A common light bulb draws about ____ amp.

• A. 1 amp
• B. 0.5 amp
• C. 2 amps
• D. 3 amps

Answer: (A)

Current in a lamp is found from the relation P = V × I, so I = P / V. A typical household bulb is around 100 watts and is used on a standard mains voltage of about 120 volts. Plugging in the numbers gives I ≈ 100 / 120 ≈ 0.83 amperes, which is close to one amp. That’s why “about one amp” is a common practical estimate for a standard bulb on a 120 V system.

Keep in mind the exact current depends on the bulb’s wattage: a 60 W bulb draws about 0.5 A, a 150 W bulb about 1.25 A, etc. The take-away is that current scales with wattage and inversely with voltage, so on typical US household voltage a standard bulb is often rounded to about one amp for quick calculations.