Question 1

How do I convert watts to amps?

Accepted Answer

For DC or AC single-phase resistive loads, divide watts by voltage: A = W / V. For AC with inductive loads (motors, compressors), include the power factor: A = W / (V x PF). For three-phase circuits, use A = W / (V x PF x sqrt(3)), where V is line-to-line voltage. A typical 1500W space heater on 120V draws 12.5A because its power factor is effectively 1.

Question 2

What is power factor and when does it matter?

Accepted Answer

Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amps). Resistive loads like heaters and incandescent bulbs have PF = 1.0. Motors, fluorescent ballasts, and most inductive equipment run around 0.80-0.90. LED drivers and modern switching supplies are typically 0.90-0.95. A lower power factor means more current is required to deliver the same real power, which is why you must include PF in AC calculations.

Question 3

Why does three-phase use sqrt(3) in the formula?

Accepted Answer

In a balanced three-phase system, the line-to-line voltage is sqrt(3) times the phase voltage. Total real power is P = sqrt(3) x V_LL x I x PF, where V_LL is line-to-line voltage and I is line current. Solving for current gives I = W / (V x PF x 1.732). This is why a 10 kW load at 480V 3-phase draws only about 14A per leg, versus 52A on 240V single-phase.

Question 4

How many amps does a 1500 watt appliance draw?

Accepted Answer

At 120V AC with PF = 1, a 1500W appliance draws 1500 / 120 = 12.5 amps. This is why space heaters and hair dryers are typically rated at 1500W - it is the maximum continuous load for a standard 15A, 120V circuit (80% of 15A = 12A continuous, but appliances can draw briefly above that). At 240V, the same 1500W would draw only 6.25A.

Question 5

Can I use the same formula for kilowatts?

Accepted Answer

Yes, just convert kW to W first by multiplying by 1000, or include 1000 in the formula: A = (kW x 1000) / V. For example, a 7.68 kW EV charger at 240V single-phase draws (7.68 x 1000) / 240 = 32 amps. Many electric vehicle chargers, HVAC units, and industrial equipment are rated in kW, so this conversion is useful for sizing breakers and wire.

Question 6

What voltage should I use for 240V appliances?

Accepted Answer

Use the nameplate voltage. In North America, residential 240V is actually 240V line-to-line (L1 to L2) from a split-phase service. Dryers, water heaters, ranges, and EV chargers all use this 240V value in the formula. In Europe and most of the world, 230V single-phase is standard. Commercial three-phase is typically 208V, 480V, or 600V line-to-line.

Question 7

Why is my amp reading higher than calculated?

Accepted Answer

Real-world meters often read slightly higher than calculated values due to inrush current at startup (motors, compressors), harmonic distortion from electronics, and actual power factor being lower than assumed. Inrush can be 3-10x the running current for a fraction of a second. For continuous operation, the calculated value should match within 5-10%. If it is consistently off, verify the PF and actual voltage at the outlet under load.

Voltage	Amps (1,500W)
12V	125 A
24V	62.5 A
120V	12.5 A
208V	7.21 A
240V	6.25 A
277V	5.42 A
480V	3.13 A

Appliance	Watts	Volts	Amps
LED bulb	10 W	120 V	0.08 A
Refrigerator	150 W	120 V	1.25 A
Microwave	1100 W	120 V	9.2 A
Hair dryer	1500 W	120 V	12.5 A
Electric dryer	5500 W	240 V	22.9 A
EV charger (L2)	7700 W	240 V	32 A

Voltage	Amps (1,500W)
12V	125 A
24V	62.5 A
120V	12.5 A
208V	7.21 A
240V	6.25 A
277V	5.42 A
480V	3.13 A

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