Receptacles

Shapes match plugs to circuits

Receptacles, plugs, and switches carry voltage and current ratings, and their shapes are designed so equipment can’t be plugged into the wrong voltage or a circuit too small for it. The wrong match can damage equipment, overheat and start a fire, or even cause an electrical explosion ¹.

The common types ¹:

• 15 A, 125 V duplex: two vertical slots of different heights and a round ground hole. The everyday receptacle.
• 20 A, 125 V: the neutral slot is T-shaped, so it accepts both 15-amp and 20-amp plugs. Code permits it only on a 125-volt, 20-amp circuit.
• 15 A, 250 V: horizontal blades, for 240-volt equipment like a window air conditioner or shop tool.

Every receptacle and switch is marked with its ratings. Switches are often rated up to 277 volts, a voltage common in commercial buildings ¹. A plug that won’t fit a receptacle is the system working: don’t modify either one.

Which wire goes where

Devices are polarized: one side is meant for the neutral ¹:

• Brass screws take the hot (black or red).
• Silver screws take the white neutral.
• The green screw takes the equipment ground.

On the face, the short slot is hot and the tall slot is neutral. Polarized plugs have one wider blade that only fits the neutral slot, so a lamp’s threaded socket shell, the part a finger might touch while changing a bulb, stays on the neutral.

A duplex has two screws on each side, joined by a breakaway tab. Use one screw per wire; feed through with pigtails or the second screw, never two wires under one ¹.

Grounding the receptacle

Receptacles must be grounded. Some are self-grounding, picking up the ground through a mounting screw and spring clip against a grounded metal box. If you’re not sure a receptacle is self-grounding, run a short pigtail from the box’s grounding to its green screw ¹. In a plastic box, the circuit’s ground wire lands directly on the green screw. The grounding path must stay continuous through every box ².

GFCI receptacles

A GFCI receptacle protects people at its own outlet and, wired through its LOAD terminals, at the ordinary outlets downstream of it on the same circuit. In Wisconsin houses, 125-volt 15- and 20-amp receptacles in bathrooms, garages, outdoors, crawl spaces, unfinished basements, kitchens, laundry areas, and near tubs and showers need GFCI protection ³. Every outdoor 120-volt receptacle must be GFCI protected ².

When replacing an old receptacle, identify LINE (power in) and LOAD (to downstream outlets) before connecting a GFCI; reversed, it won’t protect anything.

Replacing one

1. Turn off the breaker and test the outlet dead with a tester you’ve checked on a live outlet.
2. Note which wires go where; photograph them.
3. Move each wire to the same position on the new device: hot to brass, white to silver, ground to green, clockwise loops.
4. Fold the wires back, mount the device straight, install the plate, restore power, and test with a plug-in tester.

What to take from this

Receptacle shapes match plugs to voltage and current: the T-slot 20-amp, 125-volt receptacle goes only on a 20-amp circuit, and horizontal blades mean 250 volts. Hot goes to brass, the white neutral to silver, ground to green; the tall slot is neutral, which keeps polarized equipment safe. Ground every receptacle, use one wire per screw, and put GFCI protection where Wisconsin requires it.