Reversed polarity on an electrical outlet (C) Carson Dunlop AssociatesReversed Polarity at Electrical Receptacles
What is Reversed Polarity, how do we detect it and why is it dangerous?

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Reversed polarity electrical receptacles:

Definition of reversed polarity at an electrical receptacle, its causes, cures, and dangers. What happens if you reverse the hot and neutral wires at an electrical receptacle? We also explain the difference between reversed polarity and reversed LINE - LOAD connections in a building electrical circuit.

This article series describes how to choose, locate, and wire an electrical receptacle in a home. Electrical receptacles (also called electrical outlets or "plugs" or "sockets") are simple devices that are easy to install, but there are details to get right if you want to be safe.

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What is Reversed Polarity at an Electrical Receptacle and Why Is It Unsafe?

Electrical Outlet wire connections (C) D FriedmanThe hot and neutral wires must be connected to the proper terminals on the electrical receptacle.

[Click to enlarge any image]

  • The "hot" or "live" black wire (or red wire) should be connected to the brass-colored screw terminal on the electrical receptacle, usually marked BLACK or HOT. The connection shown in our photo (left) is correct.
  • The "neutral" white wire should be connected to the silver-colored screw terminal on the electrical receptacle, usually marked NEUTRAL or WHITE

Carson Dunlop Associates' sketch points out that the white wire, i.e. the neutral wire, will be connected through the receptacle's internal parts to the wide slot on the receptacle face in order to assure that the neutral wire side of an appliance being plugged-in there is properly connected.

One of our buddies argues "what the heck, it's "alternating current" - the energy is flowing rapidly back and forth at 60 cycles per second on the wires - why would direction or polarity make any difference whatsoever?"

A non-technical answer might be "Why do you think wall plugs and electrical receptacles are 'polarized' or designed with a wide plug blade slot and a narrow plug blade slot (and on many devices a ground connector too) so that the plug will only go into the receptacle one way?" Given that there is extra cost and trouble involved, surely that's not for "no reason"!

Which Wall Plug Blades Connect to Which Receptacle Slots?

Notice that the neutral blade on a wall plug (sketch at left) is the wider blade, plugs into the wider slot in the electrical receptacle, and is intended to connect electrically to the neutral wire inside the receptacle.

The more narrow wall plug blade on a polarized plug is the "hot" blade and is intended to connect electrically to the hot or live terminal in the electrical receptacle.

What Happens When Polarity is Reversed at a Receptacle?

Most electrical appliances and devices are designed so that their "on-off" switch interrupts electrical power at the point of entry into the appliance or device circuitry or components. If you switch the hot and neutral wires that may not quite be the case, and parts of the device will remain energized or potentially energized even when the electrical device switch is OFF. No electrical current may flow, but it could flow if someone touches the wrong part of the device, or damage may be caused in other circumstances as well, as we describe next.

Watch out: Reversed polarity on an electrical outlet is dangerous. If you accidentally reverse these wires the device you plug in to the receptacle may "work" but it is unsafe and risks a short circuit, shock, or fire.

Even a simple light or floor lamp is less safe with polarity reversed

While some devices such as an incandescent electric light may appear to work properly and safely regardless of which way the lamp's plug is inserted into the wall outlet, virtually all modern electrical appliances, even lamps, use polarized plugs. In the case of an electric light, the device will "work" properly in either position.

When a lamp or light fixture is connected with proper polarity, the hot wire connects to a contact at the bottom inside center of the bulb socket or screw-in base, and the neutral wire is connected to the shell that contacts the sides of the bulb when it is inserted and screwed into the socket.

But if the lamp is plugged in with its polarity reversed the metal "shell" into which the bulb screws is energized or "hot". Because this component is much easier to touch when changing a light bulb than is the connector in the internal center of the bulb base, a shock hazard is present.

Some appliances and electronic devices may be damaged if polarity is reversed

Some appliances and some electronic equipment may be damaged if left connected to a reversed-polarity electrical circuit.

We disassembled a coffee maker that had burned-up and found that the appliance had been damaged by being left connected to its receptacle with polarity reversed. The presence of live voltage at the "wrong end" of a circuit or circuit board may cause some devices on the board to remain energized even when the device has been "switched off". A result can be overheating or electrical shock hazards.

Common Causes of Reversed Electrical Polarity on 120V Circuits, Devices, Appliances in Buildings

  • The black wire was connected to the silver screw (wrong), and white wire to the brass screw (wrong) where one or more individual electrical receptacles is wired.
  • The black and white wires were reversed at a splice in a junction box somewhere in the circuit between the electrical panel and the first receptacle in the string, or between two downstream receptacles in the same circuit.
  • The black and white wires were improperly connected in the electrical panel
  • In an older electrical circuit it is no longer easy to visually determine which wire is "black" and which is "white". It's easy enough to determine which is the hot wire, using an electrical tester, neon tester, VOM or DMM, by checking for voltage or current flow between an individual wire and ground.

    and also DMM Digital Multimeter HOW TO USE
  • Someone did not understand the proper connections for the red wire in a 3-wire or multiwire branch circuit
  • Other wiring errors such as short circuits can also produce "false" or apparent "reversed polarity" on a circuit. An example is unanticipated current on a supposedly "off" neutral wire on along a ground wire in an electrical circuit.

Is Reversing Line/Load Connections the Same as Reversed Polarity?

GFCI wiring details, back view (C) Daniel FriedmanNo.

You can reverse LINE and LOAD connections on daisy-chained devices (like the GFCI receptacle shown at left) and the circuit will appear to work properly. However the circuit may not be safe or fully protected.

Line and load electrical wire connections are important to get right on certain electrical devices such as GFCIs and AFCIs. Our photograph (left) illustrates the line and load markings on the back of a GFCI electrical receptacle.

Looking at the side or back of the molded case of this and other electrical devices such as AFCIs, you will see that one pair of terminals will be marked "line" and the other "load".

"Line" terminals

The Line terminals (green arrows in photo at left) on an electrical receptacle are for the incoming hot wire - the terminal marked LINE is connected to the incoming power source or the "hot" wire (typically black or red in insulation color) that connects to the brass colored screw (marked "Black" or "Noir) at the lower left " in our photo.

And the incoming neutral (white) wire from the electrical panel connects to the "Line" and "White" or "Blanc" terminal marked at the lower right in our photo

Load" terminals"

The Load terminals (red arrows near the top of our photo at left) on an electrical receptacle are for the outgoing wires. These wires feed electrical receptacles that are located "downstream"(farther from the electrical panel) from the device. The outgoing hot or black wire (red arrow, above left in our photo) connects to the terminal marked "Load" or "Charge" and "Black" or "Noir". The outgoing white, neutral wire, connects to the terminal marked "Load" or "Charge" and "White" or "Blanc" in our photograph.

Readers of this article should also

and also SAFETY for ELECTRICAL INSPECTORS. Our photo at page top is not an example of a proper electrical outlet installation.

This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe electrical conditions. Critique and content suggestions are invited. Credit is given to content editors and contributors.

Color coding of wires to properly connect an electrical outlet (C) Carson Dunlop Associates

The illustration at left shows the typical wiring of an electrical outlet or "receptacle", courtesy of Carson Dunlop Associates.

Watch out: Electrical components in a building can easily cause an electrical shock, burn, or even death.

Even when a hot line switch is off, one terminal on the switch is still connected to the power source.

Before doing any work on the switch, the power source must be turned off by setting a circuit breaker to OFF or removing a fuse.


Continue reading at ROUTING, Securing & Protecting Electrical Wires or select a topic from the More Reading links or topic ARTICLE INDEX shown below.


Or see ELECTRICAL RECEPTACLE CONNECTION DETAILS - where to connect black, white, red, green, ground wires .

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REVERSED POLARITY at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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