Explaining MultiWire Circuits- For Fun :)

[N5XL]

In most residential electrical, the power is better described as a "split phase", or better still, a center tapped, split phase, sourced voltage. In these cases there is no phase shift because there is only one phase to deal with housewide.

Don't let the two ungrounded conductors or "hot" legs at the service panel or at that dryer outlet confuse you. Those are not separate phases. Both ungrounded conductors are derived from a one phase from the power company. How you end up with two ungrounded conductors a grounded conductor and how you measure them is where the confusion starts.

One ungrounded conductor at the house service panel comes from one end of a coil inside the transformer that makes up the secondary of the transformer. The grounded conductor (neutral), comes from a tap that happens to be connected directly in the middle of the coil on the secondary of the transformer, and the other ungrounded conductor is connected to the opposite end of the coil on the secondary of the transformer.

We started with one phase, and still have one phase that has been transformed down to 240 volts when measured across the full length of the coil secondary of the transformer. The confusing part is that there is now an extra connection at the transformer secondary (the center tap of the transformer secondary to be specific). Its not causing any phase shift, its just giving you a new reference point to confuse you in your measurements. Because most people use the neutral as a reference point, its easy to see how you could think its two separate phases. What you are really seeing on a scope is a mirror image of the same waveform.

If you had 10 taps on the secondary of the transformer, you wouldn't have 10 phases, you would still only have one phase, just split 10 ways. The number of taps on the secondary has no bearing on the phase of the system output. Remember that only one phase is being fed to the transformer from the power company...one phase in, one phase out....but that output is split, giving you two ungrounded conductors and a grounded conductor....all still single phase.

Dave

[Greg Sullivan]

Dave,

Thanks for the excellent description of how a single phase is configured to provide both 120 and 220 power. I always assumed that 220 VAC was produced by connecting accross 2 phases of a true 3 phase source. So, when I'd hear someone say "220 single phase", I thought they really meant 220 two phase.

When I am in various industrial buildings that have three phase incoming power, I've seen where 220 is derived from connecting across any two of the three phases.

Have you ever come across a house that actually has 3 phase power connected to it, or is that reserved for industrial applications?

Thanks again,
Greg Sullivan / Canyon Lake, CA.

[rain252]

N5XL,
What I still don't understand about this single phase residential.........

1.) Why are the two hot legs in a residential system said to be 180 degrees out of phase with each other? If both hot legs are in phase with each other, when I take a voltage read from leg A to leg B, wouldn't I read zero volts? If the two legs are truly one phase, how do two 120v legs add to 240?

2.) I will try to attach a simplified drawing of a residential transformer showing the primary lead terminating at the center tap of the secondary coil (neutral).
Seems that would just be a direct short to ground of the primary since the neutral is grounded, wouldn't it? Obviously it isn't, but I don't understand it. ?????

[N5XL]

I'm glad that Paul entitled this thread Explaining Multiwire circuits for FUN!! People following this thread 3 deep (as of this posting) could only be described as having fun! (or maybe crazy...) BTW, this would be a good time to get one of Russel's margaritas.

I'll take your questions point by point and try and explain the best I can whats happening. At anytime if the electricians see me getting over my head, I hope they will feel free to throw me a line.

Why are the two hot legs in a residential system said to be 180 degrees out of phase with each other?

. Two reasons. It has to do with the way that people view the waveform in the main panelbox with an oscilloscope, and the fact that in multiwire circuits, the neutral carries the difference in current flow back to the transformer. Its my opinion that the second reason offers support for the first reason in peoples thinking. Maybe it will be better if I address your next two questions and answer all three at once

If both hot legs are in phase with each other, when I take a voltage read from leg A to leg B, wouldn't I read zero volts? If the two legs are truly one phase, how do two 120v legs add to 240?

It may help you if you think of the powerpole transformer as a 240 volt secondary device only. For our discussion, lets forget that the center tap exists and just focus on the two ends of the coil that make up the secondary. As you know, one hot leg is connected to one end of the secondary coil, and the other hot leg is connected to the opposite end of that same coil. Lets pick one end of that coil and call it the "cold end" and the opposite end of the coil and call it the "hot end". Why I chose those terms will be clear in a minute.

Take your regular volt meter and set it to read AC voltage. Touch both ends of your meters leads together, and note the reading...0 volts. Now, take one of the leads..the negative lead and put it on the cold end of the secondary. Take the other meter lead..the positive lead, and put it in the same place...note the reading is 0 volts. Everything will be referenced from this one point from now on..the cold end of the coil. Since both of our meter leads are connected to this one point it reads 0 volts because no potential exists there. Now lets pretend that our secondary coil is made up with no insulation on it, and that we are able to probe the coil anywhere along its surface windings. Leave the negative end of your meter attached to the cold end of the coil, but now move the positive meter lead up the coil, away from the cold end towards the hot end and take a reading. You will have some small voltage...lets say 10v AC. Continue to move your positive meter lead up the coil towards the hot end....20v 50v...100v....120v. Right about there is where the "center tap" of our coil is located at....directly in the middle of our secondary windings. If you were to continue to probe the coil along its length towards the hot end, you will eventually get to the hot end of the coil...and 240 volts AC. Its called the hot end because thats where maximum voltage exists when referencing it from the cold end.

The secondary output of a single phase power transformer is exactly like this, with the exception that there are only three places we can probe the secondary, but its operation is exactly as described. The cold end, the middle of the coil, and the hot end are the three leads you see in a residential panelboard.

One of those ungrounded conductors in a residential panelboard is a direct connection to the cold end of our coil, and the other ungrounded conductor would be connected to the hot end of our coil. The reason you see 120 volts on either of the "hot legs" is because you are actually measuring the potential from the either end of the coil to the center tap. It has nothing to do with an electrical phase relationship, but where you are referencing your measurements from. In residential work, leave the term "phase" at the transformer because thats exactly where it belongs...on the power company side of the transformer. From the transformer onward to the house, you are single phase, or better put, split phase. Now you know where the split comes from in "split phase". If you think of the transformer secondary as a 240v coil, that just happens to have a tap on it in the center, it helps to clear up the confusion.

Why these two hot legs are said to be "out of phase" has to do with where an oscilliscope is hooked up to take the readings. If you hook your negative lead to the center tap, you are referencing the wrong part of the secondary. You have to reference the cold end of the secondary and take your readings from there always. If you reference from the center tap, the positive lead of the scope shows a sine wave, lets call it in a positive direction. If you leave the negative lead on the center tap, and move the positive lead to the other leg, you have what looks like a sine wave out of phase with the first reading, but what you are really doing is showing yourself a mirror image of the same waveform.

The other thing that further confuses people is that on multiwire circuits, its there is less current on the neutral than what flows in both circuits that returns to the transformer. The easiest way to clear this up (this is regard to mutiwire circuits only) is to simply think of circuits originating on one leg of our panelboard as having positive amps, and the circuits sourced from the other leg of our panelboard as having negative amps. Then it becomes simply a matter of adding positive an negative numbers.

Electricians...how am I doing so far? I promise I'm almost done...

I will try to attach a simplified drawing of a residential transformer showing the primary lead terminating at the center tap of the secondary coil (neutral).
Seems that would just be a direct short to ground of the primary since the neutral is grounded, wouldn't it? Obviously it isn't, but I don't understand it. ?????

Sure seems that way doesnt it. And it would be except for the fact that the primary is wound in a coil...and thats critical to why it doesn't just short out to ground. When you are talking about AC circuits that alternate 60 times a second, there is something called reactance that keeps the current from shorting directly to ground. Its exactly like resistance, but it changes its "resistance" or impedance, based on the inductance of the coil in the primary. How many turns, how tightly the turns are wound, spacing, conductor size, insulation and the frequency all factor into how much resistance is there keeping the current from just shorting to ground.

It might be easier to think of your transformer as a space heater. If you think about it, drawn on paper, a space heater is a direct short across the hot to neutral. The reason a space heater heats and doesnt short is because there is resistance in the elements. The resistance is what prevents the current from directly shorting out....its resistance is calculated to be exactly enough to cause special wire to get hot but not burn through. In your transformer drawing, the primary is designed such that, with the correct inductive reactance (impedance) to provide power to the secondary without shorting directly to ground....it gets hot, and delivers its power to the secondary, but doesnt short.

I hoped some of this helped...now I need one of Russ's drinks!!

Dave

[rain252]

That was pretty good, actually, Dave. Especially the part about "splitting" of a single phase instead of 2 phases, and why the primary side doesn't just short to ground at the neutral termination. The part that is not real intuitive to get a grip on (at least for me) is the stuff about the "180 degree out of phase" of the two hot legs. Maybe someone else can toss in some info about that and help clarify what is actually happening with that. This is a good discussion.

[Velo]

Hi all. I just found this thread looking for info on multiwire cicuits and have actually found it quite entertaining. What I'm actually looking for is to clear up some confusion over the way my home is setup. For some background info, I live in a large condo apartment in a building that was designed in the late 50's and built in the very early 60's. I don't know why but every apartment was supplied with all three phases of three phase power to each apartment. I have all THREE 100 amp lines up to the apartments breaker panel. I suspect this was done to provide enough power for the air conditioners of the day. Every single room was supplied with its own dedicated 220v (or should I say 208v?) outlet for its an air conditioner. This means the breaker panel was crammed up to the top with double pole breakers for each of these outlets, about 3/4's + of the panel capacity. The remaining slots were filled with tandem breakers since there was no room left. Tandem breakers as in the kind that have two mini breakers in a single slim breaker hooking up to a SINGLE BUS EACH (not the double pole type as in two seperate breakers attached together.

This is were my worry starts. Recently I did a complete replacement of every single light switch, receptacle, and light fixture in the apartment since they were old, ugly, or rusted and some were in pretty bad shape. While I was at it I decided to make a full wiring diagram of things since the electrical plans for the building have long been lost. What I discovered was that all the receptacles and lights were wired in multiwire fashion, with two hots sharing a neutral. The problem is that the two hots for each outlet are connected to the SAME tandem breaker. This means the two hots are on the same bus each. Under normal circumstances I would know this is a very bad thing but since it is a three phase setup I don't know if there is a reason for that. Its been that way for decades and I've never had a problem. I've checked all the neutrals and they show no signs of browning out. They are in remarkably good shape for 40 year old cable. However, given that most of the heavy pulling stuff suck as AC's are on their own dedicated circuits most of the receptacles on the multiwire configurations have very little load on them. At most a tv and a couple of lamps. Once an electrician came many years ago and actually replaced one of the tandem breakers because he found it faulty. I'd figure something like this would have jumped out at him but no. Either its ok or he had no idea it was wired in such a way. I'm just trying to figure out if I need to do something about this or not. Obviously the 220v circuits for the AC were changed down to 120v long ago. That means the extra hot cable was removed and the double pole breaker replaced for a single pole freeing up a bunch of space in the panel. These have a normal two wire setup of 1 hot and 1 neutral. What I need to figure out is if I should have the tandem breakers removed and switched for a two pole breaker accross seperate buses. Or is running an extra neutral another option to seperate the circuits?

[Paul W. Abernathy]

Velo,

Let me first start by saying this is probably a question that should be asked in a DIY forum as this is not that type of forum. Most the electricians here come to assist the home inspectors in their line of work and getting way to technical could throw an element of confussion that can harm lives if not clearly understood.

I will also say that tandems in older panels are never good simply because of the fact the panel problem has an excessive number of OCPD's that exceed the panels actual rating.

To determine if you have a problem we would need to know more about the set up, while it is very rare to have 3 phase located in a residential module it can happen and does in remodel coversions of old buildings that were at one time office locations as such.

The key to understanding 3 phase is depending on the type of set up you have be it 3 phase delta or wye system is the ability of usage.

The transformer for that building better be balanced correctly depending ont he loads but I can probably guess it is not but since as you stated very little is on the circuits you probably have not seen any issues...then again it may be fine.....my point is without us knowing more details on the actual panel setup and if possible some images of the panel.....it would be quite possibly wrong for us to comment on it....maybe Bob Badger would know more or even Greg...but to NAIL something down I know I would need more information from you.....

for example....what is the make of the panel.....Zinsco for example on their old 3 phase tandoms used different legs...just looks like it used only one....so what brand panel is it.....how are the lines labeled...in other words are they color coated..taped...what have you.....see any orange in the panel.....

My best advice is if you are really concerned.....contact a local electrician to come look at it...and one who is familiar with 3 phase....why the other guy may or maynot have seen anything really does not mean alot....he came looking to fix a breaker.....he was not their to look at other things and thats kinda how the industry goes.....

Not bustin your chops fella...really I am not...but the question can be quite complex unless we know all the varibles...

Also......I beg to ask what information do you have that tells you their are that many multiwire circuits.....if ran in cable ...are you seeing alot of 3 wire cables in the panel.......We need to know what you have done to determine this information....the same way we would approach it in the DIY forum....since i happen to moderate at one....

[Paul W. Abernathy]

P.S.....But I will leave you with this....and since I am JUMPING on a leap of faith here.....if you TRULT have multiwire circuits sharing neutrals on a single LINE...( we will leave the word phase for the actual transformer because thats where it really is...those are not phases at your panel...)

Anyway......ANYTIME you have multiple loads on the SAME line which transfers back to the phase and winding you have potential for things like unbalanced windings and other issues...the fact you have no browning of the neutral ( grounded conductor ) is probably because of your lightned loads.....but again I would suggest you contact a local electrical contractor to make that determination....he may agree with me or disagree with me....I always find it hard to give advice on 3 phase from probably 1,000 miles away and if you are REALLY concerned the piece of mind having it looked at locally will be worth it.

Unless greg or bob or even NXR-R2DR...sorry N5XL...just teasing ya....good information their....You are not fooling anyone fella...lol....thehehe

Anyway......Single Phase is dangerous enough.....messing with 3 phase and not being 100% comfortable can be DEADLY....

[Paul W. Abernathy]

Bump....Good Ole' Information