Current draw:
how many amps a fan or motor really pulls

Why you want amps and not watts

The label on a fan or a motor gives power in watts or horsepower. A wire or a circuit breaker does not care about watts; it cares about the current flowing through it, because current produces heat in the conductor. Converting a rating into amps is therefore the first step in deciding whether a circuit can carry a load.

On a direct-current circuit the conversion is a plain division of watts by volts. On household alternating current there is one more factor, the power factor, because inductive loads such as fans and motors draw current that is partly out of step with the voltage. Ignoring it leads to an underestimate.

The relationship is simple enough to do on paper, and a calculator makes it quick to try different appliances and compare them side by side.

It is the same arithmetic that sits behind the numbers on a plug top and an extension board. A 5 A socket is rated for about 1,150 W at 230 V, and a 16 A socket for about 3,680 W, before any power factor is applied. Comparing the total of your loads with those figures is a quick safety check.

The single-phase formula

Resistive loads such as an iron or a heater have a power factor of 1, so the formula reduces to watts ÷ volts. A 1,000 W heater draws 1,000 ÷ 230 = 4.35 A.

Starting from horsepower: motors

A motor's rating is its output at the shaft, not what it draws from the supply. The electrical input is higher, by the inefficiency of the motor. One metric horsepower is about 735 W, and one mechanical horsepower is 746 W, which is the figure used in this example.

Efficiency and power factor both fall when a motor runs lightly loaded, so a motor driving a small load draws less current, but not in proportion.

Typical currents on a 230 V supply

The table also shows why a bank of efficient fans is a minor load while a single heater is a major one. Twelve BLDC fans together draw less than two amps. One 1 kW heater draws more than all twelve fans plus the lights.

A three-phase example

Larger pumps and workshop motors run on three phases, where the same power is shared across three conductors and each carries less current. The formula gains a factor of √3, and the voltage is the line-to-line value.

Compare that with a single-phase 5 HP motor on 230 V, which would draw about 22 A. Three-phase supply is why large motors are fed that way: thinner cable and smaller breakers do the same job.

From amps to the electricity bill

Current tells you about the wiring. The bill depends on energy, which is power multiplied by time. A 75 W fan running 8 hours a day uses 75 × 8 ÷ 1,000 = 0.6 kWh a day, or about 18 kWh in a 30-day month. At a tariff of ₹7 per unit that is roughly ₹126 a month, which is why replacing a 75 W fan with a 30 W BLDC fan can pay for itself over a few years.

Power factor affects the amps drawn from the supply but not the kilowatt-hours that a household meter records. Larger commercial consumers are often billed on kVA or penalised for low power factor, which is why factories fit capacitors to bring it close to 1.

Starting current, wiring and breakers

At start, an induction motor draws a surge of current often five to seven times the running figure, until it reaches speed. For the 1 HP pump, a start-up of 32 A for a second or two is not unusual. Breakers for motor circuits are chosen with a time delay or a suitable curve so that they ride through this surge but still trip on a real overload.

Wire size follows the running current, with a margin, and the length of the run, since voltage drop grows with distance. A long run to a pump in a field may need a thicker cable than the current alone suggests.

• Use the nameplate current whenever you have it.
• Allow margin: a continuous load is usually kept to no more than about 80% of the breaker rating.
• Check voltage at the motor when it runs. A low supply makes motors draw more current and run hotter.
• Capacitor-run fans and pumps can lose capacitance with age and then draw more current for less output.
• For anything beyond a plug-in appliance, have a licensed electrician size the cable and protection.