Tag: parallel

In a Series circuit (loads connected in a row end to end) it’s easy to calculate total circuit resistance because you simply add up all the resistances and you have the total.

In a Parallel circuit the voltage is the same across all the loads, the amperage is simply added up but the resistance is a bit more tricky.

It gets tricky to imagine because the total circuit resistance of parallel loads goes down the more loads you add.

For example, if you have one light bulb connected to a power source, the total resistance of the circuit is just the resistance of the bulb.

Add in another bulb in PARALLEL and the resistance of the circuit goes DOWN

When you are calculating the total resistance of a parallel circuit you take each individual resistance and divide it into (not by) one. You then add up all the resistances that were divided into one and divide that sum into one. The formula looks like this for the diagram at the top of the article.

1÷Rt (total resistance)= 1÷R1 + 1÷R2 + 1÷R3

For this particular application as shown above it would be.

1÷Rt(total resistance)=1÷120 + 1÷45 + 1÷360

So 1 ÷ 120 = .0083 + 1 ÷ 45 = .022 + 1 ÷ 360 = .0028

Then we add them all up

.0083 + .022 + .0028 = .0331 

Then to find the total you divide one by the total

1 ÷ .0331 = 30.21 Ohms total 

As you will notice, 30.21 Ohms is less than the lowest resistance in the circuit. This makes sense when you think about ohms law.

The lower the resistance the higher the amps. Adding in additional parallel loads INCREASES the amperage in a circuit, and we see this ever day when we notice that compressor amps and condenser fan amps added together equals total condenser amps.

So it stands to reason if lower resistance equals higher amps and adding in more parallel loads increases the amps, then adding in more parallel loads reduces the resistance.

Another myth this busts is the idea that electricity ONLY takes the path of least resistance. Electricity actually takes all paths between positive and negative charges and every additional path (parallel circuit) just decreases the resistance between the two points of potential difference. This increases the total circuit amperage, which is why when you try to run two hair dryers on one 15a circuit the breaker trips. Two hair dryers in parallel = lower  total circuit resistance = higher amps.

Not that I would use two hair dryers….. maybe that’s why I’m almost bald.

— Bryan


I had an old timer tell me that you can never connect two transformers together because they will “fight one another”.

If you are anything like me (and heaven help you if you are), whenever someone says something like that, a cartoon in your head starts playing.

In this case, I imagine two transformers with boxing gloves on duking it out to see which one “wins”.

The truth is you can connect two transformers together so long as you are careful, but you need to know why you’re doing it and then do it properly.

Transformers have a VA (Volt-amps) rating that dictates how many volt-amps (volts x amps, which is watts simplistically but there is a more complicated reason it is called VA in transformers that we won’t get into here) the transformer can handle on the secondary.

Above we show two 75VA transformers with 24V secondary windings.

75VA÷24V=3.125A

So with a 75VA transformer, you can run a maximum of 3.125 Amps, if you needed more power you would need to either go get a larger, more expensive transformer or…. you could connect another identical one in parallel. If you connected two 75VA transformers in parallel you would then have 150VA of secondary capacity which can be necessary in some cases with multistage commercial units or some large accessories.

In this case, parallel simply means connecting the two primary and secondary windings together in the exact same way as we show above… Pretty easy

It is SUPER important to get the polarity exactly the same and use two transformers with identical winding turns in the primary and secondary and identical secondary coil impedance (resistance).

In fact, it is so important that I advise that you only do this if you have two identical model transformers.

To be even safer, connect the primary windings first and check the secondary’s against one another with a voltmeter before actually connecting them to the system. For a typical 24v secondary you can connect the two common wires to ground to act as a stable reference first then check the two R or Hot side leads to one another and then to common. They should read 0v to one another and 24v to common. If you get anything other than 0v from hot to hot then you want to recheck your primary wiring and ensure that they are exactly the same.

— Bryan

Scroll to top
Translate »

Daily Tech Tip

Get the (near) daily Tech Tip email right in your inbox!
Email address
Name