You are installing electrical service for a new 2,500 square foot home. The calculated electrical load is 28,000 watts. What is the minimum service size required in amperes at 240 volts?
Correct Answer
D) 117 amps
Current = Power ÷ Voltage = 28,000 watts ÷ 240 volts = 116.7 amps. This would require a minimum 150-amp service, but the calculated load is 117 amps.
Why This Is the Correct Answer
Using Ohm's Law: Current (Amps) = Power (Watts) ÷ Voltage (Volts) = 28,000 ÷ 240 = 116.67 amps. Rounded to a whole number, this is approximately 117 amps. The question asks for the calculated load in amps, not the next standard service size.
Why the Other Options Are Wrong
Option A: 150 amps
150 amps is a standard service size that would be selected if you were choosing the next available panel size above 117 amps. However, the question asks for the calculated load, not the standard panel size to install.
Option B: 200 amps
200 amps is a common residential service size but is larger than required by the calculated load. It would be an over-specification for this scenario.
Option C: 100 amps
100 amps is smaller than the calculated load of 116.67 amps and would be insufficient, potentially causing breaker trips or a fire hazard. Service must meet or exceed calculated load.
Memory Technique
P = IV, so I = P/V. 'PIV' — Power equals Current times Voltage. Rearrange: Current = Power ÷ Voltage. For this problem: 28,000 ÷ 240 ≈ 117 amps.
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