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03-08-2010, 11:21 AM   #16
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Quote:
 Originally Posted by secutanudu Why would you wire a receptacle from two different circuits? If you plan to hook up two high-energy devices to the same duplex receptacle? I am still having trouble undetstanding how 28v could backfeed through the live breaker...
Suppose you have the three wires. Black goes from breaker to load A, red goes from breaker to load B. White goes to both leads. Now use a heater for each load. Turn off load A leaving load B on. Current goes from the breaker to load B, through the load and back to the panel through the shared white conductor. There is current on the white conductor, many times there is enough current to knock you on your butt.

03-08-2010, 11:28 AM   #17
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Quote:
 Originally Posted by secutanudu Why would you wire a receptacle from two different circuits? If you plan to hook up two high-energy devices to the same duplex receptacle? I am still having trouble undetstanding how 28v could backfeed through the live breaker...

Suppose you have a multiwire circuit. Black wire is connected to load A from a breaker. Load B is connected to load B from a breaker. White wire is connected to both loads. Put a load on A and put a load on B. Turn off breaker A. Load B remains on. Current travels from breaker B, through the red wire, through the load and back to the panel through the white wire. The current remaining on the white can be dangerous depending upon the type and size of the load.

 03-08-2010, 12:00 PM #18 Just call me Andrew   Join Date: Mar 2009 Location: Albany, NY Posts: 2,271 Rewards Points: 1,032 The weird part is, all loads were disconnected at the time, but one breaker was on. When both breakers were off, the 28v disappeared. Also, I thought the neutral was normally safe to touch as long as it is not disconnected further down the chain. I am not advocating haphazardly touching white wires, just trying to understand what's going on. __________________ Andrew Last edited by secutanudu; 03-08-2010 at 12:03 PM.

 03-08-2010, 01:20 PM #19 Master Electrician   Join Date: Mar 2010 Location: Indiana Posts: 4,413 Rewards Points: 5,082 Open up a neutral on an energized multiwire circuit you could get zapped by 240 volts as well as destroying equipment on those circuits.
 03-08-2010, 01:33 PM #20 Member   Join Date: Nov 2007 Location: Nashua, NH, USA Posts: 8,597 Rewards Points: 2,800 Assuming the multiwire branch circuit is properly connected (red and black to opposite sides of the 120/240 volt service) the white wire will not be overloaded without overloading either the red or black and tripping its breaker. Maximum current returns on the white wire when the black is carrying maximum current and the red has no connected loads turned on (or vice versa). __________________ Stick to your lawn watering schedule until it really starts to pour. After the storm you have only the same number of rest days you always had and then you need to start watering again.
 03-08-2010, 01:48 PM #21 Master Electrician   Join Date: Mar 2010 Location: Indiana Posts: 4,413 Rewards Points: 5,082 The currents on the neutral are cancelled by each other on a multiwire circuit. The reason you can share a neutral this way is that if 20 amps flow on load A and 20 amps flow on load B there will be 0 current flow on the neutral white wire. This is also why the circuits must be on different phases other wise the neutral currents will become additive and could overload the neutral. Last edited by brric; 03-08-2010 at 02:13 PM. Reason: additional
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03-08-2010, 10:11 PM   #22
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Quote:
 Originally Posted by brric The currents on the neutral are cancelled by each other on a multiwire circuit. The reason you can share a neutral this way is that if 20 amps flow on load A and 20 amps flow on load B there will be 0 current flow on the neutral white wire. This is also why the circuits must be on different phases other wise the neutral currents will become additive and could overload the neutral.
Could you explain this in a little more detail?

I'm having a hard time understanding why having a MWBC on opposite phases cancel each other's current on the neutral wire.

Thanks, this is all great info, and is great to understand as MWBC can be very dangerous to someone who is not aware of how they operate.

 03-08-2010, 10:20 PM #23 Member   Join Date: Jan 2009 Location: South of Boston, MA Posts: 17,248 Rewards Points: 2,000 Think of it as 2 different waves "0" is the middle - the neutral It carries the difference between the 2 hots Most households are only 1 phase power But most people refer to each hot bus as a different phase Its actually the same power, the transformer merely flips it 180 degrees apart - best way I can explain 2 opposite waves cancel each other 2 of the same waves make a wave 2x as big...2x what the neutral can carry Thus the danger if they are not on a different phase In addition if a split receptacle you actually have possibilty of 240v power if you hit both hots
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03-08-2010, 10:44 PM   #24
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Quote:
 Originally Posted by Scuba_Dave Think of it as 2 different waves "0" is the middle - the neutral It carries the difference between the 2 hots Most households are only 1 phase power But most people refer to each hot bus as a different phase Its actually the same power, the transformer merely flips it 180 degrees apart - best way I can explain 2 opposite waves cancel each other 2 of the same waves make a wave 2x as big...2x what the neutral can carry Thus the danger if they are not on a different phase In addition if a split receptacle you actually have possibilty of 240v power if you hit both hots

Makes perfect sense Dave, thank you! Good description and now I remember from my reading on electrical about 180* out of phase in regards to typical residential services.

03-08-2010, 10:56 PM   #25
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Quote:
 Originally Posted by brric The currents on the neutral are cancelled by each other on a multiwire circuit. The reason you can share a neutral this way is that if 20 amps flow on load A and 20 amps flow on load B there will be 0 current flow on the neutral white wire. This is also why the circuits must be on different phases other wise the neutral currents will become additive and could overload the neutral.
I thought that if you have 20 amp on A and 20 amp on B then the neutral will carry one phase at a time, so 20 amp on the neutral.

03-09-2010, 06:18 AM   #26
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Quote:
 Originally Posted by 3 legged dog I thought that if you have 20 amp on A and 20 amp on B then the neutral will carry one phase at a time, so 20 amp on the neutral.

That is incorrect. Several of us have explained it. What more can we do?

03-09-2010, 07:19 AM   #27
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Quote:
 Originally Posted by 3 legged dog I thought that if you have 20 amp on A and 20 amp on B then the neutral will carry one phase at a time, so 20 amp on the neutral.
Picture an evenly matched tug of war
The neutral is in the middle
Both pulling the same power (20a)

If one side pulls more then the other then the neutral carries a little power

Power on opposite hot feeds is 180 out of phase with each other

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03-10-2010, 02:49 PM   #28
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Quote:
 Originally Posted by Scuba_Dave Picture an evenly matched tug of war The neutral is in the middle Both pulling the same power (20a) If one side pulls more then the other then the neutral carries a little power Power on opposite hot feeds is 180 out of phase with each other
Ah yes I see.
I was having a hard time seeing this due to the way it puts 2 loads in series on the 240 volts.
And if you step back and look, your load center is one large multi branch ckt.
Thanks S_D

03-10-2010, 02:54 PM   #29
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Quote:
 Originally Posted by 3 legged dog Ah yes I see. I was having a hard time seeing this due to the way it puts 2 loads in series on the 240 volts. And if you step back and look, your load center is one large multi branch ckt. Thanks S_D
Yes, you are correct.

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03-12-2010, 05:57 AM   #30
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I was reading the instructions for the SquareD handle ties I plan to install today...this line has me confused:

Quote:
 "NOTE: Handle tie does not provide common trip protection."
Isn't that why I'm doing it? So they trip together? Or am I misunderstanding what it's saying?

Here's the installation manual: http://static.schneider-electric.us/...840-172-01.pdf
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