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10-30-2009, 02:37 PM   #16
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Quote:
 Originally Posted by Stubbie Habits are hard to break but I'm finding it less taxing of my time to look in on the forum and respond to something that I feel I may add some drawings that will help what all you guys explain so well.
...thanks for the excellent explanation and drawings as usual!

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10-30-2009, 02:50 PM   #17

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Quote:
 Originally Posted by electro " "Unbalanced" isn't the correct term " I thought about that a little while after I posted , That term would refer to the current carrying conductors . Thanks guys . It's good to have conformation that I'm on the right page .
Electro

The grounded leg or neutral is a current carrying conductor just as the hot wires. Either can knock the morning dew off you.....

That term unbalanced is actually how most electrical textbooks explain what we have been discussing here on this thread. Example from a text book....."the shared neutral of a multi-wire branch circuit carries the unbalanced current while the balanced current cancels" another more confusing way to say it carries the difference.....
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10-30-2009, 02:51 PM   #18
yeah, right

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Quote:
 Originally Posted by busman Yeah, but who can find the code violation. It's a killer to find.
One way to look at it being a violation is that the definition of a MWBC is:

Branch Circuit, Multiwire.
A branch circuit that consists
of two or more ungrounded conductors that have a voltage
between them, and a grounded conductor that has equal
voltage between it and each ungrounded conductor of the
circuit and that is connected to the neutral or grounded
conductor of the system.

If there are two ungrounded (hot) conductors on the same leg, there's no "voltage between them". That means its not a MWBC and you can't use any of the MWBC code sections.

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10-30-2009, 02:55 PM   #19
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Quote:
 Originally Posted by Scuba_Dave In addition under NEC 2008 you have to use a 240v breaker for a MWBC, so sharing across the same buss is not possible
That's the easy way to do it. But single pole breakers with a handle tie are also allowed. A MWBC needs a common disconnect, not a common trip. 210.4(B)
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08-11-2010, 04:48 PM   #20
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Quote:
 Originally Posted by zpm That's the easy way to do it. But single pole breakers with a handle tie are also allowed. A MWBC needs a common disconnect, not a common trip. 210.4(B)

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08-11-2010, 05:14 PM   #21
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Quote:
 Originally Posted by jcollins what about 3 ph

It also needs a common disconnect.
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08-11-2010, 05:17 PM   #22
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Quote:
 Originally Posted by jbfan What about it. It also needs a common disconnect.
Which is absolutely HORRIBLE.
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 08-11-2010, 09:52 PM #23 Member   Join Date: Nov 2007 Location: Nashua, NH, USA Posts: 7,971 Rewards Points: 1,548 On a three phase system, one neutral can be shared by three hot wires, one from each of the phases. A MWBC of this kind would have three breakers with handles tied together. With a 3 phase system, and an MWBC, the math for computing the current on the neutral is somewhat more complicated. __________________ The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit. Last edited by AllanJ; 08-11-2010 at 09:59 PM.
08-12-2010, 04:07 AM   #24
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Quote:
 Originally Posted by 220/221 Multi wire branch circuit. If you have 20 amps from one hot and 20 amps from another hot, you have 40 amps on the neutral if they are on the same phase. If they are on different phases, they use the neutral at different intervals.
Actually, they don't use the neutral at all, if they are balanced loads. Draw a circuit diagram with two equal resistors on a split phase (single phase) circuit. You'll see that with phasor voltages, the voltage at the center between the two resistors is zero. Zero volts to ground equals zero current flow in the neutral. You could disconnect the neutral on a balanced circuit with no effect.

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08-12-2010, 05:14 AM   #25
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Quote:
 Originally Posted by busman Actually, they don't use the neutral at all, if they are balanced loads. Draw a circuit diagram with two equal resistors on a split phase (single phase) circuit. You'll see that with phasor voltages, the voltage at the center between the two resistors is zero. Zero volts to ground equals zero current flow in the neutral. You could disconnect the neutral on a balanced circuit with no effect. Mark
No effect other than to destroy most 120 volt equipment on the circuit.

08-12-2010, 06:09 AM   #26
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Quote:
 Originally Posted by brric No effect other than to destroy most 120 volt equipment on the circuit.
Been there, done that.
Disconnected a neutral and watch magic smoke rise from several computors!
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08-12-2010, 06:24 AM   #27
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Quote:
 Originally Posted by brric No effect other than to destroy most 120 volt equipment on the circuit.

Only if the load isn't balanced. If the loads are the same, there will be no damage (in fact no effect at all) from disconnecting a neutral.

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08-12-2010, 07:31 AM   #28
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Quote:
 Originally Posted by brric No effect other than to destroy most 120 volt equipment on the circuit.
Not if it's a balanced load.
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 08-12-2010, 08:12 AM #29 I=E/R     Join Date: May 2010 Location: Minnesota Posts: 2,052 Rewards Points: 1,000 It is surprising how confusing this topic is and it has been discussed several times in various ways. Without the neutral, the MWBC is nothing but a 240V circuit with two loads in series. The center tapped neutral forces this to two "constant voltage" circuits. Proper application of Ohms Law will explain everything.
08-12-2010, 12:33 PM   #30

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Quote:
 It is surprising how confusing this topic is ....
Why do you find it surprising?

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