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Two Way Switching Explained | How to Wire a 2 Way Light Switch

Learn how a circuit is wired to turn a light on or off from two different locations.

In this article, we’re going to show you how a circuit is wired allowing you to turn a light on or off from two different locations. We are going to refer to this light control circuit as a two way switching circuit.

SPDT switches

Before we get started, we need to introduce you to some switch terms.

If we are going to turn a light on and off from 2 different locations, we need 2 Single Pole Double Throw or SPDT switches.

SPDT Switches

A SPDT is called a 2-way switch in the European Union, and a 3-way switch in North America.

What is SPDT

A SPDT switch has 3 terminals. It will have one terminal in and 2 terminals out.

Each terminal on a SPDT switch has a name. The input terminal is called Common. The output terminals are called Normally-Open and Normally-Closed.

Operating the toggle or lever will cause the Common terminal to connect to the Normally-open or Normally-closed terminal depending on its position.

SPDT - Components and Functions

Two way switching Circuit

Staircase lighting circuit

OK…let’s start wiring! Back we go to our stairwell lighting circuit.

We have a switch at the bottom of the stairs and another switch at the top of the stairs. And, of course, our ceiling light over the middle of the stairs.

For our discussion, let’s call the switches Up and Down. Either switch can turn the light on or off.

Stairwell Lighting Circuit

Wiring in the EU and North America

Circuit wiring is very similar in the European Union and North America but there are some differences. Electrical power supply wire insulation colors are different.

In this article, we will be referring to North America cables and wire colors.

Another difference is the typical household electrical voltage level. In North America, the voltage is 115 Volts AC, while in the EU the voltage is 220 volts AC.

Circuit wiring in the European Union and North America

NMD90 14/2 & NMD90 14/3

In North America, the 2 electrical power supply wires are called Line and Neutral and originate from a Circuit Breaker in an Electrical Panel. The Line wire has black insulation and the Neutral wire has white insulation.

The cable that leaves the Electrical Panel will contain these 2 wires plus a third wire which is connected to Earth Ground. The Earth Ground wire has no insulation.

The cable is referred to as NMD90 14/2 because of the size of the conductors and the number of wires inside. NMD90 14/2 has 2 wires sized at 14 AWG.

Wire NMD90 142

Power cable wiring

The actual circuit wiring is a bit of a challenge for 2 reasons.

– The cables are located behind the walls and the electrical power cable could enter the circuit at 3 different locations.

– The cable could enter at the light or either of the switches.

SPDT power cable Wiring

Just to add more possibilities, a cable could run between the switches and another cable from a switch to the light. Or, a cable could run from a switch to the light and another cable to the other switch.

We’re not going to discuss all possibilities in this article.

We will explore a wiring circuit where:

– The power enters at the Down Switch

– There is a cable that runs between the 2 switches

– And another cable that runs from the Up Switch to the light.

All cables are hidden behind the wall and above the ceiling.

It is easy to identify our wiring circuit by looking at the wires in the ceiling. There will be 3 wires only. These wires connect directly to the light.

SPDT Switch Wiring

Before we go any further, this is a good time to remind everyone that working with electricity is very dangerous! Leave the electrical work to qualified technicians!

Earth ground wiring

OK…here we go…Each device will be contained in an electrical box in the wall and ceiling.

As we said earlier, the electrical power enters through the Down Switch electrical box.

For all electrical circuits, the bare Earth Ground wires are connected and bonded to each electrical box.

Earth Ground wiring

SPDT switch wiring

We’ve introduced a new cable. This new cable is called NMD90 14/3 and has a third red wire.

The electrical power black wire connects to the Down Switch common terminal.

The power white wire does not connect to the switch at all but is connected to the white wire of the 14/3 cable. The wire connection is completed inside the switch electrical box.

There is another wire connector securing the 14/3 white wire to the 14/2 white wire of the light cable. Again, this wire connection is completed inside the UP Switch electrical box.

The 14/3 cable connects the 2 switches. The red wire connects the switches normally open terminals, and the black wire connects the normally closed terminals.

And finally, the black wire of the light electrical box connects to the Up Switch common terminal.

SPDT switch wiring explained

Flow of Electricity in a  two way switching circuit

Let’s see if it works! With the switches in the positions shown, the electricity will flow from the line wire through the light and back to the neutral wire.

What happens if we change the Down Switch position? The electrical circuit is broken and the light goes off!

Ok…Let’s see what happens if we change the Up Switch position. If all goes well, the light will come on. With the switches in the new positions, the electricity will flow from the line wire through the light and back to the neutral wire.

Flow of Electricity in a 2-way switching circuit

Summary

Ok…let’s review what we’ve covered in this article:

– A SPDT is called a 2-way switch in the European Union, and a 3-way switch in North America.

– A SPDT switch has 3 terminals. It will have one wire in and 2 wires out.

– Each terminal on a SPDT switch has a name. The input terminal is called Common. The output terminals are called Normally-Open and Normally-Closed.

– Turning a light on and off from 2 different locations requires a 2-way switching circuit.

– A 2-way switching circuit requires 2 SPDT switches.

If you have any questions about two way switching circuit and the wiring, add them in the comments below and we will get back to you in less than 24 hours.

Got a friend, client, or colleague who could use some of this information? Please share this article.

The RealPars Team
By Ted Mortenson

By Ted Mortenson

Automation Engineer

Posted on Nov 30th, 2020

Ted Mortenson

By Ted Mortenson

Automation Engineer

Posted on Nov 30th, 2020

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