Master Electrician Exam (NEC)Load CalculationsEasy
A commercial building has a total calculated general lighting load of 30,000 VA and a total calculated receptacle load of 10,000 VA. Assuming all loads are noncontinuous, what is the minimum required service size in amperes for a 208Y/120V, three-phase system?
- A138.9 A
- B111.1 A
- C166.7 A
- D83.3 A
Show answer & explanationAnswer & explanation
Correct answer: B. 111.1 A
First, sum the total VA load: 30,000 VA (lighting) + 10,000 VA (receptacles) = 40,000 VA. For a three-phase system, the current (I) is calculated using the formula I = VA / (V * √3). So, I = 40,000 VA / (208V * 1.732) = 40,000 VA / 360.256V ≈ 111.03 A. The closest answer is 111.1 A.
Why the other options are wrong
- A. This option likely uses an incorrect voltage or miscalculates the √3 factor.
- C. This option likely results from dividing by a single-phase voltage or an incorrect multiplier.
- D. This option results from dividing by line-to-neutral voltage or using an incorrect multiplier.
Three-Phase Current Calculation
To find the current in a balanced three-phase system, divide the total apparent power (VA) by the product of the line-to-line voltage and the square root of 3 (approximately 1.732).
- Formula: I = VA / (V_LL * √3)
- V_LL is the line-to-line voltage (e.g., 208V, 480V).
- √3 is approximately 1.732.
Memory trick: Three phases need Root 3 to find the amperes.