A Beginners Guide to Calculating Current in Parallel Circuits

This is Part 2 in 3-part series about parallel and series circuits. Check out: Part 1: Calculating Resistance in Parallel Circuits, Part 3: A Beginners Guide to Batteries in Series and Parallel.

In the first post of this series, we discussed how resistors work in a parallel circuit. Today we are going to look at how current behaves.

Our example will be a DC circuit, powered by a 12V battery, with a combination of 5 resistors and series and parallel.

Is current constant in a parallel circuit? 

Yes, the total current is always constant in a parallel circuit. For example, if a current enters through point A at 0.072A, passes through multiple resistors, it will exit with the same 0.072A value at point B. What comes in, comes out.

How Does Current Flow Through a Parallel Circuit?

In a parallel circuit, our charged particles are divided up into separate branches. The number of particles (or current) in each branch is dependant on the resistance on each path. The current will always prefer to take the path of the lowest resistance.

In our example, the current will prefer to take the path of the 20 Ohm resistor, but some current will still travel through the 30 Ohm and 50 Ohm resistor.

Calculate the Total Current in a Parallel Circuit?

To calculate the total current in a parallel circuit, add the current of each individual pathway. The formula for total parallel circuit current is:

IT = I1 + I2 + I3… 

In our case, we already know the total current, we just need to calculate the current of each path:

0.072 = x + y + z

To work this out we must perform Ohms law on each individual branch. The sum of these values should equal 0.072A.

A common mistake you can make right here is to use the 12V supply voltage in your calculations. The first step is to work out the voltage drop across this part of the circuit.

From our previous post, we already know that the total resistance for this section is 9.68 Ohms. To calculate the voltage drop:

V = IR
V = 0.072 * 9.68
V = 0.697

Now that we have the voltage drop, we can work out the current across each branch.

Resistor x: 

I = V / R
I = 0.697 / 20
I = 0.034 A

Resistor y: 

I = V / R
I = 0.697 / 30
I = 0.023 A

Resistor z: 

I = V / R
I = 0.697 / 50
I = 0.014 A

Going back to our formula on the total current:

IT = I1 + I2 + I3 ...
IT = 0.034 + 0.023 + 0.014  

Total Current

IT =~ 0.072 A

Next Steps

Parallel circuits can be tricky at first, so don’t worry if you don’t; get it the first time, so try to design your own circuit, and solve it yourself using Ohms law.

Now I want to throw a test over to you. Try making your own parallel circuit. The best way to learn is by doing, and forcing yourself to solve real problems will help these rules stick.

I hope you have learned a thing or two today. If you like it, why not give it a share.

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Written By Tim M

I’m on a journey to learn all I can about electronics - and I’m sharing everything. If you like my content, check out my Honda CX500 Restoration Project

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