Neutral function in 120 or 240

[jynx1]

I'm confused on neutral completing a 120 or 240 volt circuit relative to the hot(s) and then having 0 voltage when relative to ground... How can it circumstantially have two different voltages?

Also, if we're dealing with 120 volt AC outlet 60hz single phase circuit, is the neutral still an alternating current conductor? Again and again I've read that the neutral in 120 solely carries the current to ground... that sounds like the characteristics of a dc circuit... so, if no voltage is applied to (maybe because it's a single phase circuit therefore poles donnot reverse) the neutral, the current takes the lease resistive path to ground, which in essence completes the 120 v circuit.

Is this correct? I'd like to know so I can discuss some other electrical ideas.

Thanks for any help!

 

[Smart $]

I'm confused on neutral completing a 120 or 240 volt circuit relative to the hot(s) and then having 0 voltage when relative to ground... How can it circumstantially have two different voltages?

The 120V measurements are referenced to the grounded neutral (steady-state 0 volts measured ground to neutral). Each "hot" to ground voltage is an AC voltage, but using either as the reference, the other's cycle is 180? out of phase. Measuring across the two "hots" gives you 240V.

 

[Smart $]

...

Also, if we're dealing with 120 volt AC outlet 60hz single phase circuit, is the neutral still an alternating current conductor? Again and again I've read that the neutral in 120 solely carries the current to ground... that sounds like the characteristics of a dc circuit... so, if no voltage is applied to (maybe because it's a single phase circuit therefore poles donnot reverse) the neutral, the current takes the lease resistive path to ground, which in essence completes the 120 v circuit.

Is this correct? I'd like to know so I can discuss some other electrical ideas.

Thanks for any help!

Current always returns to its source. With utility power, this is typically the service transformer. While the ground (earth) may carry a little of the current, typical premises wiring is designed to minimize such current.

Neutral conductors in two-wire circuits carry just as much current as the circuit hot. Neutral conductors in three-wire 120/240 circuits carry the unbalanced current of the two half circuits. With two identical loads connected L1-N and L2-N, there will be no current through the neutral conductor because the current through one half of the circuit is the very same current through the other half.

 

[jynx1]

Thanks for the reply.

So even though the current is alternating, it still in essence is a direct path back to the transformer because it is grounded to the center tap transformer? It can 'feed' the circuit the same as the hot, though?

 

[ActionDave]

I'm confused on neutral completing a 120 or 240 volt circuit relative to the hot(s) and then having 0 voltage when relative to ground... How can it circumstantially have two different voltages?

Also, if we're dealing with 120 volt AC outlet 60hz single phase circuit, is the neutral still an alternating current conductor? Again and again I've read that the neutral in 120 solely carries the current to ground... that sounds like the characteristics of a dc circuit... so, if no voltage is applied to (maybe because it's a single phase circuit therefore poles donnot reverse) the neutral, the current takes the lease resistive path to ground, which in essence completes the 120 v circuit.

Is this correct? I'd like to know so I can discuss some other electrical ideas.

Thanks for any help!

Read this http://forums.mikeholt.com/showthread.php?t=89023&p=708650#post708650 and see if it helps. I think it will and explains things better than I can.

 

[Smart $]

Thanks for the reply.

So even though the current is alternating, it still in essence is a direct path back to the transformer because it is grounded to the center tap transformer? It can 'feed' the circuit the same as the hot, though?

The more appropriate term is connected. The current will be the same if the neutral connection were not grounded.

A comparative analogy would be a two series-connected batteries, where each half of the transformer winding is represented by one battery, and the center tap connection is the junction point of the two batteries. How much and where the current flows is determined by the connected load.

 

[infinity]

I agree with Smart that the two battery analogy is a good way to understand how the 120/240 volt system is connected. When I occasionally use it to explain how it works you simply stress that you need to disregard the battery polarity.

 

[Electric-Light]

The "center tap" doesn't even have to be ground. You can have one end grounded and still have 0-120-240 rather than 120-0-120.

It's neutral only because its referenced to ground.

 

[david luchini]

The "center tap" doesn't even have to be ground. You can have one end grounded and still have 0-120-240 rather than 120-0-120.

It's neutral only because its referenced to ground.

I don't believe this is correct. The neutral point is the midpoint on a single-phase, three wire system. In your listed references, the neutral in the first is at "120" and the in the second is at "0." (0-120-240 and 120-0-120)

 

[Speedskater]

While it's technically correct, it's not applicable to typical NEC single phase wiring systems.

 

[kwired]

The "center tap" doesn't even have to be ground. You can have one end grounded and still have 0-120-240 rather than 120-0-120.

It's neutral only because its referenced to ground.

I agree with David.

We call the center tap neutral, mostly because it is equal to all the other points on the system.

You could have a single winding with 100 taps somewhere mid winding. You can only ground one point, whether it be one end of the winding or one of the taps. If you ground more than one point you have a short circuit between those two points.

We ground conductors that are not a "neutral" in simple two wire supplies, or in three phase three wire corner ground systems. Even in DC circuits often the negative is referred to as 'ground' but this is true only because the negative conductor of the source is actually grounded someplace. You can ground the positive terminal and things will work just fine, some electronics devices will not work, but is by design more so than the fact that it can't work. Some old automobiles or other machinery they did ground the positive instead of the negative. That just means the frame of the machine is at same potential as the positive instead of the negative, current still flows the same paths and direction (with DC).

General rules in NEC usually require the "Neutral" to be the conductor that gets grounded, but if there is no "neutral" then pretty much any point in the system can be grounded.

We ground systems to make a reference to earth, and we extend this grounded conductor to all non current carrying metallic components of our system and call it the equipment grounding conductor. The purpose of the equipment grounding conductor is to create a low resistance path so if an ungrounded conductor comes into contact with one of the grounded components there will easily be a high level of current that flows which will in turn cause overcurrent protection devices to open the circuit preventing further damages or injuries to people.

 

[jim dungar]

The "center tap" doesn't even have to be ground. You can have one end grounded and still have 0-120-240 rather than 120-0-120.

Correct.

It's neutral only because its referenced to ground.

Incorrect.
The connection to ground is immaterial.
It is the neutral because it is 'half way' between the the other conductors.

 

[Electric-Light]

Correct.


Incorrect.
The connection to ground is immaterial.
It is the neutral because it is 'half way' between the the other conductors.

Are you sure about that? Center tap is not required for neutral.

In Norway, there is 230v delta, ungrounded. All the power at receptacles come from any two of three.

In some places, they use corner grounded delta, let's say u.
u=N/L1, v=l2,w=l3. so, its still 230v in whatever way but uv and uw yields L-N, vw yields L-L.
it is neutral as far as wiring is concerned, because it is bonded to ground, so that wire has no voltage with respect to ground.

If for some reason you choose to bond one of the branch on a 208Y/120 setup, and leave the not floating you'll have....
with respect to ground... 0, 208, 208, 120v(the knot). Even though the knot is in between all the wires, it would be improper to call the knot "neutral" because, it will be 120v above ground.

 

[mivey]

Are you sure about that? Center tap is not required for neutral.

If it were not the center-tap, it would not be the neutral but just the grounded conductor. It is a loose terminology issue because some people loosely call the grounded conductor a "neutral" when it is not.

...In some places, they use corner grounded delta, let's say u...it is neutral as far as wiring is concerned

It is the grounded conductor, not a neutral.

...with respect to ground... 0, 208, 208, 120v(the knot). Even though the knot is in between all the wires, it would be improper to call the knot "neutral" because, it will be 120v above ground.

That is because of loose terminology again. The "knot" is the neutral point of the phase conductors, even if we ground one of the phase conductors. If you want to be correct, you should distinguish between a grounded conductor and a neutral conductor. If you are just talking to someone using loose terminology and everyone knows what you meant, then it is not such a big deal.

 

[petersonra]

It is not unusual for people to refer to the grounded conductor as "neutral", even when it really is not the circuit neutral.

 

[kwired]

It is not unusual for people to refer to the grounded conductor as "neutral", even when it really is not the circuit neutral.

True, but they are setting themselves up for misunderstandings when they run into a grounded conductor that is not a neutral.

 

[petersonra]

True, but they are setting themselves up for misunderstandings when they run into a grounded conductor that is not a neutral.

not unlike when they call both the EGC and the GEC "the ground".

 

[Electric-Light]

How do you properly label each wire in this wiring then?

The blue is not a center tap, yet its grounded. if voltage is derived from a non CT transformer, does the grounded end get called "grounded side" as opposed to "neutral" ?

Reds are center taps and they're 120,120 and 208 above ground, however are these "neutral" ?

 

[Electric-Light]

I suppose this is one of those semantics thing like inflammable, which means that it *is* flammable, however non-flammable and flammable are preferred, because inflammable can be misinterpreted as "it does not burn" and here technical/grammatical accuracy is of lesser importance than consequences of misunderstanding.

A conductor that is labeled "neutral" but floating more than a negligible amount off ground can lead to shocks.

 

[kwired]

How do you properly label each wire in this wiring then?

The blue is not a center tap, yet its grounded. if voltage is derived from a non CT transformer, does the grounded end get called "grounded side" as opposed to "neutral" ?

Reds are center taps and they're 120,120 and 208 above ground, however are these "neutral" ?

There is no common neutral in that drawing. Each red could possibly be called neutral for its associated winding, but they are not grounded conductors, and if you were to connect this to some panelboard for power and lighting, you would need overcurrent devices on each of the red conductors also, so the panel would have five ungrounded buses.

The blue conductor that is grounded is exactly that - the "grounded conductor". It would need to be grounded either at the source or at the first disconnect, and then all equipment grounding conductors separated from current carrying "grounded" conductors just like any other system with a grounded conductor.

Kind of a clever way to get 120 volts from all three phases of a delta system, but finding a panelboard that would work with it would likely cost more than just using a wye transformer.