Very Basic AC power question

Short Answer:  The neutral wire completes the circuit to the power source. 

Long Answer: The hot wire is one wire to the load (one example of a "load" is a train transformer). The neutral wire is the other. 

Longer Answer:  The hot wire is always at some voltage.  In a house system the hot wire oscillates.   The neutral wire is always at zero volts and connected to the ground somewhere back at your main breaker panel, or the input to the house, in older installations it may even be further upstream.  Thus the ground is a safety wire so if you have an electrical fault (i.e. the input hot wire to your transfomer shorts out to the case) it carries the current.  Otherwise your transfomer case (in this instance) is electrically live and can give you a shock. 

Even Longer answer:  At one time it was thought that if the case was not electrically conducting, it the would be safe even though there was an internal short.  But electricity can find any hidden current path, so the ground wire was required. 

You now don't have to go back to High School Physics

You always put a switch in the hot or black wire or plug that has the smaller blade or smaller slot in a socket.  This assumes your house is wired to the electrical code.  To be double sure use a double pole switch rated for 110 volts and adequate amperage, which will switch off both wires.

To better understand wiring get a few dollar copy of Wiring Simplified at Home Depot etc. based on the latest National Electric Code.

Our older post war transformers like the LW, ZW etc. do not have a polarized 110v plugs since they are supposed have the transformer frame inside enclosed and isolated inside of a non conductive bakelite/plastic case.  The none polarized allow us to phase multiple transformer and allow running trains from tracks powered my two more transformers.

But in the 1950s some dangerous products were made and sold as the electrical codes were not thought out well then.  An example, I was given a large Hallicrafters short wave tube radio from the 1950s original a polarized plug.  I plugged it in and it worked and I checked the metal case to ground with a volt meter.  With the radio switch off the case read 110v.  I reversed the pull and the case read 0 volts with the switch off.  I switched the radio on and the case read 110 volts.  So the radio was shock hazard depending on the plug position and the power switch position.  I installed a ground fault plug and power wire from an old hair dryer and wired the on/off switch on the radio in to the black hot wire and it safer now.

Charlie

Not to go off too far on a tangent here, but to further clarify some of @John Sethian's information around grounding...

The ground wire first appeared in the NEC Code in 1947.

As for the ground wire and our train transformers, even the current UL specifications for Toy Transformers in section 16 of specification 697 state that the cord, plug and unit should NOT be grounded, at least 18 gauge in size, between 5 and 10 ft long and not detachable from the transformer. All substantial non-circuit based metal components, including metal cases and frames are to be floating. This is all paraphrased as I can't directly quote the source because it is copyrighted, but you can purchase your own copy: https://standardscatalog.ul.co...aspx?productId=UL697

So the neutral wire isn't actually transmitting (carrying)   + and _ electrons out to the AC motor?  It is just returning the flow back to the  breaker box?  Only the hot wire is producing an outward current flow?

Is that it?

Thanks,

Mannyrock

@Mannyrock posted:

...If an AC power source is two wires (ignore the third ground wire), and both of those wires are alternatively generating positive and negative electrons, then why is one of the wires called the "hot" wire and the other the "neutral" wire??

Thanks

Mannyrock

It is simply a matter of convention because yes, in an AC circuit electrons flow in alternate directions for each half-cycle. "Circuit" is the operative term because electrical current must flow in a [closed] circle or loop, hence two wires connecting the voltage source and the load. Current (electron flow) flows from source to load and thence back into the source. In DC the flow is continuous and in AC the direction of current flow "alternates".

It so happens that in commercial [single phase] power distribution one "leg" or "side" (misnomer because current flows in a circular path) of the circuit is grounded, literally, by connecting that side of the circuit to a rod driven into.....the ground.  So if you touch any wire on this "side" of the circuit you will feel nothing because that wire is always at ground potential. The other wire/side of the circuit varies in voltage (with respect to ground) from -120V to +120V.

@Mannyrock posted:

So the neutral wire isn't actually transmitting (carrying)   + and _ electrons out to the AC motor?  It is just returning the flow back to the  breaker box?  Only the hot wire is producing an outward current flow?

Is that it?

Thanks,

Mannyrock

Mannyrock

The neutral wire is carrying current, either from the AC motor, or to the AC motor, depending on which half of the cycle you are in.   

The hot wire voltage is oscillating with respect to the neutral wire.  For a 120 Volt, 60 Hz system (what come sout of a standard wall plug), at one point in time the voltage between the hot wire and the neutral wire is PLUS 170 Volts.  Precisely 8.5 msec (0.0085 seconds) later, the voltage between the hot wire and the neutral wire is MINUS 170 Volts.   

TIME OUT FOR SOME EXPLANATIONS:  Why 0.0085 seconds? Because the voltage operates at 60 cycles per second.  The time between one positive peak to the next is 1/60 cycles = 0.017 seconds.  The time between the positive peak and the negative peak is half that, or 0.017/2 = 0.0085 seconds

Why 170 Volts? because the AVERAGE (in terms of power) of the voltage over one 0.015 second cycle is 120 Volts.  Note this average here is not calculated in terms of a numerical average (which would be zero), but in terms of the power,  which is calculated as the square root of the average of the square of the voltage. (Yes, that can be complicated, so take my word for it or go back to high school physics)

 NOW BACK TO YOUR QUESTION:   During the positive part of the cycle, current flows from the breaker box, through the hot wire, through the AC motor, then back through to neutral wire to the breaker. During the negative part of the cycle, current flows from the breaker box, through the neutral wire, through the AC motor, back through the hot wire, and into the breaker box.  

A QUESTION YOU MIGHT ASK:  If the voltage is oscillating (going plus and minus) why is the motor not turning one way and then the other?  The answer is there are two coils in an AC motor, rather than just a coil and a magnet.  Thus when the current changes direction, the current through both coils change direction, so the motor turns one way.  If you applied AC to a modern can motor which has a coild and a magnet, it would quickly burn up because it would try to change direction every 0.015 seconds.  As it can't, it would quickly burn up.

Well, here's my confusion:

"During the negative part of the cycle, current flows from the breaker box, through the neutral wire, through the AC motor, back through the hot wire, and into the breaker box. "

OK, so during the cycle, the neutral wire is in fact carrying current to the the AC Motor. So why then is it called neutral?  If it is carrying current, then why isn't it called HOT as well?

Mannyrock

@Mannyrock posted:

So why then is it called neutral? 

Properly installed, neutral is tied to earth ground at the panel. It is literally neutral, at 120 volts, relevant to each of the two hots, which are 240 volts apart.

Rob,

A Romex line has three wires.  A bare copper wire (which is connected at one end to your switch box, and at the other end to the ground in the breaker box.)  The other two are insulated wires, one is generally black and one is generally white.  It is my understanding that only one of these insulated wires is called the hot wire, and the other is called the neutral.  But, if the neutral wire carries current during the cycle from the breaker box to the AC device, then how can it be called neutral?  It is carrying current.  Therein lies my confusion.

Mannyrock

It's just a naming convention. Don't put too many brain cells to death over it.

Ignore the bare ground wire. The white neutral is tied to earth ground at the panel. It is LITERALLY neutral, at 120 volts potential, relevant to each of the two hots(two sets of black wires at the panel, or for 240 volt appliances black & red), which are 240 volts apart in potential.

Previous contributors are correct.    Residential power is 'BI-PHASE'    Industrial is '3 PHASE'.  If you would take the 'HOT' in one part of your residence and take  the 'HOT' from another part of the residence and measure the voltage between the TWO HOT wires you could get 220 volts. Hence the terms 'HOT' and  'NEUTRAL'.   It is a deep and somewhat   $%$#&#$^   subject.

Geez,

I think RobBoy hit the nail on the head.

Mannyrock

Try looking at the neutral as a "return" to earth be it short distance to yours(safer) or longer to the plant (best for production/distribution)

It may return a negative or positive charge as needed.

Things get more complicated at 440v where a Wye or Delta phasing can have impact on some things.

The fact we mix terminology and use it loosely between ac and dc doesn't help. Folks tend to lock into the dc frame of mind because of the easily understood +/- relationship; it is almost self explanatory. I often think if we learned ac first we would understand the big picture sooner and more thoroughly. 

Since almost everything in our houses runs on DC, have you ever wondered why the power line feeding your house is AC?

@BOB WALKER posted:

Since almost everything in our houses runs on DC, have you ever wondered why the power line feeding your house is AC?

Safer because of alternating.

Higher currents can travel longer distances in smaller supply lines than with DC.

Since when does everything in the house run on DC? What are you thinking of?

The advantage of AC is that a simple transformer can raise the voltage tremendously and the current will be very low.

That means that power can be carried long distances at high voltages and little loss from resistance. The loss is little  because the current is low.

@BOB WALKER posted:

Since almost everything in our houses runs on DC, have you ever wondered why the power line feeding your house is AC?

OK, name one appliance in your house that has a DC motor in it!

Yea Edison lost the DC vs AC battle, one of few times he was wrong.  VCR, cassette and CC player probably have DC motors like the newer trains but I would not call them appliances.  AC is better for high drain things like washing machines, vacuums, ect.

Charlie

Bob,

Edison and Tesla fought like dogs over 100 years ago about which power form to use.  Edison wanted DC, because he said it was far safer (but would require a relay station of some type every 5 miles).  Tesla wanted AC, because it could travel long distance on light lines without boosts.   As long as you didn't physically grab any bare wires, it was safe.

Tesla was offered the right to provide the first outdoor electrical lighting, at the Worlds Fair.  Can't remember the year.  1898? He strung miles of wire, and hung literally thousands of lights, to light the entire thing up.  It was the first time that almost anyone had ever seen a lightbulb work.   

Not a single person was injured during the Fair.  Tesla won.  It would be AC to power houses forever.

Mannyrock

@Choo Choo Charlie posted:

Yea Edison lost the DC vs AC battle, one of few times he was wrong.  VCR, cassette and CC player probably have DC motors like the newer trains but I would not call the appliances.  AC is better for high drain things like washing machines, vacuums, ect.

Charlie

If you want to get more even more complicated most high efficiency high draw Applinces IE Air conditioners (I’m a HVAC tech) now starting around 16 seer have fan motors that take in 220vac 1ph and convert it to DC then is sent out in a square AC 3ph fashion to drive the motor. All furnaces and air handlers built after June 2019 also MUST have a ECM motor for the blower as well. One step up from that is doing the same thing with the compressor at 17-20seer and higher. Of course now with solid state boards that’s what’s REALLY sensitive to power surges. 

AC is used for long distance transmission because it is easy to use transformers to raise the voltage at the source and then reduce it to a reasonable voltage to the customer.  The advantage of high voltage for transmission is currents are lower so there is less voltage loss due to resistance of miles of wire.  There are some DC transmission systems in use.  They also run at high voltage to avoid resistance losses.  With a DC system the voltage must be converted back to AC so transformers can be used at the customer end.  There are trade offs in cost and complexity of the two approaches.  There are both AC and DC approaches to railroad electrification.

AC and DC have both been used in homes.  Light bulbs don't care.  Older radios were designed without transformers so they would work on either AC or DC.  Some appliances could be designed for DC but why bother when AC is the prevalent power supply.  It is true that many devices must have an internal AC to DC power supply to meet their particular needs.  But, if they had to work off DC the job would be harder.  If you needed 5V DC from a 120V DC line how would you lower the voltage?  With AC a simple transformer lowers the voltage, then you can rectify and regulate it as needed.

The neutral wire question depends a lot on context.  Most will encounter this in terms of house wiring where the electrical code rules.  The only point I would add to other comments is never consider the neutral wire to be a 0V.  The current flows down one wire to the load and then back the other wire.  At any point the neutral voltage will be higher based on the current and resistance of the wire.

As an electrical component a transformer provides isolation (this does not apply to auto-transformers such as a Variac).  The two wires of the secondary have a voltage between them but are not referenced to the input side so you wouldn't measure a voltage from a secondary wire to the primary side.  However, in some devices there is an intentional connection between these domains such as the ground wire in a TMCC system.

As a vague analogy consider a water based system that pumps water some distance to a turbine and then feeds back to the input to the pump.  This is an isolated (closed) system.  Now consider you break one pipe close to the pump and connect the ends to an open reservoir.  The high pressure output of the pump flows through a pipe ("hot") to the turbine and then back through a pipe ("neutral") back to the reservoir ("ground").  At any point in the return pipe the pressure will be higher than that of the reservoir due to the pressure drop of the water flow through a finite diameter pipe. 

Like many, this discussion is like pulling that loose thread on a sweater.  Any answer leads to more topics.  Unfortunately, I only have one mode - long winded.  Sorry.

Oh yeah.  You can be misled by thinking about electrons.  Just concentrate on voltage and current.  By convention a positive current flows out of the positive terminal of a voltage source and returns to the negative terminal.

Also, don't think about things moving back and forth in an AC circuit or that the voltage and current are sinusoidal.  For AC you use the RMS value to compare to DC.  The RMS voltage or current of any signal is the value that will produce the same amount of heat in a resistor as a DC voltage or current of that value.  So 18V DC and 18V AC will produce the same heat in a resistor or lamp.  The values we commonly use (120V, 18V, etc.) are the RMS values.  (RMS refers to Root Mean Square which refers to the math used to calculate the RMS value of a given waveform.)

The problem with electrons is they have a negative charge which is inconsistent with engineering conventions.  This really messed me up until I got to engineering school.  I remember hearing the Navy was notable for teaching the wrong conventions with an electron centric approach.

Referring to the original post.  AC does not have positive and negative electrons.  Electrons are always negative.  In AC they reverse direction back and forth.   A positron is similar to an electron but with a positive charge.   Also, a given electron does not flow through the length of a wire.  Electrons drift in one direction or another.  The collective action of billions of electrons cause the effects we see.  In semiconductors you have positive holes flowing instead of electrons.   You can understand all this and the relationship to magnetism by getting into quantum physics and special relativity - but I don't recommend it!

@Mannyrock posted:

I understand the difference between generating AC power and DC power.  In high school physics, we had to learn the difference.

But, here 50 years later, I am a little confused. If an AC power source is two wires (ignore the third ground wire), and both of those wires are alternatively generating positive and negative electrons, then why is one of the wires called the "hot" wire and the other the "neutral" wire??

 

By convention, we connect one conductor from secondary of the transformer on the power pole to earth.  We also connect that same conductor to earth at the entrance of the home.  That conductor is called neutral or, per the NEC, the grounded conductor.  All of the conductors that connect to the secondary of the transformer all have a voltage potential between them.  In typical US households we have "split single phase" power.  The two outer conductors on the secondary have 240V of potential between them.  The middle conductor, or "center tap" has 120V of potential between each of the outer conductors.  The center tap is the aforementioned conductor that is connected to earth.

For current to flow in a circuit there must be a voltage potential between the conductors and a complete circuit.  When you stand on the ground - or on something with a conductive path to earth - and then also touch the neutral wire, little to no current flows.  This is because there is no voltage potential between neutral and earth since as mentioned above, neutral and earth are bonded at the service entrance to the home (and on the power pole).  No voltage = no current.

The other two conductors, those with 240V potential between them, each have 120V potential to neutral - and also earth.  If you stand on the ground - or on something with a conductive path to earth - and then also touch either of the phase conductors, current WILL flow because of that potential.  Which direction the current flows doesn't matter - left, right, up, down, back, forth, to, fro.

So why is one wire called hot and the other neutral when current flows "both ways"?  Because "hot" has 120V of potential to earth and (in a correctly wired circuit) neutral has 0V of potential.

The typical American house has a 3 wire 230 volt service. If things were properly wired, half the load is on one of the 230 volt legs and the other half of the load is carries on the other 230 volt leg.  The neutral wire only carries the imbalanced current, which you hope is near zero. As for grounding the natural, in my opinion that is a misguided attempt at safety.  I, by far, prefer to work on ungrounded systems with ground fault protection.  I believe that many more of us would be alive today if the neutral had never been grounded. 

Thanks for all of this very informative discussion.  The water flow/pump/reservoir analogy by Penn Station really helped.

Bottom line from my viewpoint:  If you have a piece of fully connected Romex coming out of your wall socket, and run it straight to a big appliance, turn the appliance on, cut the "neutral" wire running to the appliance, and put both ends in your mouth, then you are going to be shocked or worse.  Right?

So, they should have never called it a "Neutral" wire.  :-)

Mannyrock

@David Johnston posted:

The typical American house has a 3 wire 230 volt service. If things were properly wired, half the load is on one of the 230 volt legs and the other half of the load is carries on the other 230 volt leg.  The neutral wire only carries the imbalanced current, which you hope is near zero.

The current on the neutral carries the full current in every 120V (110, 115, 117, 125 etc.) branch circuit.  The only place the neutral carries no current (or the small imbalance of current) is between the service panel and the transformer. 

As for grounding the natural, in my opinion that is a misguided attempt at safety.  I, by far, prefer to work on ungrounded systems with ground fault protection.  I believe that many more of us would be alive today if the neutral had never been grounded. 

Nearly every electrical distribution system has some part of the secondary of the transformer bonded to earth.  Otherwise the voltage could float well above the rated voltage of the circuit depending on the distance from the furthest point of grounding.  The primary reason for bonding one of the conductors of a secondary is to allow a fault to earth to develop enough current to trip the over-current protection device.

One key aspect of house wiring is consideration of lightning.  That's why you have ground rods at the house and don't just count on the third wire of the service line.  The electrical code captures years of real world experience of electric shock and fires and evolves as new hazards are discovered so history won't repeat itself.

Branch circuits are assigned to one side or the other in the electrical box.  As loads are switched on and off independently it is very unlikely there will be complete balance in the power demand to the service line.  In an individual branch the neutral will carry the full return current for that branch.  An appliance like a dryer that uses 230V with a double breaker might be balanced, but that appliance may have 120 Volt components like a lamp so perfect balance may not be achieved.

Branch circuits generally use bee and flower topology.  So you may have a high current load ahead of the outlet you are looking at.  In that case your neutral could be at the potential of the neutral side of the load. There is also the possibility of wiring errors or other faults.  Further wire color can be deceiving on connection to switches and three way wiring. Never assume the neutral is benign.  Note the neutral is not disconnected by circuit breakers so proceed with caution.  In my young and stupid days I'd replace outlets with the power on just for the challenge.  There a lot of wild and crazy things you can do with electricity: Tesla coils, Jacob's ladders, and general hijinks...

When I referred to a floating secondary I meant inside equipment connected to an outlet.  By using an isolation transformer you can avoid current paths to the house wiring; you only have to worry about the secondary voltages.  Another fun item is radios and TVs with a hot chassis.   These had power cord interlocks but there was also the cheater cord.  An isolation transformer can be used when servicing this equipment.

On the other hand don't be overly concerned.  A simple tester is available to show if an outlet is wired correctly as is a simple voltage present indicator.

I think the worst offense in the US is lamp sockets which are marginally insulated and subject to miswiring even if a plug with one wide blade is used. 

You guys do "long winded" well though, full of tasty bits with just enough sauce;  I'm eating it up easily, I hope they aren't full 😋

Whew!  After reading through all this, I think I will go back to pushing my trains around by hand. 

BRIO!

I don't know how they figured all that out before Google was invented.

I will concede that most motorized appliances contain AC motors. Every electronic device requires DC. Think how many rectifiers you probably have working in your house.

@BOB WALKER posted:

I will concede that most motorized appliances contain AC motors. Every electronic device requires DC. Think how many rectifiers you probably have working in your house.

 

But todays electronics are also a fairly new advancements over vacuum tubes and relays, etc.   

It boils down to efficient delivery to the consumer in the past.

If they figure out a way for dc to exceed ac in overall safety and efficiency, it will change again; by demand.

Did I mention I like schematics for the "picture worth 1000 words" aspect 

Hey GVDobler,

The electrons have collided in my gray with all this info....

...........so I ask now........where at In Country ?