Receptacles -- # of wires per screw

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Is there a specific electrical code that states how many wires can be attached per screw when wiring outlet receptacles?
I've always heard that when wiring receptacles, one shouldn't attach more than one wire to each screw. I haven't been able to locate anything about that in the 2008 National Electrical Code. It may be in there, but so far, I haven't been able to find it.
Part of why I am curious is that I notice that typical outlet receptacles have two silver screws, two gold screws, and only one green grounding screw. And that started me wondering, "Why aren't there two green grounding screws?" For outlets that are in the middle of a circuit, there are typically six wires in the outlet box -- two black, two white, and two grounds. So, wouldn't it be easier if the receptacle had 6 screws? -- two for the two black wires, two for the two white wires, and two for the two ground wires?
Is it possible that the code allows two ground wires to be connected to the one green grounding screw, but does not permit two wires per screw for the white and black wires?
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On 2/9/2009 9:24 PM RonABC spake thus:

I don't have the answer to this, and would like to know what (if anything) the NEC has to say about it.
My own guideline is only 1 wire per screw, including ground screws: use pigtails when more connections are needed. No need to ask for trouble.
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RonABC wrote:

It may be a function of how the devices are designed rather than an NEC ruling. If they were not designed and/or UL listed to have more than one wire that in itself I believe would fall under some general NEC guidelines. But not sure what section?

I am pretty sure you must wirenut the grounds. Using the special green wirenuts with the feed hole makes this much easier. As far as I know NEC does not require pigtails on the other wires but some local codes do. Pigtails are better as a device failure is not as likely to take out other devices down stream.
I prefer back feed type outlets which will take as many as 4 wires on each side. Not as good as using pigtails but better and much easier than wrapping wires around screws.
On occasion I will feed 2+ outlets or switches with a single wire. I strip enough insulation off to wrap around a screw then continue the same wire to the next outlet/switch.
Kevin
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Hi! What's a "back feed type outlet"?
Thanks!
David
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I have heard of "back stab" outlets. Perhaps that's the term we're seeking?
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A backfeed outlet has holes in the back. You strip the wire and "stab" it into the hole. It has spring clips inside that grab the wire.
They save a lot of time because you just strip and stab. But many professional electricians won't use the backstab method because over time the copper clips heat up and create a bad connection. Once they start heating up the extra resistance creates even more heat until the connection fails. Most common symptom of this is intermittent operation. Backstabbing has a much higher failure rate than wire wrapping around the screws.
It's a shortcut that can bite you down the road.

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RonABC wrote:

the multiple cable outlet should be pigtailed out anyway. Only ONE black, ONE white, and ONE bare go to the outlet. To daisy chain them using the multiple screws is sloppy and asking for trouble.
s
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On 2/9/2009 10:12 PM Steve Barker spake thus:

Are you saying not to use both screws on a duplex outlet? I do this all the time, and it seems perfectly safe and sensible, and saves a lot of unnecessary pigtailing.
Unless I'm misunderstanding what you said.
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09 Feb 2009 22:47:38 -0800, David Nebenzahl wrote:

By placing the receptacle in parallel with the circuit by pigtailing, downstream outlets are unaffected by a problem with the receptacle, or its outright removal.
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Seth Goodman

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On 2/10/2009 5:37 AM Seth Goodman spake thus:

But the only problem with the receptacle that could affect downstream devices would be if the metal "bridge" between the two screws somehow burned or melted out; pretty unlikely to happen. I still think this is a perfectly good wiring method.
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David Nebenzahl wrote:

You're not misunderstanding.
to daisy chain from outlet to outlet using the double set of screws will lead to trouble down the line. Especially if you have to take an outlet out of the mix. Then you'd be killing everyone down stream also.
s
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On 2/10/2009 6:32 PM Steve Barker spake thus:

Why? When you work on the circuit, you need to disconnect the power anyhow. If you need to remove an outlet, you can simply wire-nut the two sets of wires together and everyone's back in business. I don't see the problem.
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RonABC wrote:

If the receptacle had 2 green screws, or if you attached two wires to the one screw, removing the receptacle would leave all the downstream devices ungrounded until you hook it back up. With only one ground wire going to it, the downstream devices are not dependent on it for their grounds.
(I too *love* those heavy-duty back-wired outlets with all the clamp terminals on the back. They work great for tying a bunch of cables together -- but not the ground conductors.)
Bob
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zxcvbob wrote:

Yes. The code requirement is at 250.148-B.
For a multiwire branch circuit (with a common neutral), if you break the neutral while the circuit is hot the downstream voltages can shift. As above, I believe the neutral has to be spliced, with a single wire to a receptacle. (Briefly looking, I couldn't find a code requirement.)
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bud-- wrote:

Thanks. Apparently, that is the exact code requirement that I was wondering about.
Here's what is says (from the 2008 National Electrical Code):
"250.148(B) Grounding Continuity. The arrangement of grounding connections shall be such that the disconnection or the removal of a receptacle, luminaire, or other device fed from the box does not interfere with or interrupt the grounding continuity."
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300.13
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One wire, one screw. Not sure how two wires would even fit under one screw, and if you did it would probably not be very secure. Sometimes you have to use a pigtail. For most electrical outlets there are two screws for the hot, two screws for the neutral, and one for the ground.
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There are a couple of code sections that would apply: Mostly in section 110 that require equipment be installed as per manufacturer instructions, installed in a workmanlike manner, also 110.14A, which mentions that terminals for more than one conductor shall be so identified
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RBM wrote:

Thanks. I think that's it. I don't think I've ever seen anything on any electrical terminals that identify them as being for more than one conductor, but I'll have to try looking at different devices and maybe the manufacturer specifications for each device.
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Personally, I'm curious about receptacles with screw clamps. Each clamp has two holes where wires can be inserted, and there are 4 clamps. That could be one pile of wires. I prefer to use pigtails on any receptacle with more than four wires.

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