Resistance in Parallel Circuits

The load is the part of an electrical circuit that does something. Examples include motor windings (inductive) and filaments in lightbulbs (resistive). When you run multiple loads in parallel with each other, you connect each side to the power supply.

In a series circuit, the loads are connected to each other, not each to the power supply. The electrons must move through each load as they proceed to the final load. You can add up the resistances of each load to get your total circuit resistance.

In a parallel circuit, the voltage is the same across all the loads. That’s why you can get 120v to the lightbulb in your bathroom and in your bedroom all the way across the house. For amperage, you add the amperage of each circuit to get the total circuit amperage. However, the resistance DECREASES for each load you add to the parallel circuit.

If you connect a single lightbulb to a circuit, the total circuit resistance is just that of the bulb (and an insignificant amount from the conductors). When you add another bulb to the circuit, the resistance decreases; you can calculate the total circuit resistance by dividing each load’s resistance (R) into 1 (1/R). Then, you add those values together to get total resistance (Rt) and divide that sum into 1 (1/Rt).

The idea is to get you to understand the relationship between resistance, voltage, and amperage in a parallel circuit. You have more current in a parallel circuit, so you will have lower resistance. The more parallel circuits you add, the more current you will draw; in turn, your total circuit resistance will be lower.

In a parallel circuit, electricity does NOT take the path of least resistance; it takes all available paths. The amount of current is just proportional to the resistance.

You can read our HVAC School article on Parallel Circuit Resistance HERE.

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