CSLB C-10 Electrical ContractorPlanning and EstimatingMedium
A client is planning a new commercial office building. The blueprints specify a 208V, 3-phase, 4-wire feeder for general power and lighting. The total connected load for this feeder is calculated to be 75 kVA. What is the minimum required feeder ampacity?
- A175 A
- B250 A
- C285 A
- D208 A
Show answer & explanationAnswer & explanation
Correct answer: D. 208 A
For a 3-phase circuit, the current (I) is calculated using the formula: I = VA / (√3 * V). Here, VA = 75 kVA = 75,000 VA, and V = 208V. So, I = 75,000 VA / (1.732 * 208V) = 75,000 VA / 360.256 = 208.2 Amps. The minimum required feeder ampacity is 208 Amps (rounded up to the nearest whole number).
Why the other options are wrong
- A. Incorrect. This is lower than the calculated ampacity.
- B. Incorrect. This is an overestimation.
- C. Incorrect. This is a significant overestimation.
Three-Phase Current Calculation
The method of determining the current in a 3-phase electrical circuit, typically used for sizing conductors and overcurrent protective devices based on the total apparent power (VA or kVA) and line-to-line voltage.
- Uses the formula I = VA / (√3 * V).
- √3 (square root of 3) is approximately 1.732.
- Crucial for proper sizing of electrical components in 3-phase systems.
Memory trick: Three-phase amps: 'KVA' and 'VOLTS' with a 'ROOT-3' twist.