Electricity 101 Ohm's Law, Circuit Capacities, and Appliance Wattages

Terms | Circuit Elements | Voltage
Amperage | Wattage | Resistance | Electricity Types | Electricity Sources
Ohm's Law | Wire Gauge Capacity & Uses | Circuit Capacities
Appliance Wattages

"Ohm's Law"

There is a fundamental relationship between voltage, amperage, and resistance in a closed functional circuit. This relationship is known as "Ohm's Law."

Ohm's Law states:
Current is directly proportional to the applied voltage. Current is proportional to the circuit's resistance.

What the heck does that mean?

ohm's Law

When the circuit voltage increases, the current increases.
When the circuit voltage decreases, the current decreases.
When the circuit resistance increases, the current decreases.
When the circuit resistance decreases, the current increases.

Wattage also has a direct relationship with voltage, amperage, and resistance in the circuit. Current and amperage are two words meaning basically the same thing. However, amperage is the measurement used to communicate the amount of current flowing past a given point in one second. The symbol of I for current can also be used in place of A for amperage.

A Guide to Wire Sizes and a Few of Their Uses.

Wire Size

Wire Capacity & Uses


60 amps, 240 volts; Central air conditioner, electric furnace.


40 amps, 240 volts; Electric range, wall oven.


30 amps, 240 volts; Dryer, window air conditioner.


20 amps, 120 volts; washers, receptacles, microwaves.


15 amps, 120 volts; light fixtures, receptacles.


Light-duty extension cords


Thermostats, doorbells, security systems.

To Determine Safe Circuit Capacities Multiply the Amperage Rating by Voltage.

Amps x Volts

Total Capacity

Safe Capacity

15 A x 120 V

1800 watts

1440 watts

20 A x 12 V

240 watts

1920 watts

25 A x 120 V

3000 watts

272% watts

30 A x 120 V

3600 watts

2880 watts

20 A x 240 V

4800 watts

3840 watts

30 A x 240 V

7200 watts

5760 watts

Typical Wattages of Home Appliances


725 (frost-free, 16 cubic feet)

Clothes Washer


Clothes Dryer




Microwave Oven


Coffee Maker




Hair Dryer


Heater (Portable)


Clothes Iron


Vacuum cleaner


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