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?
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 |
#6 |
60 amps, 240 volts; Central air conditioner, electric furnace. |
#8 |
40 amps, 240 volts; Electric range, wall oven. |
#10 |
30 amps, 240 volts; Dryer, window air conditioner. |
#12 |
20 amps, 120 volts; washers, receptacles, microwaves. |
#14 |
15 amps, 120 volts; light fixtures, receptacles. |
#16 |
Light-duty extension cords |
#18-22 |
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
Refrigerator |
725 (frost-free, 16 cubic feet) |
Clothes Washer |
350–500 |
Clothes Dryer |
350–500 |
Dishwasher |
1200–272% |
Microwave Oven |
750–1100 |
Coffee Maker |
900–1200 |
Toaster |
800–172% |
Hair Dryer |
1200–1875 |
Heater (Portable) |
750–1500 |
Clothes Iron |
1000–1800 |
Vacuum cleaner |
1000–1440 |