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Electrical switches: Does it matter which wire it's on? Question
Most everyday electrical wiring has two wires, usually one white and one black/red. When installing a switch, does it matter which wire I put the switch on?
3 answers
This answer is from a US/Canada perspective. Other countries vary in wire colors (exception: bare (uninsulated), green, green/yellow are always ground) and standard practices.
Below is a full "how do I wire a switch" answer. But the implied question is, I believe, something like:
I have an empty switch box. It has two black wires and two white wires. Right now the blacks are together and the whites are together and the light is always on. I want to install a switch so that I can turn the light on/off as needed. When installing a switch, does it matter which wire I put the switch on?
And the answer to that question, in the US/Canada, is:
Black is hot. Neutral is white. For safety reasons, switches are installed on hot, not neutral. So connect the switch to the black wires and leave the neutral wires connected together.
which is the same as a "In + Out" two-cable setup with /2 cables, as described below.
There are references to the NEC, the National Electrical Code. This sets the standard across the US. A similar code is used in Canada. The NEC is updated every 3 years and each state adopts a newer version of the NEC at a different time, so some changes can take a while to propagate across the country. In addition, most changes are grandfathered so you don't need to change existing installations to meet the current code unless you are adding circuits or making substantial changes. Many jurisdictions (state, county, city) have their own modifications to the NEC, sometimes making things stricter (e.g., large cities that require conduit instead of non-metallic cable for residential wiring) and sometimes making things easier (e.g., delaying adoption of GFCI protection on large circuits).
There are two very different ways a switch can be wired:
In + Out
This is either two cables or two sets of wires, one coming in from the breaker/panel (or from a previous junction in the circuit) with always hot, neutral and ground and one going out to the switched device with switched hot, neutral and ground. I won't say anything more about ground because (a) ground is everywhere (except in very old installations) and (b) all grounds always connect together and, if you use a metal box (as you should, but some people use plastic for reasons), to the box. The out cable/wires may also include a wire for ongoing hot to additional switches or unswitched devices.
If these wires are in cables then with a 2-wire cable, referred to as /2, has:
- black = hot or switched hot
- white = neutral
- bare = ground
A 3-wire cable, referred to as /3, has:
- black = hot
- red = switched hot (note: technically black could be switched hot and red hot, but convention is to use red for switched hot)
- white = neutral
If the wires are in conduit then you can use the same colors as in cables, but you can also use:
- blue, yellow (not green/yellow), pink, and a few other colors (I can't remember at the moment)
- gray for neutral
- green for ground
Although some jurisdictions may limit colors, particularly in residential use, the NEC does not.
With this type of wiring (in + out, two cables or conduits), the typical configurations are:
- In - Hot (black) to one switch connection. If it is a simple switch then it does not matter which connection. If it is a smart switch then it may matter which connection.
- Out - Switched hot (black or red) to the other switch connection.
- Neutral (white) wires together. Neutral also connected to the switch if it requires power (many, but not all, smart switches, dimmers, timers, motion sensors, etc.)
With metal boxes, switches do not normally need ground wires connected. With plastic boxes, switches with a ground connection (older ones may not have one) are connected to the ground wires.
At the switched device, switched hot (black or red) and neutral (white) connect to the appropriate connections on the device and may also chain to additional devices.
Switch Loop
A switch loop is one cable or group of wires that goes from the breaker/panel (or other junctions along the way) to the switched device (e.g., light or fan) and from there to the switch. Essentially hot and switched hot form a loop through the switch.
Switch loops used to have an advantage in wire cost because they did not require a neutral wire to go down to the switch. However, many newer switches (smart switches, dimmers, timers, motion sensors, etc.) require neutral and the NEC now requires neutral in most switch boxes if wired using cables. If wired using individual wires in conduit then a neutral can be added later.
- Old-style switch loop
An old-style switch loop is not valid for new work. However, if you are replacing an existing switch then you can still use it, unless you need neutral.
This has one cable or one pair of wires (ignoring ground, as noted above). If it is a cable then white will be hot and black will be switched hot. There are reasons for this, but it is required by the NEC. The white wire should be marked on each end to indicate it is not neutral, typically using a piece of black electrical tape. If it is individual wires in conduit then the two wires will typically be black and red, but could be any valid hot colors (i.e., not white, gray, bare, green or green/yellow).
With an old-style switch loop, black and white each go to a connection on the switch. If it is a simple switch then it doesn't matter which wire goes to which connection. If it is a smart switch, dimmer, timer, motion sensor, etc. that does not require neutral then it might matter which wire goes to which connection - check the installation instructions.
- Switch loop with neutral
A switch loop with neutral can be wired with a /3 cable, with black = hot, white = neutral and red = switched hot. It can also be wired with individual wires in conduit, with white = neutral and any two valid hot colors for hot and switched hot.
The hot (usually black) wire goes to one connection on the switch. It doesn't matter which connection with a simple switch, but it may matter on a smart switch, dimmer, timer, motion sensor, etc. The switched hot (usually red) wire goes to the other connection on the switch.
If the switch requires neutral, connect the white neutral wire to the appropriate connection. If the switch does not require neutral then cap the white wire with a small wire nut.
When using a switch loop, at the device you will have two cables or sets of wires - In and Switch Loop. The In hot wire and Switch Loop hot wire are connected together. The In neutral, Switch Loop neutral (if new switch loop with neutral) and device neutral are connected together. The Switch Loop switched hot wire connects to the hot connection on the device.
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The switch will work either way. However, there are significant safety benefits to installing switches on the hot wire.
For residential AC in the United States, only one wire actually has a potential difference (the "hot" wire, usually red or black) while the other wire is neutral (usually gray or white). Note that "neutral" is not the same as "safe to touch" - what the grid considers neutral may be quite different from the ground, so you should not go touching neutral wires unprotected. But it is generally not expected that it has the same powerful voltage as the hot wire.
When current is passing through devices, it is quite visible. It is obvious that you shouldn't try to stick piece of wire into the housing of a lightbulb which is on. And indeed, when the switch is on, the entire circuit is dangerous.
When the switch is off, there is no current, but the voltage still propagates through the parts of the circuit connected to the hot wire (and the grid). Those parts (before the switch) are very dangerous. The rest of the circuit (after the switch) is significantly less dangerous. If you put the switch on the neutral wire, then the dangerous part will include the device (eg. lightbulb). Unlike current, voltage is hard to see unaided, and with the switch open the lighbulb will appear to be off despite having a significant voltage. The moment someone touches the housing, the electrons will rush through that person to the ground, and electrocute them in the process. This is arguably a surprising result to most people, because they would not expect to be electrocuted when "the switch is off".
For this reason, putting the switch on the hot wire improves safety in two ways:
- Reduce accidents when people accidentally touch the contacts of an electrical device which is off
- Doesn't mislead people into thinking a device is safe to work on "because the switch is off and the device seems unpowered"
Most everyday electrical wiring has two wires
Well, no. Nowadays you usually get hot, neutral (or "return"), and ground. At least here in North America where houses get power from the secondary of a center-tapped 240 V transformer.
usually one white and one black/red
That depends on jurisdiction. Here in the US, it's black for hot, white for neutral, and green for ground. In the past this wasn't always the rule, so you need to be more careful with old wiring.
When installing a switch, does it matter which wire I put the switch on?
Absolutely! This will be spelled out in any competent electrical code. You always want to switch the hot wire or wires. For a typical 120 V circuit in a US house, that would be the black wire. For 240 V appliances there are two hots, so they both need to be switched.
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