Understanding the Neutral?

[DirtyLunka]

Let me start by saying I'm a newbie here and really like the site. I have 2 years in the trade and I am starting to ask questions towards the theory more than the code and construction. If the neutral carries the unbalanced load how is that it does not trip the breaker with current going back on a grounded conductor? Also can you ever get a voltage reading on the neutral? I have been driving some of the journeyman crazy with my questions, but I want to know how electricity works rather than just put it together.

 

[charlie b]

You are talking about a multi-wire branch circuit, in which a single neutral is shared with two (or three) ungrounded conductors. The current returning to the panel via the neutral conductor is the same current that left the panel via one of the two (or three) breakers and via one of the two (or three) ungrounded conductors. Since it passed through the load, before heading back via the neutral, there was enough resistance to keep the current well below the value that would trip the breaker.

Yes, you can get a voltage reading on the neutral. The closer to the panel that you take the measurement, the closer your reading will be to zero volts.

Welcome to the forum.

 

[DirtyLunka]

Thanks for the quick response Charlie. What if it is not a multiwire branch circuit and it is just a dedicated circuit? Does the neutral still carry the unbalanced load back to the panel?

 

[LarryFine]

Dirt, welcome to the zoo. :smile:

Your question is really a hodge-podge of several different topics. You first need to understand simple electrical flow, but here's a little ditty I typed up some time back. See if it helps, then come back and ask some more questions:

http://forums.mikeholt.com/showpost.php?p=708650&postcount=4

 

[charlie b]

What if it is not a multiwire branch circuit and it is just a dedicated circuit? Does the neutral still carry the unbalanced load back to the panel?

I don't have time to check out the link that Larry gave. So I'll just mention that the term "unbalanced load" does not apply, unless you are in a situation in which some load will be on one phase, and some load will be on another phase, and the two phases share a neutra. If you have one ungrounded conductor serving a load, and if you have one grounded (i.e., "neutral") conductor returning from that load, then that neutral is not carrying the "unbalanced load," but rather is carrying all of the load.

 

[roger]

Thanks for the quick response Charlie. What if it is not a multiwire branch circuit and it is just a dedicated circuit? Does the neutral still carry the unbalanced load back to the panel?

If you are referring to a two wire circuit, the grounded conductor carries the same current as the ungrounded conductor.

Click HERE for some basic illustrations of a single phase system neutral.

Three phase Wye systems get a little more involved.

Roger

 

[220/221]

Learn to follow the path.

Basically,

A 120V circuit flows from the ungrounded source, thru the hot wire, thru the light and then heads back on the grounded conductor.

A 240V circuit will flow from one ungrounded conductor, thru the heating element and back on the other grounded conductor. The ungrounded conductors are out of phase so the electricty is flowing in different directions on the ungrounded conductors 60 time a second

Technically....ask someone else.

 

[DirtyLunka]

Thanks guys! Still not a pro on this but your replies definitely opened up my eyes on this subject.

 

[K8MHZ]

I don't have time to check out the link that Larry gave. So I'll just mention that the term "unbalanced load" does not apply, unless you are in a situation in which some load will be on one phase, and some load will be on another phase, and the two phases share a neutra. If you have one ungrounded conductor serving a load, and if you have one grounded (i.e., "neutral") conductor returning from that load, then that neutral is not carrying the "unbalanced load," but rather is carrying all of the load.

Are you forgetting the use of a neutral in a single phase 120 / 240 volt multi-tapped transformer type service?

The NEC differentiates between those two types of neutral applications when it comes to fill and derating calculations.

 

[don_resqcapt19]

Are you forgetting the use of a neutral in a single phase 120 / 240 volt multi-tapped transformer type service?

The NEC differentiates between those two types of neutral applications when it comes to fill and derating calculations.

How does the use of the conductor or type of system have anything to do with the fill rules?

 

[musthafa]

Understanding the Neutral

Understanding the Neutral

I am also a new member (Sept 2008); nice to see your post.

Your post has three issues:-
1. the neutral
2. the neutral in a multi wire circuit
3. Balanced load of a multi wire circuit
4. Tripping during unbalanced neutral current

I understand that Charlie b and our Moderator Roger have given you good explanation. I hope the following small brief also will help understand your question.

Electrically, Neutral is a reference point or a path which is common to a multi group( multi phase) power source. It shares the return path of each group (supply and return) of the circuit in to the power source. It has equal affinity to each the circuit if these circuit are in equpal power and behaviour (this happens when we have a equal/balanced load). It has no special affinity OR it has zero special affinity to any of the circuit. At this time the current throug the nuetral is zero FOR A BALANCE MULTI CIRCUIT LOAD. It need not to take any special efforts to bear the net current of the banced circuit.

This is not the case when the multi circuits having different power and behaviour are connected to a power source. The neutral has to carry more current as a return from the higher power load which is different in quantity and bebaviour than the other circuit load. Now the load is unbalanced; and the neutral carries this unbalanced load. Deffinitely neutral need to show special affinity to this load and has to do special effort to carry this current; work has to be done by the neutral and definitely there will be some voltage drop and we can measuere this voltage with a precise measuring equipment.

When we are talking about a single circuit (two wires - supply and return) theoretically, we do not have phase wire and neutral wire. We have one supply and one return; and each one carry the current in both direction in 60/50 time per second depending of the system Frequency.

The concept of phase and neutral comes only and when we connect this circuit to a multi phase power source. The wire which we connected to the NEUTRAL of the source obtain the reference as neutral and the other connected to the system bears the reference as phase. As far as this referred neutral is only the part of single cicuit, we do not have shared neutral for this circuit. So the question of balance or unbalance load does not arise.

Eventhough we connect this two wires to a dedicated single source of the secondary the shared neutral does not exists.

For a multi phase circuit, if we provide appropriate monitoring device with the neutral, the breaker should trip. It depends upon the requirement. In residential and small commercial installation we do not use this protection normally. We do this system in Main Switchgear to safeguard the transformal neutral current.

Feel free to comment; it will enhance my knowledge also.

 

[quogueelectric]

Let me start by saying I'm a newbie here and really like the site. I have 2 years in the trade and I am starting to ask questions towards the theory more than the code and construction. If the neutral carries the unbalanced load how is that it does not trip the breaker with current going back on a grounded conductor? Also can you ever get a voltage reading on the neutral? I have been driving some of the journeyman crazy with my questions, but I want to know how electricity works rather than just put it together.

The resistance of the load limits the current to within the range of the breaker protecting the circuit. Without a load there is no current flow and the other extreme when there is a short circuit you only have the resistance of the wire in the entire closed loop limiting the max current from any particular voltage source.

 

[Besoeker]

Let me start by saying I'm a newbie here and really like the site. I have 2 years in the trade and I am starting to ask questions towards the theory more than the code and construction. If the neutral carries the unbalanced load how is that it does not trip the breaker with current going back on a grounded conductor? Also can you ever get a voltage reading on the neutral? I have been driving some of the journeyman crazy with my questions, but I want to know how electricity works rather than just put it together.

Ground and neutral are two quite different things.
For a three phase system, the neutral will carry the (vector) sum of the phase currents back to a point on the supply, possibly the star point of a transformer.
The ground connection sets the potential of the system with respect to ground. It is not a current carrying conductor in normal operation.

 

[ronaldrc]

If you can see through all my clutter on these examples they might help this is a single phase neutral and how it works to balances out.

http://home.comcast.net/~ronaldrc/wsb/The_Edison_Neutral.html

 

[glene77is]

Sorry, wrong part of messages.

 

[glene77is]

What if it is not a multiwire branch circuit and it is just a dedicated circuit?
Does the neutral still carry the unbalanced load back to the panel?

It is not spoken of in that manner.
But, In this case,
YOU would call the whole circuit 'unbalanced' and
YOU would say that all the 'unbalanced' current flows through the neutral.

...