ground wire

October 22, 2009 1:22 PM

what is a ground wire? Plain English please. I have a microwave that needs an extension. So I got the 3-pronged male/female units. But my wire is only 2. What wire should I get, and why? What difference does it make? What is a ground, and why do some things require it? Where should a ground go?
posted by ebesan to Technology (25 answers total)

Best answer: An excellent explanation from How Stuff Works:

If you look around your house, what you will find is that just about every appliance with a metal case has a three-prong outlet. This may also include some things, like your computer, that have a metal-encased power supply inside even if the device itself comes in a plastic case. The idea behind grounding is to protect the people who use metal-encased appliances from electric shock. The casing is connected directly to the ground prong.

Let's say that a wire comes loose inside an ungrounded metal case, and the loose wire touches the metal case. If the loose wire is hot, then the metal case is now hot, and anyone who touches it will get a potentially fatal shock. With the case grounded, the electricity from the hot wire flows straight to ground, and this trips the fuse in the fuse box. Now the appliance won't work, but it won't kill you either.

What happens if you cut off the ground prong or use a cheater plug so you can plug a three-prong appliance into a two-prong outlet? Nothing really -- the appliance will still operate. What you have done, however, is disable an important safety feature that protects you from electric shock if a wire comes loose.

posted by cerebus19 at 1:25 PM on October 22, 2009 [6 favorites]

It helps prevents shorts, melting and fire. I'm not sure by your description what has 2 prongs or 3.

How many prongs does your microwave cord have?

How many prongs does your extension cord have?

How many holes does your outlet have.
posted by JeremiahBritt at 1:26 PM on October 22, 2009

Also, you may be able to use an adapter like this one if and only if your two prong outlets are properly designed and installed to take one. You can verify this by installing the adapter (they're dirt cheap) and testing it with an outlet tester like this one. Remember to cut the power to the outlet at the circuit breaker before installing the adapter.
posted by jedicus at 1:42 PM on October 22, 2009

Response by poster: the MW has 3.
I need to make a short extension, to a 3-prong outlet; so I should get cord w/ 3 wires? but then which one is the ground wire? or does it matter?
posted by ebesan at 1:42 PM on October 22, 2009

I need to make a short extension, to a 3-prong outlet; so I should get cord w/ 3 wires? but then which one is the ground wire? or does it matter?

Yes, you want a 3-wired cord. The ground wire is the "extra" one that two-prong plugs don't have - I'm assuming you're in the US; it's the one that's round, as opposed to the two flat ones.
posted by Tomorrowful at 1:44 PM on October 22, 2009 [1 favorite]

Shorrt answer: A ground wire is so called because it allows electricity to, literally, flow into the ground (that is, the earth underneath your house). By doing this, it completes the electrical circuit through the earth rather than doing so by some different, unpleasant pathway (e.g., through you or through some part of the house where the resulting heat generation could cause damage).

In some other English speaking countries, it's actually called an "earth wire" rather than a "ground wire" connection.
posted by darkstar at 1:46 PM on October 22, 2009

It's also important to make sure the extension cord can carry as much current (measured in amperes) as the microwave wants to pull. If you have an 1100 watt microwave oven, then at 110 volts that's 10 amperes and you'd probably want a 20 amp extension cord just to have safe margin.

If you were to use a 5-amp cord, you would be at serious risk of starting a fire from the cord overheating and burning whatever it's sitting on.
posted by Chocolate Pickle at 1:48 PM on October 22, 2009 [1 favorite]

Plugging a microwave into an extension cord is a good way to start a fire. Most electricians would probably say never do it, but I am with Chocolate Pickle; I think it is safe enough as long as you use the proper extension cord.
posted by caddis at 1:51 PM on October 22, 2009

The prongs on your microwave cord make a little "face" The twowo flat ones are the eyes and the ground wire is the mouth.
posted by Acacia at 2:08 PM on October 22, 2009 [2 favorites]

Two, not twowo. I hiccuped.
posted by Acacia at 2:10 PM on October 22, 2009

Are you planning to actually make the cord longer as opposed to plugging the microwave into a cord and plugging the cord into the wall? Please don't do that.

Use the right extension cord like Cop and caddis have suggested.
posted by soelo at 2:43 PM on October 22, 2009

uh, Cop = CP = Chocolate Pickle
posted by soelo at 2:45 PM on October 22, 2009

ebesan: I need to make a short extension, to a 3-prong outlet; so I should get cord w/ 3 wires? but then which one is the ground wire? or does it matter?

As soelo says. Please, if you need to ask these questions, you shouldn't be making anything. Buy a short extension cord, or get an electrician to either make one or replace your appliance cord.

We're not saying you're too dumb to do it, we're not part of a conspiracy from The Man to keep you down, and we're not part of the International Brotherhood of Electricians, Locksmiths, Plumbers, and Miscellaneous Overchargers (Chapter #113). It's not hard to do (around here you can buy the bits in K-Mart, & instructions are on the packet!), but we're trying to keep you safe.
posted by Pinback at 3:11 PM on October 22, 2009 [1 favorite]

Wait, are you trying to actually make the microwave cord longer? Please, please, please, do not do this. The risk of fire is huge. This is what extension cords are for. You plug the microwave into the extension cord, and the extension cord into the wall.

If you have already cut the power cord for the microwave, please call an electrician to fix it for you.
posted by hwyengr at 3:13 PM on October 22, 2009 [2 favorites]

You said that your MW has 3 prongs and that you "I have a microwave that needs an extension. So I got the 3-pronged male/female units. But my wire is only 2.". It sounds to me as if you are trying to make your own extension cord with 2 wire cable, thus bypassing the ground. That would not be a good idea for reasons others have stated. Why not simply buy an assembled extension cord with an appropriate rating?
posted by Neiltupper at 3:58 PM on October 22, 2009

Yes, please do not make anything for this purpose. I teach this stuff and I wouldn't dream of making an extension for something like a microwave.

As others have said, it's even borderline to buy an extension cord. The microwave may draw 10-15 amps when running (which is a lot for an extension cord) but the current at the instant you hit "start" may well be much higher that that. You run the risk of accidentally building a spot-welder.

If you must use an extension cord, buy a heavy duty short cord made for air conditioners. Note that it's not safe to connect more than one of these together, either.
posted by range at 4:40 PM on October 22, 2009 [1 favorite]

Note that the air conditioner cord linked is for 220V, so not suitable at all. A similar 110V one might be.
posted by alexei at 6:07 PM on October 22, 2009

N'thing everyone above - if you have to ask, you shouldn't make your own extension cable.


That said, if you got your hands on a proper 12 gauge, 3-wire cable, and some 15a Edison plugs you would identify the ground by its color coding:

Color jacket on your wire ----> Color on your plug
Black ----> Gold screw. This is the hot wire.
White ----> Silver screw. This is the neutral wire. Don't be fooled by the name, this also carries live current.
Green ----> Green screw. Ground.

If you're not in the US, it's usually
Brown --> Hot
Blue --> Neutral
Green --> Ground


Again, don't screw around with live current if you're not sure what you're doing.
posted by Wulfhere at 6:08 PM on October 22, 2009

And if you don't want to make one, here's a few products that might work for you:
2 footer.
25 footer.
posted by Wulfhere at 6:12 PM on October 22, 2009

Response by poster: no, I didn't cut anything.
just need 2 more ft.. and extensions are usually longer.
i'll buy a heavy-duty.
the main point of my question was just to understand the reasoning behind grounding.
thanks
posted by ebesan at 6:12 PM on October 22, 2009

It's perfectly safe to run a microwave on a correctly rated extension cord as long as you don't do something stupid like running it behind your fridge or under a rug or something. You won't find one rated for 20 amps at 120 volts, which would be overkill anyways, but a 12 gauge cord a few feet in length would work for any unit. Air conditioner cords are a good choice because they are rated for 15 amps and only have a single outlet making for a compact installation however you can find power bars with 15A ratings too if you could use a few more outlets near your microwave.
posted by Mitheral at 7:00 PM on October 22, 2009

Note that the air conditioner cord linked is for 220V

Whoops -- you're absolutely right; I looked too fast and checked amp rating but not plug. As others have said already, get something rated for 15A (typically marked "air conditioner") with a single receptacle and you'll be fine. I've found these in 3' lengths before.
posted by range at 8:05 PM on October 22, 2009

Do not make your own cord! You obviously do not understand even the most basic aspects of this and if you make one the wrong way you may very well start a fire. If you must do this, purchase a short extension cord rated for the proper wattage. Three foot cords in this rating are readily available. You should go to a hardware store and tell them exactly what you want to do and ask for their professional advice on which cord to use. This is not an area in which you want to be making mistakes. They will burn your house down.
posted by caddis at 9:34 PM on October 22, 2009

Response by poster: thanks everyone.
bought a simple extension from home depot.
but this has been informative...where can i learn more....'electrical for dummies'?
posted by ebesan at 5:53 PM on October 23, 2009

Response by poster: not 'simple', I mean a short, rated one.
posted by ebesan at 7:01 PM on October 24, 2009