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I have a lamp that has a damaged plug. I have a replacement plug, but I'm not sure which wire on the cord should be connected to hot plug and which wire to the neutral plug. I know that the plug had a polarity (it had one large blade and one small blade), but I don't know which wire was the "hot" wire. The cord doesn't have a ground wire.

Is there a way to know which wire should be connected to "hot" or neutral?

This is in the United States.

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enter image description here

The shell of the bulb socket (i.e. the part with the threads on it) should be neutral. There are many ways it's possible for you to be on contact with the shell:

  • while handling a fixture, i.e. to keep it from falling or to steady it
  • while screwing in a bulb
  • using cheap LED bulbs with metallic heat sinks (many of them either bond the heat-sink to the screw-base shell, or the insulation is so thin it could easily breach there.)

All of this is for nought, if your lamp does not have a polarized plug: note the extra-wide blade on one side in this photo. This keys it, to make it impossible to insert wrong in a NEMA 5-15, 5-20 or 1-15 socket.

enter image description here

If it doesn't, you should get a polarized plug or cord with plug. On a cord with a pre-molded plug, there will usually be ribbing or marking, and that should be on the neutral side, which is the extra-wide blade.

The ribbing is meaningless on a user-attached plug, or non-polarized plug.

To be sure, either

  • measure with an ohmmeter (zero ohms from shell to the wide blade on the lamp's plug, infinity to tip in all switch positions)
  • measure with a voltmeter, with lamp plugged in, from the shell to holes on a nearby receptacle - near 0V to ground (that could also mean a bad connection) and 120V away from hot (sure, but more dangerous to measure).
  • dismantle the lamp and visually follow the wire, noting the rib/marking on the wire. Check the plug to make sure the rib/mark does indeed go to the wide blade.

Lamp cords with pre-molded plugs can be readily had from a proper electrical supply, McMaster-Carr, or by hacking an extension cord of appropriate gauge (typically 18 AWG unless it's one heck of a lamp). Use cords that are white, translucent yellow, brown or black - avoid obvious "I hacked an extension cord" colors like dark green.

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In the united States most lamps are wired with STP cord. STP cord is polarity marked.

enter image description here

In this photo you can see the "rib" or raised stripe. The other conductor of the cord will be smooth. The rib is neutral and is associated to the wide blade of the plug.

Now that you know the secret walk around your house and look at lampcords, they all follow that convention.

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    Never trust the ribbing thing! Always test with a multimeter. – Carl Witthoft Dec 9 '16 at 14:18
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    What's the down vote for? I challenge any of you to show me a molded on plug where the rib is not the wide blade or neutral? – Tyson Dec 9 '16 at 15:24
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    I didn't down-vote, but I've seen enough code violations to agree with @CarlWitthoft here. Use a multimeter or 2-pole voltage seeker. – Mast Dec 9 '16 at 17:47
  • I didn't downvote either, and I do endorse the method of physically looking at how the cable is connected in the lamp and the plug. Nothing coming out of China would surprise me, but every plug I've seen is sided, i.e. the two wires meet the plug flat and go to the obvious side. In fact, looking at the lamp At My Desk, I see the plug is non-polarized (!) but one side of the cord has ribs. Oy vay, I guess I better get a cord and walk my talk! – Harper Dec 9 '16 at 18:52
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You all forgot to mention which of the two screws on the socket is the Hot Connection. It is the Copper Screw, not the Steel Screw. The Copper screw connects to the center copper tab inside the socket that looks like a "tongue". The Steel Screw connects to the outer threaded part of the socket and it needs to be Neutral. If you try to change a "burned out" light bulb without turning off the socket and its Hot, you could inadvertently touch the bulb's metal threads and get shocked or worse.

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    Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Sep 21 at 20:12

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