k2x

The other students in my iec class hate when i ask questions because they want to get out early so i thought I'd ask a few here.

We learned that in a multi wire/ 3 wire/ two hots with a shared neutral , that if the loads are balanced on the A and B phase that there would be no current returning on the neutral. My question is that if I would open the neutral, say at the panel , would the two circuits still operate as if the neutral was connected?? Or , if there is no current on the neutral, what is on the neutral?? Thanks...

augie47

If you disconnected the neutral, the loads connected to the phase conductor would still operate as long as the current could find a path (from A to B, thru the loads or vice-versa). Unfortunately the loads would see a voltage based on the resistance of the respective loads. It is fairly common for this to occur with a loose or lost utility neutral. When the neutral is lost, unless the loads are totally equal, you will find high voltage on some, low on other. Usually the tell-tale fact is demonstrated by some lights getting bright and some dim.

raider1

I agree with Augie.

It really drives me nuts when people have this attitude in an apprentice class. These apprentices always want to get out early and not take advantage of the oppurtunity to learn about the trade. These are the same guys that come back after failing the journeymans test and say "you never taught me the stuff on the test."

Chris

augie47

We like to fuss at each other and argue over obscure Code sections , but, IMHO, the bottom line on these forums is that they can be a great help to folks like k2x in supplementing his classes. I commend him for taking the time to come here and ask. Hopefully, there will be justice and he will excel over the less industrious classmates.

480sparky

Click here to see a Powerpoint on what happens when you open the neutral of a multi-wire.

__________________
In winter, why do we try to keep the house as warm as it was in summer when we complained about the heat?

Loose Neutral

When you remove the neutral you will have a 240v circuit if it's single phase 208 or 480 for three phase. depending on transformer hookup and supplied voltages. The phases are 180 degrees out of phase or 120 out for three phase. When the neutral is hooked up any unbalanced current will go thru the neutral. unbalanced 3 phase neutral will really hurt. As far as asking questions, that is the time to ask. Missing details early on will hurt in the long run. Someone once said
" He who asks a question is a fool for a minute, he doesn't is a fool for life".

John Valdes

Screw the guys in class. Ask the questions. Let them leave if they want. I know you want to get along with the others, but you have to draw the line somewhere. I agree this forum can be an enhancement to your apprenticeship program. I wish I had a venue such as this when I did my apprenticeship. You will be fine. Be your own man. Ask! Ask! Ask!

alimaj

I wish to express my happiness at the encouragement extended to k2x, this is the way it should be , and I endorse the sentiments.

waco

Good question. The loads connected to the "hot" wires would be in series with one another across a 240 volt source. Bright and dim lights is a symptom.

Personally, I think saying the neutral is there to carry the "unbalance" of the loads is wrong when the circuit is a multiwire. In multiwire circuits, the neutral is there to serve as the circuit return for 120 volt loads.

Many 240 volt appliances include 120 volt loads such as timers, lights, and motors, but the 240 volt part (to my mind) is never unbalanced and never knows there is a neutral.

Loose Neutral

.........

raider1

In a multiwire branch circuit the neutral does not carry the full load of each 120 volt leg. The neutral conductor only carrys the unbalanced current between the 2 ungrounded conductors.

For instance if phase 1 has a load of 10 amps and phase 2 has 8 amps the neutral conductor only has a current load of 2 amps.

To say the neutral conductor of a multiwire branch circuit does not carry the unbalanced current is wrong.

Chris

waco

I don't agree. The loads across a single phase, multiwire circuit are independent of each "leg" and don't even know each other exist. If the neutral is lost, then they do interact, but then it is no longer a multiwire circuit.

k2x

Thanks for that link 480. I'll try to digest it over the next couple of days and get back to you. Thanks to the others too for responding.... Night shift tonight.. Gotta go...

InPhase277

The laws of physics don't need you to agree. If what you are saying is true, then the current in the neutral would be the sum of the currents in each leg. In reality, this isn't so. The neutral carries the difference between the two loads, so some current from each load must travel through the other load.

There is no need to argue this when a test is easily done. I have done it many times for my apprentices. All you need is two 100 W lamps and a source of 240 V, and a clamp on ammeter.

brian john

Waco has a skewed view of life, he is always right. Check any long post he is involved in and you see a man that can't see the truth for his ego.

Or is is a SH*T monger and like to stir things up.

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Brian John
Leesburg, VA

Magnettica

Another way to describe what happens if the neutral is disconnected is the circuit goes from being a parallel circuit with the neutral connected, to a series circuit without the neutral connection. You would have to apply the rules of series circuits and parallel circuits to understand what happens when the neutral gets lifted.

Toronto Sparky

Hey that reminds me of a service call I went to at a mall.
Who ever installed the parking lot lights ran 3- #6 in 1" conduit powered with three phase 600v. They then took them up the pole to three 347v ballasts then they fed each ballast with one phase and tied all the neutrals together and left them floating.
The mall claims they had been working like that for years.. Seems when the mall was built 600v ballasts very easy to come by. Not so much now..

5volts

He goes by his own book.

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raider1

LOL

Chris

waco

I have probably forgotten more electronic and electrical theory than you have ever known, but, if insults are all you have left, feel free.

Fact is, my ego doesn't demand I be right at all and I'm completely at ease with the uncertainties of the trade and I still learn something new every day. But not about meggers and not about center tapped, single phase sources.

There is no such thing as an unbalance in the 120 volt loads of a single phase, 240 volt system. They don't even know one another exist.