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#129054 - 07/07/04 06:28 PM EE question on neutrals  
PEdoubleNIZZLE  Offline
Joined: Jun 2004
Posts: 176
McKeesport, PA, USA
I have an electrical engineering question on shared conductors (such as the neutral on a 120/240 single phase service) (also, this is theoretical so assume all numbers are perfect and not rounded)
Lets say between the red hot and the neutral, there is a 120V 20A load, and between the black hot and neutral there is also a 120V 20A load. Is that neutral carrying a total of zero amps or fourty amps. I have thought it out and I can see how it could be either answer. Can someone help me with this?

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#129055 - 07/07/04 07:53 PM Re: EE question on neutrals  
Ron  Offline
Joined: Mar 2002
Posts: 582
White Plains, NY
If the red hot and black hot are on different phases of a 240V single phase residential system, then the neutral caries 0A. If the red and black hots are on the same phase, then the neutral carries 40A.

Different phases (180 degrees out of phase) allows the neutral current to cancel in this example.


#129056 - 07/07/04 08:39 PM Re: EE question on neutrals  
ElectricAL  Offline
Joined: Oct 2001
Posts: 597
Minneapolis, MN USA
Think of the transformer that is supplying the service. It will be a single primary winding with a single winding secondary. The connection at the primary is not important for the question you are considering.

240V is on the secondary from one end of the winding to the other end of the winding.

Putting a centertap in the secondary winding gives 120V above and 120V below. For the instant that the Line to Line voltage is at its max, the difference from line to line will be 240V, therefore, the 120V above and below the centertap add to each other.

Now, ground the centertap.

The line to line is still 240V. The centertap to line is still 120V. One of the voltages has to negative 120V while the other is positive in order to get a total of 240V.

So, paying attention to the direction of the current, one 20A line current must come out of the winding, while the other 20A line current is entering the winding on the other end of the winding. There is no current flowing from the centertap to or from the neutral.

Another way to get at this is to think of the current in each loop. . .half of the secondary winding, line, load, neutral, centertap; and the other half being identical. Both loops 20A currents will travel the same direction clockwise, then counter clockwise, alternating.

The neutral (common between the two loops) has two 20A currents in it going in exactly opposite directions, cancelling each other.

Hope some of that connected somehow. [Linked Image]

Edited for Sp. & clarity.

[This message has been edited by ElectricAL (edited 07-07-2004).]

Al Hildenbrand

#129057 - 07/07/04 09:32 PM Re: EE question on neutrals  
PEdoubleNIZZLE  Offline
Joined: Jun 2004
Posts: 176
McKeesport, PA, USA
Ron and ElectricAl,
Thanks for clearing me up on this. I have been trying to figure it out on my own but I could never figure out, until now, whether to add or subtract amps. But then I realized (Thanks to ElectricAl) That since one line would be pos and the other line would be neg. (At top of AC sine wave curve), the neutral is common and would be the neg. and pos. respectively to the 2 lines. Looks like I should break down the diagrams into the 2 directions of flow for AC.
Thanks again,

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