220 volt double pole breakers (US)

[ke5frf]

This furthermore means that in actuality, there are three neutrals, and the 240 neutral isn't at the same potential as the 120 neutral. Wow, I understand how the 120/240 voltages are obtained but this totally blows my mind, because I never heard of this before.

Are both 120 circuits used in the lighting and outlets? They can't be! That would mean that there is 120 volts potential between different neutrals in my home.

RIGHT?

 

[Mark_R]

This furthermore means that in actuality, there are three neutrals, and the 240 neutral isn't at the same potential as the 120 neutral. Wow, I understand how the 120/240 voltages are obtained but this totally blows my mind, because I never heard of this before.

Are both 120 circuits used in the lighting and outlets? They can't be! That would mean that there is 120 volts potential between different neutrals in my home.

RIGHT?

Woah, back up the wagon! You had it there a minute

There is only one neutral, which is generally referenced to ground. All the neutrals are the same, it's the same buss bar in the panel.

Both 120 circuits are used in the house. If you took a meter with 50' leads and went to the hot side of different outlets in your house (the smaller one, right side if the D shaped ground is down) you would find ~240 between some and ~0 between others, as you are alternately connecting to the same end of the 240v winding or opposite ends.

This is where Sceadwian would have a 50-50 chance of vaporizing his double male ended extension cord.

Visualize a voltage divider resistor network with 2 resistors, 10V end to end, 5V from the center to either end.

 

[tcmtech]

All of the common/neutral lines are all connected to the same point in the main panel box. (All white wires typically.) The bare copper wires (some times green too) are the earth grounds and they are typically tied to the same buss bars as the common lines.

The 240 volt power comes from two power supplying lines being 180 degrees out of phase from each other due to the center tap of the main supply transformer being the common neutral line. If the center tap is ignored in the circuit its just a 240 volt source with a circuit breaker on each line.

For a 240 volt powered device like an electric motor two power circuits are needed but not the actual common/neutral thats coming from the center tap. The common/neutral line is just connected as a safety or earth ground connection only. It does not serve a purpose other wise.

 

[Mark_R]

Lets say you have two 100 watt 120v lamps in your house connected on one of the 120 volt legs and two the other. you could go to your service and disconnect the neutral and nothing would change. In this situation the neutral is carrying no amperage at all. You have the analogy of the perfect voltage divider in the DC world.

Lets say you shut off one of the lights, without the neutral connection the potential of the neutral buss would move 40 volts towards the leg with the single bulb, giving you 80v on that leg and 160 on the other (not good). Re- connect the neutral and it would pull the neutral buss back to 0v. In doing so, the neutral carries the difference in the current of the two legs.

 

[ke5frf]

All of the common/neutral lines are all connected to the same point in the main panel box. (All white wires typically.) The bare copper wires (some times green too) are the earth grounds and they are typically tied to the same buss bars as the common lines.

The 240 volt power comes from two power supplying lines being 180 degrees out of phase from each other due to the center tap of the main supply transformer being the common neutral line. If the center tap is ignored in the circuit its just a 240 volt source with a circuit breaker on each line.

For a 240 volt powered device like an electric motor two power circuits are needed but not the actual common/neutral thats coming from the center tap. The common/neutral line is just connected as a safety or earth ground connection only. It does not serve a purpose other wise.

Y'all don't get me wrong, I definitely know the prong configurations and color codes in household wiring, even that red is an alternate hot sometimes, and that white is often taped with black electrical tape to represent hot in series light switches if my memory serves.

But how can all the neutrals be tied together in the breaker panel? That makes NO sense given the center tapped transformer revelation. Especially given Mark R's confirmation that neutrals are not all at the same potential. I just don't see how it is possible for the 240 neutral to be tied in with the 120 neutral(s), which represent 120 volts of potential between the three of them, 60 between 240 neutral and each 120!

I can see now why I am not the only person who gets this wrong, I've talked to MANY people with various levels of knowledge of electronics and electricity, and I have always felt that this wasn't the most clear topic.

 

[Mark_R]

In a nutshell ==> Shared neutral - Wikipedia, the free encyclopedia

 

[Mark_R]

But how can all the neutrals be tied together in the breaker panel? That makes NO sense given the center tapped transformer revelation. Especially given Mark R's confirmation that neutrals are not all at the same potential.

If I wasn't clear I'm sorry, but that is definitely NOT what I'm saying.

ALL THE NEUTRALS ARE THE SAME. THEY GO BACK TO THE NEUTRAL BUSS IN THE PANEL AND THEN ON TO THE CENTER TAP OF THE UTILITY TRANSFORMER.

 

[tcmtech]

Mark R is confused or you misread what he said.
Trust me all domestic US power systems have all the common/neutral and ground lines returning to the same point the power transformers center tap is tied too.
In a 240 volt powered device the center tap is not normally used as a power return line. Its just there as a earthing or ground return so that if one of the live feed lines shorts to the body of the device it take the power to ground and trips the breakers and shuts off the power.

 

[Mark_R]

I just found this ==>

The best explanation I have ever seen, with diagrams, the math, etc. I'm printing it out to give to my apprentices.

 

[RCinFLA]

One thing that has not been mentioned is the 'ganged' dual 240 v line breakers have the handles mechanically tied together so if either one trips, they both open up.

A 20 amp breaker is a 20 amp breaker and a 40 amp breaker is a 40 amp breaker. Doesn't matter what line voltage they are tied in.

To Euro's, it is a little (but not exactly) like putting two 20 amp breakers in series. Still a net 20 amp breaker.

The 240vac center tapped grounded transformer is best description. Two, handle tied together breakers are on the two hot ends of transformer.

Arguably, it make the U.S. residential connection a little more protected for lightning strikes and a little safer to accidental contact to hot plug line.

 

[ke5frf]

Thanks for all the information guys, a lot of learning in one day. That is what I love about this field, I don't think I'll ever know it all, and I can't help but admire those who know things that I don't, and I love sharing my knowledge with those who can learn from me.

 

[Sceadwian]

I was wondering about that RCinFLA, cause a 220 motor load won't like 110 voltage if only one line trips, it would cause a massive drop in torque on the motor and dramatically increased heating.

 

[Mark_R]

I was wondering about that RCinFLA, cause a 220 motor load won't like 110 voltage if only one line trips, it would cause a massive drop in torque on the motor and dramatically increased heating.

it's a series circuit, if one half trips the current stops.

 

[Sceadwian]

Right'o Mark =) That went right over my head =O Although Neutral/phase faults could still be messy.

 

[RCinFLA]

Not exactly,

An example,

A 240 vac pump develops a winding to case short near one end of the 240v winding. Lets say it is at a point that would normally be 20 volts away from one end. Because the case is grounded, the side nearest the winding short will draw high current tripping that side's breaker which pulls the other side mechanically with it. It is important that the case be grounded for safety, even though actual motor operation doesn't require it.

In Europe if the same short developed 20 volts away from the grounded power line end, the motor may continue to run, drawing slightly more current but not enough to pop the breaker. At least until motor overheats and goes into a further failure mode.

 

[Sceadwian]

That's how ground fault interupts work, even a small amount of current few mas) through the ground will trip an emergency breaker.

I have an old school Electrolux vacume, 10+amps two wire, no ground. I zap myself with it CONSTANTLY from leaked flux. Tempted to cover the whole outside of the case with foil and ground it. I HATE static zaps, even though it's not quiet static.

 

[RCinFLA]

The shock from old vacuum cleaner is likely because of a winding to case or connecting wire to case short. If you had it on GFI outlet you wouldn't get shocked very long.

GFI's work on principle that current in one wire must exactly equal current out the other.

A torroid transformer with a large number of turns feeds a detection amplifier. A couple of turns from each power line is wound in opposite field generating direction through the torroid.

If the two main power lines' current exactly equal, their fields in the torroid exactly cancel out creating no output to the sense amplifier.

They are pretty remarkable. Most can detect a 10 microampere delta on a 10 ampere current flow.

Kids today never have had the pleasure of a five tube A.M radio without power transformer that rectified the direct A.C. mains. Touching the brass volume control shaft after the plastic knob went missing.

 

[Sceadwian]

The shock is like a static discharge, very brief duration, probably 170 volts, but the capacitance is high enough to cause a Pavlovian reaction of avoidance.

 

[tcmtech]

I had a customer come in while I worked as a service tech for a local welding supply center who had an unusual problem that no electrician he talked to and had look at it ever actually solved.

He did blown in insulation work for a living and at times he would get massive shocks at random. One day they would be constant the next not a single one.

He told me what he was doing and how the insulation blower system was set up. GFI's and everything.
What we determined is the problem came from the insulation itself and not the actual machine wiring. When the insulation was dry or he was using a longer hose he would get the shocks.

I told him to take a long piece of some very flexible stranded copper wire I had and wrap it around his hoses in a gradual spiral of about one turn per 3 -4 feet all the way to the end. Then tape it in place and run a short stainless steel screw right through the wire into the hose just to the point of breaking through the inside in 2 foot intervals all the way from end to the other end and then put 3 of them around the nozzle end. The other end was grounded to the metal blower case.

He came in about two weeks later and said he had never gotten shocked again! Then bought the last 100 feet of the fine stranded wire I had in stock and was going to refit all of his blowers and hoses with the same setup.

The fine insulation material was causing massive static build up as it traveled through the hoses at high speed and would build it up to the point it would jump to the nearest grounded object which was either him or the house he was blowing it into.
The multiple screws along the way kept the static from building up along the hose and the final set of screws worked as a collection point for any remaining charge before it left the nozzle.

Thats why vacuum cleaners have three prong cords and a wire thats inside the hoses. The static build up has to go someplace or it will continue charging up until it finds the operator and jumps to them!

 

[ke5frf]

I really appreciate the discussion on this thread. Wow, it took a little mental leverage to force the old notions out of my mind, but I see how the 240 with center tapped transformer works now. I will tell you how my incorrect notion has been reinforced all this time. In my work, I do not deal with building power, we have electricians for that. I only troubleshoot "wall powered" equipment, 95% of which is 120 volt. I do have a handful of 240 volt devices, however, but I still do not fool with building circuits. One device in particular that is 240 powered is a high temperature distillation apparatus, the heater elements driven by 240 volts at 5 amps. Traditionally to troubleshoot the temperature control system when it fails, we check voltage from chassis to each power terminal (neutral to both hots) and then from terminal to terminal...and if the meter reads 120 from neutral to both hots and 240 across, we conclude that the element is open when it isn't heating.

In my mind, I always concluded that neutral/ground was PART of the circuit, just as it is with 120 volts. I had never been told differently, so I perpetuated this thought. And since the two 120 volt "hots" were connected opposite to the load, in order for there to be any potential between them I had to see them as out of phase with each other. But my mistake was defining the potential with respect to neutral, as I am accustomed to doing with 120 volt circuits. I never thought of the center tap of the transformer permitting this to work.

And I see now how neutral to ground permits all three breaker panel supplies to share the neutral connection. The danger that Scaedwian was referring too, and I think Mark R touched upon, is trying to supply power to a device by paralleling two 120 volt hot connections from different circuits. I can see that this would cause a dead 240 volt short circuit which would be a bad, bad thing. DO NOT TRY THIS AT HOME.

I do have ONE MORE point of confusion, and this caught my eye on the ALL ABOUT CIRCUITS single phase power tutorial:

The astute observer will note that the neutral wire only has to carry the difference of current between the two loads back to the source. In the above case, with perfectly “balanced” loads consuming equal amounts of power, the neutral wire carries zero current.

OK, this is my last question, but it is very critical in understanding this.

The all about circuits tutorial offers the explanation that the two 120 volt branches in your home are IN EFFECT a series load on the 240 transformer. The neutral connection is provided to prevent an open circuit from shutting down power to everything in case of a failure in one branch. It provides an either/or path.

So, we are defining the neutral connection as having zero current, because in effect even the two 120 volt branches work together as a series load would off the 240 secondary. But in order for the current to be zero, the load has to be balanced if I read this correctly. But that will never be!!!! considering that if I plug in even so much as a night light on one branch with all circuits open on the other, wouldn't the load become imbalanced, thus the neutral would have a detectable current flow, am I not correct???

I can visualize in my mind with a perefectly balanced load on each branch, how the current at neutral would cancel out. (Am I not correct that the direction of flow would be out of phase 180 degrees on the neutral, and this is why it becomes nil? But if one half of the transformer secondary is open circuit, with no current flow, then the other branch would have no opposing current at neutral to cancel out with, correct? So wouldn't this be a dangerous scenario considering we all consider neutral to have no potential. I might think twice in the future before touching a neutral wire!

But obviously, this is not the case, something in my understanding is amiss!!!....so I would appreciate the final piece to the puzzle that answers this and I'll be satisfied.