Understanding the multi wire branch circuit/shared neutral

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K2X

Senior Member
Location
Colorado Springs
In our very large class last night I got a few questions wrong and to boil it down i think if i could understand the following I could understand why I got those questions wrong.

I have a 120/240 1 phase 3 wire panel and I run a multi wire branch circuit off breaker 1 and 3 and use a shared neutral with a 10 amp load on 1 and a 10 amp load on 3. So I've learned that if the neutral becomes open at the panel that my circuit becomes a series circuit and i am able to all the math on that senerio.

But, if I close the neutral on that circuit, i no longer have a series circuit? , even though the two loads are balanced and there are no amps flowing through the neutral.

So open neutral =no amps on the neutral, closed neutral= no amps on the neutral??

And what i really don't understand is if my closed neutral gives me a parallel circuit than why are my 10a and 10a not adding to 20 amps on breaker 1 and 20a on breaker 3??

Thanks for any help and the help in the past.:)
 

gar

Senior Member
090930-1127 EST

K2X:

I believe from your description that breakers 1 and 3 are 180 deg out of phase with each other.

Draw a schematic of a center tapped secondary with outputs A, B, and N (neutral and the transformer center tap). Instantaneously when A is positive by x volts B will be negative by x volts, and vice versa. If at any instant Van = y, then Vbn = -y, or Vnb = y. Notice the ordering of the subscript defines what point is the reference point. My choice here is that the second letter of the subscript is the reference point. Note: Vab = 2*y.

Your closed neutral is not a parallel circuit. You have two different voltage sources. If your first load is connected between output A and N, then its voltage source is Van. While your second load is connected between B and N, note B is not the same as A, and its source voltage is Vbn.

When you open the neutral the two voltage sources add together, Van + Vnb = Vab. Now you have a single voltage source supplying two loads in series. If Vab = 2*Van and you have two identical loads in series, then the voltage at the midpoint of the loads is Vab/2 = Van or Vnb. Since the midpoint of the loads has exactly the same voltage relative to point B as the midpoint of the transformer center tap there is no voltage difference between N and the load midpoint. Thus, with no voltage difference there will be no current flow when a neutral wire is connected between the transformer center tap and the load midpoint.

.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
But, if I close the neutral on that circuit, I no longer have a series circuit?
That is correct. For two things to be in series, the current flowing through the first must have no other path available then to go through the second. When you ?close the neutral,? as you call it, current going through the first load has two available paths. One path would take it through the second load, and the other path would take it via the neutral wire. The fact that the loads are balanced, and therefore none of the current actually flows through that second path, does not change the fact that the second path is available. All you would have to do is to reduce the load on one of the sides, thus creating an imbalance, and you would get current flowing in the neutral.

And what I really don't understand is if my closed neutral gives me a parallel circuit than . . . .
It doesn?t, as Gar has already mentioned. For two items to be in parallel, they must be connected to each other on both their left sides and their right sides. When you ?close the neutral,? the two loads are connected to each other on their ?downstream? sides (i.e., at the point of connection to the shared neutral). But they are not connected to each other on their ?upstream? sides, since they run to different breakers on the panel.
 

K2X

Senior Member
Location
Colorado Springs
Great!! Thanks for the responses. I think I'll be able to get it now, (after a few more readings). .. And thanks Larry for the pictorial of the center tap. That really clears up a lot of things. We'll have transformers next semester but it's nice to get a perspective on some of this. :)
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
And thanks Larry for the pictorial of the center tap. :)
The only "pictoral" was the image drawn in your brain, but, you're welcome.

And, you're also welcome to print out that post and share it with your class.
 

K2X

Senior Member
Location
Colorado Springs
The only "pictoral" was the image drawn in your brain, but, you're welcome.
It's a good picture... in my head. It will be very handy when we get to transformer theory and beyond.

And, you're also welcome to print out that post and share it with your class.
Thanks!! I''ll print a bunch up...
 
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