MWBC= more heat or Less heat?

[hardworkingstiff]

I have to disagree, with equal loading on both sides of the neutral it does in fact 'bypass' the neutral. The neutral could be removed with no change at all in the circuit operation.

I tend to agree with Dennis. If the neutral is removed, the circuit will work, but any fluctuations on one of the loads will affect the voltage on the other load where if the neutral is connected, the loads will maintain the voltage independent (except for voltage drop) of the conditions of the other load.

With a neutral you have 2 independent loads, without the neutral have a series circuit.

Nothing ever remains equal. Technically you are correct, but practically, it is a problem.

 

[hardworkingstiff]

So explain to me how the circuit would work with a MWBC feeding 2 100 watt bulbs with no neutral connected. What am I missing. No neutral makes it 220 volts and the neutral has to be connected to the socket to work. :-?

Now I am confused.

Assuming each lamp has the EXACT same resistance, the 240-volts will drop 120-volts across each lamp because its a series circuit.

 

[iwire]

Nothing ever remains equal. Technically you are correct, but practically, it is a problem.

I agree, there is a real world out there but Denis did not make that distinction. It is important to know that the circuit will function without the neutral. The neutral is in fact 'bypassed' during balanced operation.

I have worked with a fair amount of factory made equipment that would run two 120 volt lamps in series supplied from 208, even though each lamp will have some differences it worked just fine. :smile:

 

[Dennis Alwon]

I agree, there is a real world out there but Denis did not make that distinction. It is important to know that the circuit will function without the neutral. The neutral is in fact 'bypassed' during balanced operation.

I have worked with a fair amount of factory made equipment that would run two 120 volt lamps in series supplied from 208, even though each lamp will have some differences it worked just fine. :smile:

Sorry but I didn't realize we were talking about a series connection here. I know that 2- 100 watt bulbs in series will burn across a 240v line. I use to do that as a kid with my dad all the time.

 

[hardworkingstiff]

I have worked with a fair amount of factory made equipment that would run two 120 volt lamps in series supplied from 208, even though each lamp will have some differences it worked just fine. :smile:

I assume they were incadescent lamps. 104-volts (+/-) to each lamp?

It really get's interesting if you were to hook a neutral to make it a MWBC instead of a series circuit, then you would get 120-volts instead of 104-volts. That starts the brain thinking. :grin:

 

[infinity]

I assume they were incadescent lamps. 104-volts (+/-) to each lamp?

It really get's interesting if you were to hook a neutral to make it a MWBC instead of a series circuit, then you would get 120-volts instead of 104-volts. That starts the brain thinking. :grin:

You're correct and as with any example it's the supply system is that's important. For a single phase system 120/240 volt system, with the neutral disconnected, the VD across each lamp would be 120 volts, for a 208/120 WYE system, neutral disconnected, it would be 104 volts across each lamp due to the phase angle.

 

[iwire]

I assume they were incandescent lamps. 104-volts (+/-) to each lamp?

Yes, about 5,550 of them. :grin:

This is exactly like one of the machines.

It really get's interesting if you were to hook a neutral to make it a MWBC instead of a series circuit, then you would get 120-volts instead of 104-volts. That starts the brain thinking. :grin:

My brain was already having a hard time. :grin:

The lights flashed an alternating pattern so the circuit was wired

A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B

In normal parallel that would be easy to work with but now add the series circuits with German color codes and it was not that simple to work with.

 

[iwire]

Sorry but I didn't realize we were talking about a series connection here. I know that 2- 100 watt bulbs in series will burn across a 240v line. I use to do that as a kid with my dad all the time.

Not trying to bust chops here Dennis.

A balanced MWBC is pretty much a series circuit.

 

[Dennis Alwon]

Not trying to bust chops here Dennis.

Well, I did think you were being picky but I now understand what you were trying to say from your statement below. :wink:

A balanced MWBC is pretty much a series circuit.

 

[ActionDave]

A balanced MWBC is pretty much a series circuit.

This makes sense. Now can anybody explain the neutral-180 degree voodoo Dennis mentioned.

 

[hardworkingstiff]

This makes sense. Now can anybody explain the neutral-180 degree voodoo Dennis mentioned.

For single-phase supply (120/240).

The transformer steps the voltage down to 240-volts with the neutral being connected 1/2-way between the 2 points of the 240-volt side of the transformer.

Let's say you stop time when the A leg is negative and the B leg is positive. Let's also say you can stand in the mid-point (the neutral connection) and look at the A leg connection point. The electrons are coming towards you since A leg is negative. Let's say you turned around and looked at B leg, you would have to turn 180 degrees to see B leg. The electrons would be flowing away from you toward B leg since it was positive. Since you turn 180 degrees, they (A&B legs) are 180 degrees out of phase in respect to the neutral point.

This is probably not scientific, but I think it works.

 

[PetrosA]

So explain to me how the circuit would work with a MWBC feeding 2 100 watt bulbs with no neutral connected. What am I missing. No neutral makes it 220 volts and the neutral has to be connected to the socket to work. :-?

Now I am confused.

With a single phase service and 2 pole breaker, take two pigtails, wire nut the whites together, hook the blacks up to the breaker, put two 100W bulbs in, and turn the breaker on. The bulbs will light up correctly, because by tying the whites together with equal loads you're "recreating" the neutral point that exists between the two legs at the transformer. What's more, if you took a tester lead and nutted it to the two whites and tested to the neutral bar you should get a zero volts reading. It's the equal load on the two legs that recreates the neutral point - change the loads and the neutral point will wander away from where it should be. Connecting the whites to the neutral bar stabilizes the voltage to 120V when you have varying loads.

Note: this won't work with non-linear loads like fluorescent ballasts which is why neutrals can burn out when shared with that kind of load if not upsized.

Hehe, after reading the rest of the post I see my input was no needed I still think it's interesting to understand the neutral point at the center of a series circuit though...

 

[ActionDave]

For single-phase supply (120/240).

The transformer steps the voltage down to 240-volts with the neutral being connected 1/2-way between the 2 points of the 240-volt side of the transformer.

Let's say you stop time when the A leg is negative and the B leg is positive. Let's also say you can stand in the mid-point (the neutral connection) and look at the A leg connection point. The electrons are coming towards you since A leg is negative. Let's say you turned around and looked at B leg, you would have to turn 180 degrees to see B leg. The electrons would be flowing away from you toward B leg since it was positive. Since you turn 180 degrees, they (A&B legs) are 180 degrees out of phase in respect to the neutral point.

I see what you mean, this is like Larry's battries with the plate in the middle. But I'm picturing A as 120V max above the neutral and B as 120V max below the neutral. Is this correct?

 

[hardworkingstiff]

I see what you mean, this is like Larry's battries with the plate in the middle. But I'm picturing A as 120V max above the neutral and B as 120V max below the neutral. Is this correct?

At the correct point in time it is. It will be at this point about 60 times per second (at 60HZ).

Edit: (Actually, the max would be closer to 170-volts, but we are dealing with the DC equivalent of the sine wave so we call it 120-volts {I believe it's RMS=Root Mean Square}

 

[480sparky]

.... But I'm picturing A as 120V max above the neutral and B as 120V max below the neutral. Is this correct?

I don't think so. With or without the neutral, the current waveform would be the same on both legs.

 

[hardworkingstiff]

I don't think so. With or without the neutral, the current waveform would be the same on both legs.

I thought he was talking about voltage not current.

 

[480sparky]

I thought he was talking about voltage not current.

I would think they would be the same, as they are part of the same circuit. I don't see how you could have +120v on one part of a circuit, and -120v on another.

 

[ActionDave]

I was talking about voltage, and it seems like I got it. Moving on to current-

I don't think so. With or without the neutral, the current waveform would be the same on both legs.

Huh? In a simple circut doesn't current move with voltage?

 

[480sparky]

.........Huh? In a simple circut doesn't current move with voltage?

Well, it should, but not always. There are instances of voltage leading current, and current leading voltage.

But back to the subject....... if leg A and leg B of a single phase circuit are on a MWBC, they are part of the same circuit. I don't see how you can have 120volts above zero on one part, and 120volts below zero on another.

 

[ActionDave]

Because that's what everybody draws a picture of- a line with a sign wave crossing it.