Discussion On MWBC - Electrical - Page 3 - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum Discussion on MWBC

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03-31-2010, 09:15 PM   #31
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Quote:
 Originally Posted by jlmran If no current flows back through neutral, then why would losing the neutral create a problem?
So long as the loads (resistance wise) on all legs of the MWBC were the same and stayed the same, losing the neutral is not a problem. Except that in real life situations the loads don't stay the same.

With different loads on each side of the (120/240 volt) MWBC, current must flow through the neutral to have each side's loads receive 120 volts.

All of the loads on the L1 side of the MWBC present a net resistance of so many ohms. All of the loads on the L2 side present a resistance of so many ohms. With the neutral connection broken, the higher the resistance of one side relative to the other, the larger the share of the 240 volts that side gets. (An example was described earlier in this thread).
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Last edited by AllanJ; 03-31-2010 at 09:35 PM.

03-31-2010, 09:17 PM   #32
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Quote:
 Originally Posted by jlmran If no current flows back through neutral, then why would losing the neutral create a problem? No one ever do this? There have been a substantial amount of posts on this website about MWBC. Someone is doing it.
people don't do it for the simple reason that brric stated above....lose the neutral and you've got 240V across your load.
The more i think about it the more i understand why more places are specing us to have sperate neutrals even with a three phase system.
the reason you have no current in the neutral is that you have 120V 180 degrees out of whack with each other on the lines. So when you have 4 amps one light in essence you have -4 amps on the other....electricity travel in a sine wave going from positive to negative 60 times a second in North America. so the 4 and -4 get adding together because the current in a circuit going out equals what's comming back and gives you zero. Disconnect that neutral and you have 120V 180 degrees out of phase with each other and that gives you 120+ and 120-.....difference of 240V.

04-01-2010, 08:49 AM   #33
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Quote:
 Originally Posted by andrew79 people don't do it for the simple reason that brric stated above....lose the neutral and you've got 240V across your load. The more i think about it the more i understand why more places are specing us to have sperate neutrals even with a three phase system. the reason you have no current in the neutral is that you have 120V 180 degrees out of whack with each other on the lines. So when you have 4 amps one light in essence you have -4 amps on the other....electricity travel in a sine wave going from positive to negative 60 times a second in North America. so the 4 and -4 get adding together because the current in a circuit going out equals what's comming back and gives you zero. Disconnect that neutral and you have 120V 180 degrees out of phase with each other and that gives you 120+ and 120-.....difference of 240V.
1. With or without the neutral, you have 240v across the loads. The difference in having the neutral is that you insure that you have only 120v across each load when the loads are unbalanced.

2. Three phase systems are a completely different animal that what most people have comming into their home. When you have three phase power, you can start talking about having your loads in either Delta or Wye configurations and the presence or absence of a neutral is different than the discussion of the neutral for the typical home.

In the typical home, you only have ONE phase power. The power company always generates three phase power, and that's why you always see three large conductors on high power lines and many other power lines. But once you get to the individual home, each house is served by a transformer that steps down the voltage to 240v (with the center tap being the neutral). That transformer (typically that green box in your or your neigbor's yard, or the round cyclindar on the power pole) is fed by only one of the three phases, with each transformer in the neiborhood randomly getting one of the three phases.

04-01-2010, 09:52 AM   #34
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Quote:
 Originally Posted by HooKooDooKu 1. With or without the neutral, you have 240v across the loads. The difference in having the neutral is that you insure that you have only 120v across each load when the loads are unbalanced. 2. Three phase systems are a completely different animal that what most people have comming into their home. When you have three phase power, you can start talking about having your loads in either Delta or Wye configurations and the presence or absence of a neutral is different than the discussion of the neutral for the typical home. In the typical home, you only have ONE phase power. The power company always generates three phase power, and that's why you always see three large conductors on high power lines and many other power lines. But once you get to the individual home, each house is served by a transformer that steps down the voltage to 240v (with the center tap being the neutral). That transformer (typically that green box in your or your neigbor's yard, or the round cyclindar on the power pole) is fed by only one of the three phases, with each transformer in the neiborhood randomly getting one of the three phases.
What?? Get your facts straight...the green box is in my yard.

Thank you for the lesson on power distribution in the U.S. I'm in Oklahoma, and MANY rural parts of Oklahoma are still serviced by single phase distribution lines.

04-01-2010, 10:47 AM   #35
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Quote:
 Originally Posted by jlmran MANY rural parts of Oklahoma are still serviced by single phase distribution lines.
Now we're getting into exact details. Obviously the POCO isn't going to feed all three phases to an area (be it a neiborhood, a rural area, etc) that has the capacity to be serviced by a single phase. No point in running three wires to an area when only one will do.

But the main point was that while the POCO generates 3 phase power, the typical home is being serviced by only one of those 3 phases.

But some people (me included years ago) have confussed 3 phase power and the L1/L2 that we have coming into our home. In the case of 3 phase power, each leg is 120 degrees out of phase with the others. But because L1/L2 coming into your house is single phase power, and most circuits are 120v (L1 to neutral or L2 to neutral) L1 and L2 are out of phase by 180 degrees. (phase, phase, phase... phase gets used in lots of ways and it can be easy to get confused).

 04-01-2010, 12:21 PM #36 Master Electrician   Join Date: Mar 2010 Location: Toronto Ontario Posts: 1,165 Rewards Points: 500 i work on 3-phase everyday and it's good to clear that up as alot of people won't know that "out of phase" and different "phase" are two totally different things. i was assuming that the loads were equal in wich case the neutral will be at 0. but your loads on three phase are never wired up in delta or wye connections....we only use that for transformer connections to step voltages up and down and maybe on the occasional motor. I'm sure in some instances there is alot of use to them but in most factories and office towers they're wired almost the same as house from the panel out. There's just more stuff . If you check the diagram that we're discussing you'll see that there is in fact a second "virtual phase" created by the way its wired. In this sense it will behave exactly the same as a three phase circuit would if you lose the neutral. Loose the neutral one the diagram and you get 240V running in the circuit. Lose the neutral on a 120v three phase system and you'd have 240V between any two legs of the system. Last edited by andrew79; 04-01-2010 at 12:29 PM.
04-01-2010, 02:35 PM   #37
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Quote:
 Originally Posted by andrew79 Lose the neutral on a 120v three phase system and you'd have 240V between any two legs of the system.
Wouldn't you have 208 volts?

Last edited by jlmran; 04-01-2010 at 03:23 PM.

 04-01-2010, 02:43 PM #38 Master Electrician   Join Date: Mar 2010 Location: Toronto Ontario Posts: 1,165 Rewards Points: 500 yup...sorry i've just always refered to it as 240V but your absolutely right.
 04-01-2010, 03:15 PM #39 Master Electrician   Join Date: Mar 2010 Location: Indiana Posts: 4,413 Rewards Points: 5,082 Only if it's a 120/208Y system.
04-01-2010, 03:23 PM   #40
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Quote:
 Originally Posted by brric Only if it's a 120/208Y system.
Doesn't the mere presence of a neutral dictate a wye system?

 04-01-2010, 03:24 PM #41 Master Electrician   Join Date: Mar 2010 Location: Indiana Posts: 4,413 Rewards Points: 5,082 What about a 277/480 Y?
04-01-2010, 03:43 PM   #42
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Quote:
 Originally Posted by brric What about a 277/480 Y?
I was referring to the question quoted in post #37. He identified the loss of a neutral in a 120v three phase system?

04-01-2010, 08:45 PM   #43
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Quote:
 Originally Posted by andrew79 take your total load which is 140W and divide your votage into it(240V). This will give you the current running through the whole system being as it's series circuit. Now take the wattage of each light individually and divide it by the current to get the voltage dropped across each light. The larger load takes up more of the voltage.

While a 100 watt bulb will take most of the voltage dropped, its operating voltage will be less than its rating, so it will light with a lower brightness.

Quote:
 Originally Posted by 3 legged dog the 40 watt bulb survives the experiment.
Apparently you have never conducted this actual experiment.

I just wired 2 light bulbs in series on a 120 volt circuit. My Fluke measured the source at 125 Volts for this experiment. I measured .3 Amps with them so connected. That's 37.5 watts total load.

With both bulbs connected, the 100 watt bulb had 23 volts across it, and it barely glowed. The 40 watt bulb had 102 volts across it, and burned almost normal level.

Now, double the voltage, and what do you think will happen here?

The 40 watt bulb will get much brighter, with over 200 Volts on its filament, and burn out in short order. The 100 watt bulb will glow a bit more, with 46 volts or so until the 40 watt bulb blows, at which time the 100 watt bulb will go out.

Once the power is restored with the neutral properly connected, the 100 watt bulb will once again light normally, and the 40 watt bulb will need to be replaced.

The same thing happens in instances where the neutral conductor is cut from its source, and the voltages "float" -- the leg of the service that has the most load -- will operate at lower voltage, while the leg with the least load will see its voltage increase.

Take a scenario where one leg has a large load on it, such as a refrigerator compressor motor, and the other leg as a rather small load imposed on it, such as an electronic clock display on an oven. The electronic guts will fry with the 200+ voltage on it, while the fridge will barely notice anything. Once power is restored properly, ½ the things in the house will still operate, while the other ½ will have extensive damages.

Things like computers and many newer model TV sets if they have the newer universal power supplies in them, which operate normally with any input voltage between 105 and 240 will not have any problems after such an event.

Just to confirm my assumption here, I will do the same experiment tomorrow, with an actual 120/240 volt source. I'll have both bulbs connected normally, and then cut the neutral wire and report the results here. If I can measure the operating voltage on both bulbs before one of them blows, I'll include that information as well.
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Last edited by kbsparky; 04-01-2010 at 09:24 PM. Reason: Revised explanation

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04-01-2010, 09:31 PM   #44
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Quote:
 Originally Posted by jlmran Doesn't the mere presence of a neutral dictate a wye system?
No. You can have a delta-connected system, with both single phase 120/240 volts, and 3 phase 240 Volts available on the same service. This type of service has what is commonly called a hi-leg, with about 208 Volts to ground on it.
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04-01-2010, 09:35 PM   #45
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Quote:
 Originally Posted by kbsparky Your logic is flawed. While a 100 watt bulb will take most of the voltage dropped, its operating voltage will be less than its rating, so it will light with a lower brightness. Apparently you have never conducted this actual experiment. I just wired 2 light bulbs in series on a 120 volt circuit. My Fluke measured the source at 125 Volts for this experiment. I measured .3 Amps with them so connected. That's 37.5 watts total load. With both bulbs connected, the 100 watt bulb had 23 volts across it, and it barely glowed. The 40 watt bulb had 102 volts across it, and burned almost normal level. Now, double the voltage, and what do you think will happen here? The 40 watt bulb will get much brighter, with over 200 Volts on its filament, and burn out in short order. The 100 watt bulb will glow a bit more, with 46 volts or so until the 40 watt bulb blows, at which time the 100 watt bulb will go out. Once the power is restored with the neutral properly connected, the 100 watt bulb will once again light normally, and the 40 watt bulb will need to be replaced. The same thing happens in instances where the neutral conductor is cut from its source, and the voltages "float" -- the leg of the service that has the most load -- will operate at lower voltage, while the leg with the least load will see its voltage increase. Take a scenario where one leg has a large load on it, such as a refrigerator compressor motor, and the other leg as a rather small load imposed on it, such as an electronic clock display on an oven. The electronic guts will fry with the 200+ voltage on it, while the fridge will barely notice anything. Once power is restored properly, ½ the things in the house will still operate, while the other ½ will have extensive damages. Things like computers and many newer model TV sets if they have the newer universal power supplies in them, which operate normally with any input voltage between 105 and 240 will not have any problems after such an event. Just to confirm my assumption here, I will do the same experiment tomorrow, with an actual 120/240 volt source. I'll have both bulbs connected normally, and then cut the neutral wire and report the results here. If I can measure the operating voltage on both bulbs before one of them blows, I'll include that information as well.
Wow...I simply asked for a discussion. You'll have a full research paper ready for peer review and publication once you're finished. It is kind of neat that this question has generated such a response in finding the answer. I had similar plans to perform the experiment you mention, but I'll wait to hear your results. Thanks.

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