Why is One Side of an Outlet/Plug Larger than the Other?

July 23, 2010 | by Fred (email) |

outletYou’ve probably noticed that in all homes built since the early 80s, the standard configuration for a receptacle is a large slot on the left, a smaller slot on the right, and a roughly circular hole in the middle underneath the two of them.

You probably know that the center hole at the bottom is the ground wire, but probably don’t know why one slot is larger than the other, or how grounding works exactly.

How Electric Circuits Work

The larger, left slot in a receptacle is neutral and the smaller right slot is hot.  In all electric circuits, electricity flows from a point of higher electrical potential to a point of lower potential.  The hot wire in a house is the source of electrical potential. It’s the one that will shock you if you touch it, and that’s why we call it hot.

When you plug in an appliance or light bulb, the current flows out the hot side of the plug to the appliance or light, and then back out of the appliance on the neutral wire into the left side of the plug, completing the circuit.

Polarized Receptacles

Remember that in order for an electrical appliance to run, a complete circuit must exist through the appliance.  This means a switch could be placed anywhere in the circuit and still prevent the appliance from running.  For example, on a toaster, the switch could be placed either before or after the heating wires in the circuit and it would be equally effective.  However, it would not be equally safe.

Let’s say that we have a toaster with a switch installed after the heating wires.  A piece of bread gets stuck in the toaster and you (unwisely) start rooting around inside of it with a metal fork.  Since those wires are electrified (they are connected to the hot wire), as soon as you provide an alternate path for the electrical current to exit–e.g., through the fork, through your body, out your feet to the ground–you get shocked.

Take the same toaster and put the switch before those heating wires.  Now, when you go rooting with a form, the wires aren’t electrified at all, and you won’t get shocked.

If it isn’t obvious already, the reason one side of a plug is larger than the other (a.k.a. polarized) is to make sure that the switch inside a device is always in the circuit before the motor or heating wires or other electrical parts.  This is a safety mechanism just in case you accidentally provide an alternate exit path for the current.

Stay Tuned

Now you know about polarization.  Stay tuned for how grounding works and why we need it.

(photo: oxymoron)

5 Responses
  1. Regina says:

    Thanks! This website is the only website on the Internet they answered my question. And I thought it was important to know also because I’m in college working towards my goal of becoming an electrical engineer. That that bit of info was very helpful.

  2. HANDYMAN51 says:

    Is it allowed in the National Electrical code to replace polarized outlets with newer ” grounded” outlets? Does a grounding wire need to be run to the box? If not allowed, are polarized outlets still available, but in newer colors, and allowed for replacement use?

  3. poiboybf says:

    Thanks! It seems to me that as long as the circuit is complete (the machine is on), electricity is flowing through both wires. Wouldn’t the toaster still shock you if it was still on? Or am I thinking about it incorrectly?

  4. HANDYMAN51 says:

    Nicely done, and very nicely explained. We have quite a few of these polarized outlets in our home.

  5. RetiredEl15 says:

    I have a 2500 W generator that has a duplex receptacle in which the smaller slots are the neutral ( common ) wire, and the larger , T shaped slot is the HOT. Between the 2 smaller slots, the voltage is ( 0 ) zero, indicating to me that it is the neutral , since , between the two larger, T shaped slots the voltage is 226 Vac. The voltage between the smaller slot and each of the larger, T shaped, slots the voltage is 113 Vac. My point is, that, this is completely opposite to what I expected. Is this common on generators.

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