Which two electrical ratings are commonly specified for lamps?

• A. Current / Resistance
• B. Resistance / Voltage
• C. Voltage / Current
• D. Voltage / Power

Answer: (D)

Lamps are designed to run from a specific supply voltage and to dissipate a certain amount of power. The voltage rating tells you the safe and intended electrical supply, while the power rating indicates how much energy per second the lamp uses, which also relates to brightness. For a given voltage, the current drawn is I = P/V, so knowing voltage and power lets you calculate current. The resistance of a lamp, especially a hot filament, changes with temperature and isn’t a fixed specification, so it isn’t the standard rating shown. That’s why the common electrical specifications you’ll see for lamps are voltage and power.