MWBC current question
- Thread starter cppoly
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wwhitney
Senior Member
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- Berkeley, CA
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- Retired
That depends on the phase angle between phases A and B.
If it is a 120V/240V system where the phase angle is 180 degrees, then the neutral current is just the arithmetic difference, or 5A.
If is a wye system where the phase angle is 120 degrees, then you need to use vector math. The vector sum 15/0 deg + 10/120 deg is 13.2/41deg, so the neutral current is 13.2A at 221 degrees.
Here I've assumed the current is in phase with the voltage. If not, the answers change.
Cheers, Wayne
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
As Wayne stated it is not so simple but I am going to bet you have a residence with single phase 120/240..... If so it is N1-N2= 15-10 = 5amps
3 phase has a more complicated equation
PaulMmn
Senior Member
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- Union, KY, USA
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- EIT - Engineer in Training, Lafayette College
PaulMmn
Senior Member
- Location
- Union, KY, USA
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- EIT - Engineer in Training, Lafayette College
If I remember enough about vectors... and I don't.
You're supposed to add vectors by combining them head to tail... and that ends up with more than 13.2A, if my lines are to scale!
You're supposed to add vectors by combining them head to tail... and that ends up with more than 13.2A, if my lines are to scale!
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- Placerville, CA, USA
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- Retired PV System Designer
The lines on the attached diagram are not to scale. But I believe 13.2 is correct. 15 and 15 would produce 15. 15 and 0 produces 15. In between you get less than 15.
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- Connecticut
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- Engineer
Sqrt(10^2 + 8.66^2) = 13.2
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- Placerville, CA, USA
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- Retired PV System Designer
The line between them represents the difference. The distance of that line from the origin does not really mean much of anything. And that distance is NOT 13.2.
Thanks for responses. My question is for 208/120V 3 phase, 4 wire systems in commercial buildings. I know 3 circuit MWBC has 0 current if all phases are balanced but was curious on what it would be for unbalanced loads for 2 circuits. The key takeaway for me though is that the currents are not additive in this scenario (i.e. 15A + 10A = 25A) on the neutral. And at the very least should NOT be higher than the largest phase current (15A is the maximum current the neutral wire would see - WORST case) right?
So a 2 circuit MWBC shouldn't have any concern on neutral current in this scenario. So that dedicated homeruns are not necessary.
So a 2 circuit MWBC shouldn't have any concern on neutral current in this scenario. So that dedicated homeruns are not necessary.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
That assumes the loads on both have the same power factor. Which may or may not be the case.
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- Connecticut
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- Engineer
The neutral current could potentially be higher than the largest phase current, again depending on the power factor.
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- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Sure but has anyone really seen this.... I guess the issue would be with nonlinear loads
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- Connecticut
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- Engineer
A 10A motor load with 0.9pf on A-N plus a 15A resistive load on B-N would give approx 17.3A on the neutral. Just basic loads, nothing to do with nonlinear.
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- Licensed Electrician
I believe you, but we are not burning up wires anywhere are we? That's the real question.
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