Light switches do not need a neutral wire. The electrical box in which a light switch was installed did not usually have a neutral wire. However, In the United States, the National Electrical Code has required that the neutral wire should be in the switch box.

Devices that homeowners sometimes want to install in place of the original breaker require a neutral wire. For example, timers usually require a neutral wire.

If the breaker box does not have a neutral wire, it may be impossible or at least difficult to run a neutral wire into an electrical box that does not have a neutral wire.

Why Do Light Switches Not Have Neutral Wires

Switches do not have a neutral wire because only the supply end of electrical devices is designed to be switched to prevent leaving “hot” ungrounded devices that could cause an electric shock. Having a neutral wire is most likely a return from an outlet controlled by a switch.

Though, nowadays the National Electrical Code has ensured that neutral wires are in all switch boxes.

This does not mean that the neutral is connected to the switch, it is for when a box mounted device requires a neutral wire.

According to NEC codes, since most switches do not need a neutral wire to operate. They only switch the hot wire, and you are not allowed to switch or open the neutral.

Modern electronic switches need the neutral to power the electronic circuits they use to control the remote control and its WiFi functions.
Because of this, the current NEC code for new wiring requires that the circuit neutral be routed to most switches.

There are some exceptions, such as when there are multiple switches for lights or outlets and they are all in the same room and within sight of each other, not all switches must have a neutral.

A standard switch loop consists of a hot wire to the switch, a switchable return wire, and a ground wire. With a non-metallic cable, you have a black wire, a white wire, and a bare ground wire.

The National Electrical Code requires that the white wire renamed any color other than white or gray, be brought to the switch.

So with conduit, you will have a black wire, another black wire (a white wire with black duct tape on it, since that is what electricians always wear), and a bare ground wire.

If you have conduit going to the junction box, they’ll just run two black wires for the circuit breaker and a green one for the ground.

The code now requires a neutral at most breaker locations, which is usually a white wire but can also be gray. This was not the case with your wiring installation, and it is still not required if a conduit is used in the installation.

So most junction boxes probably had one black wire for hot power, another black wire for return to the load, and a red wire for return to the other load and, of course, the green ground wire.

Wiring Methods

The primary method of wiring used in homes is NM, or Romex, cable. This cable is made with ground and two or more wires in descending order of color: white, black, red, and blue. (Sometimes white with a red stripe).

You have no choice which colors go into Romex. So sometimes you have to use white as the hot color.

A rare way to run wires in houses is “loose wires in conduit.” In this case, a pipe is laid between the junction boxes, and the individual wires run along the pipe. Electricians can choose any color of wire.

In addition, this method of wiring does not allow the white or gray wire(neutral wire) to be used as a hot wire.

Credit:

https://www.quora.com/What-happens-if-a-light-switch-doesnt-have-a-neutral-wire?top_ans=290320779