Homes in North America are supplied with a 120/240V single-phase electrical service. The incoming 240V power is split into two legs. Each leg can provide 120V hot-to-neutral, and the two legs together will supply 240V, which is used to supply heavy loads such as air conditioning compressors and cookstoves.
In every home, there are branch circuits that supply the lights, receptacles and built-in appliances with electricity. The wires, or conductors, in those circuits are classified based on wiring type, size, and color, and it is the color of the wire’s insulation which designates the function of the wire. Understanding the information those colors convey helps you work with greater safety and ease when troubleshooting an electrical problem or making repairs or improvements to your electrical system. The connections should not be made haphazardly. In the United States, the color of the wire used must comply with the requirements of the National Electrical Code (NEC).
A Word of Caution: Always use a line voltage meter or circuit tester to determine if a wire is "hot" or "live." Never rely on a wire's color alone.
Depending on where you live, the branch circuit wiring in your home will be made with individual conductors run through a hollow metal conduit, or with one of three types of cable. A cable is a factory-made assembly of conductors in an outer jacket or sheath. Plastic sheathed, or Type NM (NonMetallic), cable, is widely used for home wiring circuits. Modern Type NM cables have different color sheathing which indicates the size of the conductors: White for 14AWG (American Wire Gauge) conductors, Yellow for 12 AWG conductors, and Orange for 10 AWG conductors. Type NM cable will also have markings on the cable sheath of the cable, indicating the size and type of the conductors in it, along with other information. “14-2/G”, for example, shows that that cable has two insulated, current-carrying conductors and one grounding conductor, size 14 AWG.
The other two types of cable, which are often used to extend wiring in a system wired with metal conduit, are Type MC (Metal Clad) and Type AC (Armor Clad). Both Type MC and Type AC are sheathed in a flexible metal coil. Type AC cable, commonly called BX, does not have a grounding conductor. The metal sheath, plus a thin aluminum bonding wire, provide the required path to ground. Type MC cable does have a grounding conductor. In recent years, manufacturers have begun spraying color codes onto cables with metal sheathing. The colored marks correspond to the colors of the current-carrying conductors in that cable.
Black and Red Wires
According to NEC, a black or red wire must be used as an ungrounded conductor or hot wire. In fact, any color wire except white, gray or green may be used in conduit to carry ungrounded power. No wire with any of those colors can be used for establishing a ground or neutral connection.
Because a red-colored wire also serves as an ungrounded conductor, these wires are used to connect the second leg of the electrical system to loads which require 240 volts. The red wire is also used as a by-pass in a two-way switched lighting circuit, and will often be used also as an interconnection or communication wire in a circuit that requires it, such as a smoke detector circuit.
Blue, Yellow, and Other Colors
In systems where individual conductors, or wires, are run through a conduit, wires that are blue or yellow, or any color except white, gray or green are sometimes installed to designate a specific use. For example, if power needs to be run from a ceiling box down to a box with three switches and then brought back to the ceiling to power three different sets of lights, a black wire could be used to take the power down to the switches and three wires with other colors—say red, blue, and yellow—could be used to bring the power up from each of the switches. That makes it easier to tell, at a glance, which switch is being used to control each set of lights.
A white wire is typically used as the neutral wire or grounded conductor. For conductors as small as those found in residential branch circuits, only a white or gray wire may be used for that purpose.
In the past, a white wire in a cable assembly was allowed to be used to carry ungrounded potential—to function as a hot wire—in a switch leg or when connecting a set of 3-way switches. This changed with the adoption of the 2011 cycle of the NEC, which requires the presence of a grounded conductor in every switch box, to facilitate the installation of lighted switches, motion detectors, timers, and similar energy-saving controls. In new work, the white wire in a cable assembly cannot be used to carry ungrounded power. Because that requirement was only adopted in 2011, though, a homeowner may find that one has been used that way in an existing installation. In those cases, the white wire should be marked with red or black electrical tape or permanent marker—or any color other than gray or green—to show that it is being used as a hot wire and is carrying ungrounded potential. That wasn’t always done, so be careful of white wires in switch boxes. Since a neutral should never be switched, assume that any white wire that you find connected to a switch is a hot wire until you test it and determine that it isn’t.
The White Wire in a 240V Circuit
There is one case in which a white may be used to carry ungrounded potential that is still in conformity with the most recent cycle of the NEC. That is when a cable is used to feed a straight 240V load. If an appliance does not need or use 120V power, but only needs and uses 240V power, a standard 2-conductor cable, which has a black, white and bare set of wires, may be used to connect that appliance. In that case, the black wire is connected to one pole of the two-pole 240V breaker in the panel. The white wire is redesignated as an ungrounded conductor by using black or red (or any color except gray or green) electrical tape or permanent marker and connected to the other pole of the breaker, and the bare ground wire is connected to the ground busbar. At the appliance end, the white wire is marked again and the two insulated wires are connected to the two input terminals for ungrounded power on the appliance, or on its disconnect switch, and the ground wire is connected to the frame of the appliance or the box that the disconnect is in.
This wiring method—two hots and a ground—was common for many years to supply most of the larger appliances in our homes, so it is still found in place, and in service, today. With the increased use of electrical and electronic controls in many appliances, though, a four-wire service which includes a neutral conductor is being required for a new circuit. Most new cooking appliances—ranges, wall ovens, and cooktops—are being built to use that wiring. So are the newer electric dryers. They may, however, be capable of working with either type of wiring. If you are replacing an older cooking appliance or dryer, the instructions may tell you that you can, by replacing the cord on the appliance, connect it to an existing three-wire 240V circuit.
There are still 240V appliances that only need two hot wires and a ground. An electric water heater and the outdoor condensing unit, or compressor, for central air conditioning, are two of those.
The important thing to keep in mind here is that the white wire in a 240V circuit may be carrying ungrounded power, and it may not have been marked to show that it is. Don’t assume that because it’s white, it can’t shock you. Always test to be safe.
Green and Bare Wires
Wires with green colored insulation and non-insulated wires are equipment grounding conductors or ground wires. The equipment grounding conductor, or EGC, is an important addition to residential wiring that became widely adopted in the 1970s. This separate set of conductors is connected directly to the earth. The EGC wiring carries no power at all when everything is working normally. But if there’s a fault, and ungrounded power is present where it could damage an appliance or shock someone, a properly connected EGC provides that power with a low-resistance path to earth. Since the power is trying to return to its source, it will take that path. This may trip a circuit breaker or blow a fuse but it will greatly reduce the risks of appliance damage, a fire, or electrocution to a human being. For the EGC to function properly, green and bare wires need to be connected together, to devices such as switches and receptacles, and to any metal electrical box.