When you plug in multiple appliances to a circuit, the current in the circuit increases, causing the resistance to decrease and potentially overheating and tripping the circuit breaker.
To make sure that your circuit breaker does not trip, you have to use ohm's law calculator and enter known parameters to calculate the load on the circuit.
Appliances with heating elements tend to draw more power and put more load on the circuit then other.
For example an appliance with the heating element has 120V voltage and 1000W power.
On the page above choose appropriate set of values, in this case choose P/V and the selected parameters will light up blue, enter the values into the corresponding input fields and press calculate.
Using this calculator the result is 8.3A current.
That means that circuit breaker has to be rated 10A in order to operate this appliance and be able plug in another electronic device that draws no more than 1A.
