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Old 04-05-2008, 06:46 PM   #1
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Default MWBC & Receptacle termination questions...

OK, guys, in my studies I've come across a couple things that I don't get. Maybe I'm missing something simple??? Here goes:

1) First off, why is it good practice to land ungrounded and grounded wires diagonally opposed on a receptacle? Here's a picture:



--------------------

2) Second, I've learned that when hooking up a chain of receptacles in a MWBC it is necessary to pigtail all your connections rather than jump off of the receptacle's screw terminals so that if one rec gets disconnected, you don't risk sending 240V down the grounded wire. I don't understand how that's possible. Here's some pictures:



^"normal" (Question 1 wiring method not used due to ease of drawing...)



^disconnected rec

Am I missing something?

Thanx a bunch!

-mac
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Old 04-05-2008, 06:54 PM   #2
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OK, #1 - never heard of that one. IMO, landing diagonally is meaningless, and does nothing.
#2 - You do not have to pigtail all your connections, but you MUST pigtail the neutral. (and EGCs of course) If one comes loose, you will have unbalanced voltage. Depending on loads, it could be 150v and 90v, it could be 200v and 40v.
Remember that the MWBC is 240 v between black and red. The white "divides" the voltage. (Bad terminology, but the best I can think of right now)
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Old 04-05-2008, 07:01 PM   #3
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OK, #1 - never heard of that one. IMO, landing diagonally is meaningless, and does nothing.
#2 - You do not have to pigtail all your connections, but you MUST pigtail the neutral. (and EGCs of course) If one comes loose, you will have unbalanced voltage. Depending on loads, it could be 150v and 90v, it could be 200v and 40v.
Remember that the MWBC is 240 v between black and red. The white "divides" the voltage. (Bad terminology, but the best I can think of right now)
How about "the neutral centers the voltage?" Or the neutral provides the L1 and L2 with a reference to 120 volts, since between L1 & L2 there are 240 volts.


Where the neutral is shown disconnected, nothing would happen.
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Old 04-05-2008, 07:01 PM   #4
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Quote:
Originally Posted by JohnJ0906 View Post
OK, #1 - never heard of that one. IMO, landing diagonally is meaningless, and does nothing.
#2 - You do not have to pigtail all your connections, but you MUST pigtail the neutral. (and EGCs of course) If one comes loose, you will have unbalanced voltage. Depending on loads, it could be 150v and 90v, it could be 200v and 40v.
Remember that the MWBC is 240 v between black and red. The white "divides" the voltage. (Bad terminology, but the best I can think of right now)

I agree with #1, they are on a common plate there is no difference. Terminology aside the point is not losing a neutral because of one bad device.
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Old 04-05-2008, 07:14 PM   #5
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Agree with John on both points.
The first three receptacles in that middle image MUST have the neutrals spliced.

300.13 Mechanical and Electrical Continuity Conductors
(B) Device Removal
In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.
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Old 04-05-2008, 07:24 PM   #6
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Here's why:

http://www.code-elec.com/userimages/Lost Neutral.ppt
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Old 04-05-2008, 07:46 PM   #7
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Agree with #1. I've never heard of that either.

Nice PowerPoint 480.
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Old 04-05-2008, 10:15 PM   #8
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OK, here's a quote:

...When wiring two-wire circuits, it is the usual practice to end two different grounded wires at these two (receptacle screw) terminals, as is done especially when wiring with cable. But in wiring three-wire (MWBC) circuits, this must not be done because removing a receptacle, such as for replacement, would then result in a break in the neutral wire during the time there is no connection to the receptacle. This would in turn place all the receptacles connected to one leg (beyond the receptacle temporarily removed) in series with those connected to the other leg, all at 240 volts.

So, I imagine that in my 3rd picture, for the ^above situation to be true, the electrician would have connected the two neutrals together - which I left open in my diagram. Correct?

---

The book suggests pigtailing all chained wire connections. In y'alls own experience, is this good advice / practice?

---

As far as diagonally opposing hots and groundeds, just about every diagram in every text book I've read / looked at shows it done that way. At work, this is the method we are instructed to use. I asked my boss about it a while back but have forgotten the rationale - I'll ask him again.

--

Very nice PPT. My books use just about the same diagram, but they're not so detailed with the math and all...

--thanx guys
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Old 04-05-2008, 10:32 PM   #9
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I always pigtail, even on 2 wire circuits. Makes it much easier to push the device back into the box, especially with solid wire.

Rob
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Old 04-05-2008, 10:46 PM   #10
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#1 is a personal preference of someone that taught you, most likely. Many electricians, inspectors and engineers have specific ideas on what they like or want and instill this in to the work ethic of others.

Depending where the neutral/grounded conductor is open you can have anywhere between 120 VAC to 240 VAC on the appliances depending on the resistance of the other equipment on the two circuits involved. What you end up with is a the two 120 VAC circuits in series.

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Old 04-06-2008, 07:45 AM   #11
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Quote:
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Nice Powerpoint 480
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Old 04-09-2008, 06:52 PM   #12
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OK, I asked the boss why he wants hot / grounded wires terminated in diagonally opposite screws on rec's and he said that the little break-away tab is a fusible link and it acts as a secondary method of protection i.e. if the rec is overloaded but the breaker / fuse does not trip, the link will burn out (thus protecting your circuit / appliance) if wired in the criss-cross method but not if wired straight through.

Comments?

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Old 04-09-2008, 06:56 PM   #13
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Quote:
Originally Posted by Mackie View Post
OK, I asked the boss why he wants hot / grounded wires terminated in diagonally opposite screws on rec's and he said that the little break-away tab is a fusible link and it acts as a secondary method of protection i.e. if the rec is overloaded but the breaker / fuse does not trip, the link will burn out (thus protecting your circuit / appliance) if wired in the criss-cross method but not if wired straight through.

Comments?

Comments?????
YOUR BOSS IS OFF HIS NUT!!! And you can tell him I said that!

A fusible link!
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Old 04-09-2008, 07:23 PM   #14
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Quote:
Originally Posted by Speedy Petey View Post
Comments?????
YOUR BOSS IS OFF HIS NUT!!! And you can tell him I said that!

A fusible link!
I'll forward the post...
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Old 04-09-2008, 07:44 PM   #15
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I wouldn't trust that tab to be a fuse whatsoever. I do however do the cross method myself because it makes it easier to push the device into the box IMO
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Old 04-10-2008, 05:48 PM   #16
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Well, this gives me something to research...

I see it all the time in books and illustrations and stuff but they never say why to use that method. There has to be some good reason behind it because it is kinda inconvenient and makes things a bit more confusing...



*he-he...* ^upside down

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Old 04-10-2008, 06:51 PM   #17
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Why to "tie-in" is a combination of code and practicality.*

The grounding because it's bad practice to 'loop' one around the ground screw; anyone who has done service has cussed the mecahinc that produced that 2" piece of wire they had to deal with.
(Always pigtail off the ground screw)

The grounded (now) because it is code and (past) because a failure won't drop out the rest of the circuit.

The ungrounded because a failure won't drop out the rest of the circuit and (now) since you have to do the other two you may as well do the third as well.

Also, if you are ulling stranded and have cheap grade devices it is real convienent to splice in solid at the devices.


* My opinion; you do as you like.
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Old 04-10-2008, 11:41 PM   #18
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I have never cared if the neutral and "hot" are diagonally connected. I've never had an inspector appear to care, one way or another.

I have "looped" the EGC around the first receptacle screw, then the other in double duplex setups. I think the point is to guard against open EGCs -- anywhere.

In electricity, one cannot "send" voltage. The voltage at any point in a circuit is relative to some other point in that circuit. I can see no way that an open neutral can result in the voltage on one side of the receptacle going to 240 since that voltage exists only between the "phase" or ungrounded conductors.

Trying to visualize if a "hot" leg could revert to the full secondary voltage if the neutral was lost. I suppose that it could, but since it would be just one side of the circuit, I don't see how it would matter to the connected equipment. Of course, been a very long time since I had all that AC theory!
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