Question for sparkies

[Brad Buchanan]

This is a pretty easy question but I have my doubts:

Does the neutral conductor in a circuit carry the same current as the hot conductor(s)?

Thanks,

Brad

 

[Ted in NE-OH]

Yes unless you have a fault, short to ground.

 

[Russ from MN]

In a 110 V circuit, yes.

 

[Bob]

Not always!

A typical clothes dryer with a 4-wire plug would be one example.

One "hot" leg carries the heater load plus the 120 Volt motor load, and possibly a tiny control circuit load.

The other leg simply carries the heater load.

The neutral carries the motor load (and possibly a tiny control load),

The GROUND (HOPEFULLY) carries NO load.

 

[Geo-TH,In]

If you are talking about the electric service to a house, just measure it. The neutral wire is even smaller than the 2 power wires. If you have just two equal 110v loads, one on each power wire, guess what your neutral current will be?

 

[David G]

What are your doubts?

 

[buickanddeere]

The neutral caries the unbalanced current when used with two lines single phase and with two or three line with three phase.

 

[MarkB_MI]

There are exceptions. A notable one is a "three-wire" 120 volt connection. This is when two 120 volt circuits on opposite legs share the neutral. If the two hots are drawing the same current, there will be no current in the neutral. The primary advantage of this setup is reduced voltage drop.

Kirchoff's Current Law says the sum of the currents into a node equals the sum of the currents out of the node. If the "node" is your appliance, the hot and cold currents will be the same UNLESS there is another current path (e.g. a ground fault).

Ground Fault Circuit Interrupters work by sensing very small differences between the hot and neutral currents.

 

[John T]

Brad, you ask "Does the neutral conductor in a circuit carry the same current as the hot conductor(s)?"

THE CORRECT ANSWER IT DEPENDS

1) In a 120 volt single phase three wire (Line, Neutral, Ground) BRANCH CIRCUIT........... YES BUT SUBJECT TO any leakage or shorts which may be carrying a part of the return current elsewhere. IE an alternate (other then Neutral conductor)return path!!!

2) In a 120/240 volt single phase four wire (L1, L2, Neutral, Ground) MULTI WIRE BRANCH CIRCUIT, the Neutral carries ONLY the imbalance which may be ZERO AMPS if L1 & L2 were each carrying the same current (each are 180 out of phase remember), but as high as any one of L1 or L2 if the other carries nothing. STILL SOMEWHAT SUBJECT HOWEVER to non linear loads such as highly inductive (like transformers in power supplies or motors etc) in which case Harmonic Currents come into play. THEREFORE I never specified Multi Wire Branch Circuits in an office serving say a lot of computers although critical sizing of the Neutral might take care of the Harmonic Currents.

So the general answer is usually YES, the Neutral (in a 120 volt 3 wire circuit) carries all the return current which is the same the outgoing line conductor is supplying UNLESS there is a short or a fault or leakage returning that current via an alternate path.

John T

 

[dr sportster]

Your question relates to why it is important to properly place multi wire circuits onto breakers at the panel.If you don't use phase A and phase B ,but they somehow both get on phase A the current overloads the neutral as no cancellation occurs.

 

[John T]

If I recall correctly we always used TWO POLE BREAKERS with the tie for that very reason, they fed off the two different phases. Also if one circuit trips out the other does also so Billy Bob doesn't get electrocuted when he works at that receptacle not knowing it has two legs of 120 OUCH...........

John T

 

[buickanddeere]

(quoted from post at 10:08:52 11/03/14) Your question relates to why it is important to properly place multi wire circuits onto breakers at the panel.If you don't use phase A and phase B ,but they somehow both get on phase A the current overloads the neutral as no cancellation occurs.

A single phase 120/240V panel does not have two phases. It has a center tapped secondary on the utility transformer with the neutral in the middle and line 1 and line 2 on each end. Line 1 and line 2 are 180 degrees apart electrically which is why they have 240V between them.
A 120/208 three phase panel supplied from a Wye transformer secondary does have three phases. 120 degrees apart.

 

[Brad Buchanan]

Hey guys.

Thanks for all the informative replies.

I asked the question because I am wiring one side of my shop and was pulling wire through 2 1 inch conduit.

I figured it out and my rotary phase converter required #6 copper for the 240 volt load. The control circuits run on 120 volts which I pulled from one of the hot conductors.

I was actually wondering if I could have pulled a #12 conductor through for the neutral however I used a #6 tagged white.

I am always amazed by the amount of info on this board.

Thanks again,

Brad

 

[Dusty MI]

I don't know how your control circuit is wired/works, but most likely it should be fuse protected at 15 or 20 amps.

Dusty

 

[dr sportster]

Yeah thanks ,I went to apprentice shcool too.

 

[Brad Buchanan]

Hey John.

If I understand the theory correctly all the current (less induction and other losses) in a double pole 240 volt circuit is carried by L1 and L2.

Thus if I clamp my inductive ammeter over L1 and then L2 and add the readings the sum will be different than the current flowing through the neutral (assuming that the double pole circuits are drawing power).

Do I understand this correctly?


Brad

 

[Brad Buchanan]

Hey Dusty,

Actually I have a small breaker box near the
load and it has an additional breaker for the
240 volt circuit and a single pole breaker for
the control circuits.

Probably a bit of overkill but I wanted it to be
as safe as possible

Brad

 

[Brad Buchanan]

Hey Doc.

I was taught to 'balance the panel' in relation to 120 volt circuits.

Theoretically if you had an entrance panel and located ALL of the 120 volt on one side and all were drawing their rated current would this overload the neutral?

Would the main sense the overcurrent in the neutral?

What other effect might be noted?

Brad

 

[Brad Buchanan]

Hey Mark.

A three wire 120 volt circuit is a concept that I haven't heard of.

Would not connecting the two hot leads to any two breakers on opposite sides of the single phase panel qualify?

I don't get the impression that you are suggesting using a double pole breaker for two 120 volt circuits.

Brad

 

[John T]

"If I understand the theory correctly all the current (less induction and other losses) in a double pole 240 volt circuit is carried by L1 and L2."

If its a 240 ONLY circuit, L1 & L2 240 VAC, and NO Neutral, NO 120 volt loads from either L to Neutral, of course all the current is in L's you dont even have a Neutral to such a 240 volt only load

"Thus if I clamp my inductive ammeter over L1 and then L2 and add the readings the sum will be different than the current flowing through the neutral (assuming that the double pole circuits are drawing power)."

Not sure I understand your question?? If you look at the wires from your panel out to the transformer on the pole, if the 120 volt loads on Leg 1 are the exact same as the 120 volt loads on L2, NEUTRAL CURRENT IS ZERO. If theres 100 amps MORE current on L1 then L2, Neutral current is 100. BUT THIS ALSO APPLIES IF YOU HAD A 120/240 VOLT MULTI WIRE BRANCH CIRCUIT but those aren't favored.

"Do I understand this correctly?"

Sorry, Im not sure lol

 

[MarkB_MI]

The "three-wire" or "multi-wire" circuit just eliminates one neutral conductor. Typically you use something like 12-3 Romex, which has black, white, red and bare conductors. So there's only one neutral (white) conductor which is shared by the two hot (black and red) circuits. When I've encountered it, the two circuits had individual single pole breakers. But John T says it's a better practice to use a double-pole breaker, which makes sense to me; I don't know if it's code, though.

 

[Dusty MI]

A simple example.
A panel with 2- 120 volt circuits, 1 on L-1, and 1 on L-2, no other loads.
L-1 has a 10 amp load
L-2 has a 15 amp load
Then the neutral is carrying 5 amps

Dusty

 

[buickanddeere]

(quoted from post at 08:24:39 11/05/14) A simple example.
A panel with 2- 120 volt circuits, 1 on L-1, and 1 on L-2, no other loads.
L-1 has a 10 amp load
L-2 has a 15 amp load
Then the neutral is carrying 5 amps

Dusty

Less line losses as wasted heat too.