(3) and 210.20(A), for household cooking appliances, the branch circuit OCPD must be sized at 100% of the calculated load. Thus, a 50 A OCPD is required. For 6 AWG THHN copper, its ampacity at 90°C is 75 A, which is sufficient. The 60°C column ampacity for 6 AWG is 55 A, also sufficient.
Why the other options are wrong
- A. This is too low for a 12 kW range (12000W/240V = 50A).
- B. This is too low for a 12 kW range.
- C. While 60 amperes would protect the conductors, the question asks for the *minimum* size required for the load.
Electric Range OCPD Sizing
The overcurrent protective device for an individual household electric range branch circuit is sized at 100% of the calculated load, as per NEC 210.19(A)(3) and 210.20(A).
- Calculate current (I = P/E).
- OCPD at 100% of calculated load.
- Conductor ampacity must support the load.
Memory trick: Cooking 'appliances' need 'right-sized' 'power' protection.