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Two prong AC plug - which is live and which is neutral

dullard

Elite Member
May 21, 2001
22,208
626
121
Quick question, with a two prong plug is the wider plug live or neutral? It really doesn't matter with AC since current will flow in both directions, but what is the standard in the US?
 

FoBoT

No Lifer
Apr 30, 2001
63,091
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fobot.com
it can matter for certain devices

You've probably noticed that plugs often have one prong that is wider that the other. This is done so that it can be properly aligned with a receptacle. The smaller narrower pin is for the "hot" black wire and the larger wider one is for the "neutral" white wire. This is very important when it comes to wiring new appliance plugs or receptacles as it assures that the current flow is proper through that device. Of particular note are certain devices or appliances that have circuitry, such as, coffee makers, alarm clocks, computers, etc. If the hot and neutral wires are reversed, damage can occur to these appliances. Do not "clip" polarized plugs to fit into a two (2) prong receptacle or a receptacle not designed for polarized plugs.
linked
 

dullard

Elite Member
May 21, 2001
22,208
626
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Thanks, I did a quick google search and kept coming up with millions of useless links. I should have clarified - the direction doesn't matter for my application (but I just wanted to hook up the wires with the proper colors in case someone else ever works on it).
 

nebula

Golden Member
Apr 4, 2001
1,315
3
0
Originally posted by: FoBoT
it can matter for certain devices

You've probably noticed that plugs often have one prong that is wider that the other. This is done so that it can be properly aligned with a receptacle. The smaller narrower pin is for the "hot" black wire and the larger wider one is for the "neutral" white wire. This is very important when it comes to wiring new appliance plugs or receptacles as it assures that the current flow is proper through that device. Of particular note are certain devices or appliances that have circuitry, such as, coffee makers, alarm clocks, computers, etc. If the hot and neutral wires are reversed, damage can occur to these appliances. Do not "clip" polarized plugs to fit into a two (2) prong receptacle or a receptacle not designed for polarized plugs.
linked
I want some more evidence! I just don't believe the fact that reversing the plug for a alarm clock can harm it. Like the OP said, current in AC "flows" both ways, so how can that matter. "it assures that the current flow is proper ", heh?

Besides, it's a Canadian website, I don't trust it. j/k :)
 
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LeetViet

Platinum Member
Mar 6, 2003
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I lived all my life without knowing this, you'd think someone would've given you a heads up.
 

Eli

Super Moderator | Elite Member
Super Moderator
Oct 9, 1999
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Originally posted by: nebula
Originally posted by: FoBoT
it can matter for certain devices

You've probably noticed that plugs often have one prong that is wider that the other. This is done so that it can be properly aligned with a receptacle. The smaller narrower pin is for the "hot" black wire and the larger wider one is for the "neutral" white wire. This is very important when it comes to wiring new appliance plugs or receptacles as it assures that the current flow is proper through that device. Of particular note are certain devices or appliances that have circuitry, such as, coffee makers, alarm clocks, computers, etc. If the hot and neutral wires are reversed, damage can occur to these appliances. Do not "clip" polarized plugs to fit into a two (2) prong receptacle or a receptacle not designed for polarized plugs.
linked
I want some more evidence! I just don't believe the fact that reversing the plug for a alarm clock can harm it. Like the OP said, current in AC "flows" both ways, so how can that matter. "it assures that the current flow is proper ", heh?

Besides, it's a Canadian website, I don't trust it. j/k :)
Yeah, I dunno....

We live in a house with two prong, unpolarized plugs.. and I've never paid any attention to which way I plug anything in...

Although we do have to use those 2 > 3 prong adapters in many cases, and they are polarized.. but whos to say whoever wired the sockets connected the wires the "right" way?...

Dunno.
 

FoBoT

No Lifer
Apr 30, 2001
63,091
10
76
fobot.com
i have blown up light bulbs that were in fixtures that had the polarity reversed

i cannot explain why, i am not a physicist
 

Analog

Lifer
Jan 7, 2002
12,755
1
0
Originally posted by: FoBoT
it can matter for certain devices

You've probably noticed that plugs often have one prong that is wider that the other. This is done so that it can be properly aligned with a receptacle. The smaller narrower pin is for the "hot" black wire and the larger wider one is for the "neutral" white wire. This is very important when it comes to wiring new appliance plugs or receptacles as it assures that the current flow is proper through that device. Of particular note are certain devices or appliances that have circuitry, such as, coffee makers, alarm clocks, computers, etc. If the hot and neutral wires are reversed, damage can occur to these appliances. Do not "clip" polarized plugs to fit into a two (2) prong receptacle or a receptacle not designed for polarized plugs.
linked

One side is at the same potential as ground, i.e. the neutral, which is the fat side. Touching the neutral in a properly wired system will not shock you. Never defeat a polarized plug, as you may set yourself up for a shock, because the chassis may become energized. Clipping a ground is also dangerous, since this could cause a floating chassis which would not trip the breaker in case of a fault condition.

So, to answer your question, the neutral is the fat side, the narrow side is hot. That is, of course, if it was properly wired in the first place. How can you check this? If you have a multimeter, check between the fat side and ground. You should get 0V. Check from the narrow to ground - you should get 120V. Check from the narrow to fat side, you should get 120V. That would mean you have a properly wired system. You can buy small testers that plug into an outlet and with three diagnostic lights, indicate a properly wired system. Most electrical inspectors carry such devices. :)
 

Harvey

Administrator<br>Elite Member
Administrator
Oct 9, 1999
35,052
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Originally posted by: FoBoT
i have blown up light bulbs that were in fixtures that had the polarity reversed

i cannot explain why, i am not a physicist
Or an electrician. This is not possible unless something inside the lamp was shorted, and it still would not make any difference which way the plug was inserted. A light bulb is a two terminal device, and neither side of the line should be connected to any exterior part of a lamp or light fixture.

You can have differences if you have a couple of components in an audio set up with interconnecting wires, such as from a tuner, CD player, etc.. Sometimes, when the cables are not hooked up, you can feel a slight tingle by touching one and running your finger along a metal surface of the other, but I've never seen it at a dangerous level. It's one way I check to see if all components in a stereo are plugged in the same way to minimize hum.

The same trick works with musical instrument electronics, but I've actually gotten shocked from that. Stepping up to the mic with your guitar in hand can be a "shocking" experience. :Q
 

nebula

Golden Member
Apr 4, 2001
1,315
3
0
Originally posted by: yellowfiero
Originally posted by: FoBoT
it can matter for certain devices

You've probably noticed that plugs often have one prong that is wider that the other. This is done so that it can be properly aligned with a receptacle. The smaller narrower pin is for the "hot" black wire and the larger wider one is for the "neutral" white wire. This is very important when it comes to wiring new appliance plugs or receptacles as it assures that the current flow is proper through that device. Of particular note are certain devices or appliances that have circuitry, such as, coffee makers, alarm clocks, computers, etc. If the hot and neutral wires are reversed, damage can occur to these appliances. Do not "clip" polarized plugs to fit into a two (2) prong receptacle or a receptacle not designed for polarized plugs.
linked

One side is at the same potential as ground, i.e. the neutral, which is the fat side. Touching the neutral in a properly wired system will not shock you. Never defeat a polarized plug, as you may set yourself up for a shock, because the chassis may become energized. Clipping a ground is also dangerous, since this could cause a floating chassis which would not trip the breaker in case of a fault condition.

So, to answer your question, the neutral is the fat side, the narrow side is hot. That is, of course, if it was properly wired in the first place. How can you check this? If you have a multimeter, check between the fat side and ground. You should get 0V. Check from the narrow to ground - you should get 120V. Check from the narrow to fat side, you should get 120V. That would mean you have a properly wired system. You can buy small testers that plug into an outlet and with three diagnostic lights, indicate a properly wired system. Most electrical inspectors carry such devices. :)
I was waiting for your response yellowfiero! But you didn't answer why it would matter which way you plug in your clock. Either way it's still 120V potential and an AC circuit or transformer shouldn't care about direction of electron flow.
 

nebula

Golden Member
Apr 4, 2001
1,315
3
0
LOL! I just reread this:

"This is very important when it comes to wiring new appliance plugs or receptacles as it assures that the current flow is proper through that device. Of particular note are certain devices or appliances that have circuitry, such as, coffee makers, alarm clocks, computers, etc."

I guess if you're plugging something in, you should make sure it has circuitry. :confused:
 

Analog

Lifer
Jan 7, 2002
12,755
1
0
Originally posted by: nebula
Originally posted by: yellowfiero
Originally posted by: FoBoT
it can matter for certain devices

You've probably noticed that plugs often have one prong that is wider that the other. This is done so that it can be properly aligned with a receptacle. The smaller narrower pin is for the "hot" black wire and the larger wider one is for the "neutral" white wire. This is very important when it comes to wiring new appliance plugs or receptacles as it assures that the current flow is proper through that device. Of particular note are certain devices or appliances that have circuitry, such as, coffee makers, alarm clocks, computers, etc. If the hot and neutral wires are reversed, damage can occur to these appliances. Do not "clip" polarized plugs to fit into a two (2) prong receptacle or a receptacle not designed for polarized plugs.
linked

One side is at the same potential as ground, i.e. the neutral, which is the fat side. Touching the neutral in a properly wired system will not shock you. Never defeat a polarized plug, as you may set yourself up for a shock, because the chassis may become energized. Clipping a ground is also dangerous, since this could cause a floating chassis which would not trip the breaker in case of a fault condition.

So, to answer your question, the neutral is the fat side, the narrow side is hot. That is, of course, if it was properly wired in the first place. How can you check this? If you have a multimeter, check between the fat side and ground. You should get 0V. Check from the narrow to ground - you should get 120V. Check from the narrow to fat side, you should get 120V. That would mean you have a properly wired system. You can buy small testers that plug into an outlet and with three diagnostic lights, indicate a properly wired system. Most electrical inspectors carry such devices. :)
I was waiting for your response yellowfiero! But you didn't answer why it would matter which way you plug in your clock. Either way it's still 120V potential and an AC circuit or transformer shouldn't care about direction of electron flow.
you are 100% correct, it shouldn't matter on devices that are isolated, such as drills, transformers etc. However, devices that have metallic chassis, are usually grounded at the chassis. That way, if there is an internal short, the chassis would never get 'hot'. If an internal wire on a grounded chassis system were to touch the chassis, there would be a spark, and then the breaker would trip. Imagine if someone defeated the ground on such a system. The shorted wire to the chassis would energize the enclosure, and the next person that would touch the enclosure would get a shock at the minimum. It would also pose as a fire hazard. This is especially true in devices that deal with water, such as dishwashers and coffee machines.
 

beer

Lifer
Jun 27, 2000
11,169
0
0
Yellowfiero -

Wouldn't diodes in the electronic devices act up if you reversed current flow?
 

compudog

Diamond Member
Apr 25, 2001
5,782
0
71
Originally posted by: Elemental007
Yellowfiero -

Wouldn't diodes in the electronic devices act up if you reversed current flow?
In an AC circuit, the polarity is changing 60 times per second (60Hz) Most electronic devices take the 120VAC AC current and transform it to a lower voltage and DC to operate the electronics. You can use a bridge of four diodes and convert sine wave AC to square wave DC.

But that's not the point of this thread. As has been said, a polarized plug is "hot" on the small blade and "neutral" on the big blade and the big blade is at the same electrical potential as the ground. Which is why you don't want to defeat the big and little sides and you should always plug them in the correct "polarity." As yellowfiero stated, it's a safety feature.
 

Eli

Super Moderator | Elite Member
Super Moderator
Oct 9, 1999
50,425
5
81
Our house has strange wiring. I've talked about it before.

We have concrete floors. If I push my feet into the floor, and grasp one lead of an LED, it will light when I touch the other lead to a metal object that is plugged in and grounded through the chassis.

If I hold one lead of my volt meter and touch the other lead to said metal, it reads 8 - 12VAC depending on how hard I press my feet against the floor. :p

I get quite a zap from the dryer if I go to get my clothes without putting my slippers on. :Q
 

compudog

Diamond Member
Apr 25, 2001
5,782
0
71
Eli, it sounds like the main ground from your panel box is getting leakage from one side of the line. It's either in your panel box itself or from some device in your house that has a short, but not enough of one to blow a fuse/breaker. I would do about isolating the problem by disconnecting each circuit at the panel one at a time while checking with the voltage you mentioned with your meter.
 

Analog

Lifer
Jan 7, 2002
12,755
1
0
Originally posted by: compudog
Eli, it sounds like the main ground from your panel box is getting leakage from one side of the line. It's either in your panel box itself or from some device in your house that has a short, but not enough of one to blow a fuse/breaker. I would do about isolating the problem by disconnecting each circuit at the panel one at a time while checking with the voltage you mentioned with your meter.
That would be a good start.
 

Analog

Lifer
Jan 7, 2002
12,755
1
0
Originally posted by: Elemental007
Yellowfiero -

Wouldn't diodes in the electronic devices act up if you reversed current flow?
yes, in a DC circuit (as mentioned above). In this case, with AC, polarization is the only avenue to make sure the right wire gets to the right circuit element.
 

Eli

Super Moderator | Elite Member
Super Moderator
Oct 9, 1999
50,425
5
81
Originally posted by: yellowfiero
Originally posted by: compudog
Eli, it sounds like the main ground from your panel box is getting leakage from one side of the line. It's either in your panel box itself or from some device in your house that has a short, but not enough of one to blow a fuse/breaker. I would do about isolating the problem by disconnecting each circuit at the panel one at a time while checking with the voltage you mentioned with your meter.
That would be a good start.
I'll have to try that sometime. It's never really been a problem it seems. Now that I have carpeting in my room, I don't ever notice it anymore. It used to happen all the time when I would play my guitar. Quite annoying.. lol

Since I get ~10VAC leakage off the 120V lines, it's probably around 20V when I touch the dryer in my bare feet. It actually kinda hurts. You can feel the 60Hz. :D It's like.. BZZZZZZZ.. lol

I used to love to grab my guitar or my computer case and then kiss my girlfriend when she was in bare feet...... LOL.

Then say.. "Was my kiss shocking, sweety? ;)" lol...
 

samitea

Junior Member
Nov 26, 2018
1
0
6
Thanks, I did a quick google search and kept coming up with millions of useless links. I should have clarified - the direction doesn't matter for my application (but I just wanted to hook up the wires with the proper colors in case someone else ever works on it).
It really does matter. Almost never for how a device operates and always for safety. For instance if you confuse hot and neutral when wiring a lamp, then when you screw in a light bulb the metal screw part is hot and can more easily electrocute you.
 

nakedfrog

Lifer
Apr 3, 2001
49,290
2,239
126
WTH, first the other thread about plugging computers into an ungrounded outlet, and now this one gets necro'd by a random?
 

pcgeek11

Lifer
Jun 12, 2005
16,094
1,337
126
i have blown up light bulbs that were in fixtures that had the polarity reversed

i cannot explain why, i am not a physicist
It wasn't due to the polarity as AC doesn't have a set polarity. It is a sine wave. The polarity shifts at 60hz in the USA.

The reason that the wires are polarized is for an example a lamp. You can switch either wire Neutral or the Hot wire and the results will be the same the light will turn off. But if the switch is connected to the White ( neutral ) wire then when the lamp is off the socket will still be hot. The same holds true for any appliance that id opened with the switch off if the neutral is attached to the switch the internals would still be hot with the switch off.
 

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