I'm a residential electrician and I'm getting into the commercial side of it. I know that if I have a lighting circuit that's pulling 10 amps, then 10 amps is flowing through the neutral back to the panel. So if I have multiple circuits that's pulling 10 amps and is sharing the same neutral; say circuit #1, #3, and#5, that will be 30 amps for the neutral to handle. If it's #12 wire then it will be too small for 30 amps. Not to mention if the lighting circuits are continuos. Does it have something to do with the phases?
neutral load
- Thread starter eric1973
- Start date
- Status
Eric, you are correct that in a two wire circuit the grounded conductor does carry the same as the ungrounded conductor, this is also true when using two phases and the grounded conductor of a wye system, however, barring any additive harmonics, the neutral conductor would be carrying zero amps if it were common to three equally loaded phases of a wye system.
Roger
Roger
- Location
- Logan, Utah
Yes, when you have a multiwire branch circuit then the neutral only carrys the unbalanced load of the ungrounded conductors. So in you example of three ungrounded conductors (hots) sharing one neutral each one of them drawing 10 amps you would have a neutral current of 0 because each phase is balanced.
Here is a thread on multiwire branch circuits and approximating neutral current
The formula for finding neutral current in a three phase system is:
The square root of I squared A + I squared B + I squared C - (IA * IB)-(IB * IC)-(IC * IA)
Where I = Amps
*= Multiply
Hope I haven't confused you too much.
Chris
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
That?s true, however confusing. The explanation comes from the fact that ?One Amp on Phase A? is not the same as ?One Amp on Phase B,? and neither is the same as ?One Amp on Phase C.? So you don?t add 10 plus 10 plus 10 to get 30. Like Roger said, when you add the currents from three balanced phases, the net result (i.e., the neutral current) is zero.
On the other hand, I can think of two difference examples of a ?worst case? imbalance. One example has Phase A and Phase B equally loaded (say, 10 amps each), and no load on Phase C. The other example has Phase A loaded (say at 10 amps), with no load on either Phase B or C. In both cases, the load on the neutral is the same (10 amps).
So you see, as long as you don?t overload any of the phase conductors, you won?t get an overload on the neutral. But as Roger said, if there are harmonics present, that statement is no longer true. The neutral load can exceed the phase load, when harmonics are present.
On the other hand, I can think of two difference examples of a ?worst case? imbalance. One example has Phase A and Phase B equally loaded (say, 10 amps each), and no load on Phase C. The other example has Phase A loaded (say at 10 amps), with no load on either Phase B or C. In both cases, the load on the neutral is the same (10 amps).
So you see, as long as you don?t overload any of the phase conductors, you won?t get an overload on the neutral. But as Roger said, if there are harmonics present, that statement is no longer true. The neutral load can exceed the phase load, when harmonics are present.
brian john
Senior Member
- Location
- Leesburg, VA
Eric:
A common problem we see is multiwire circuits not properly installed. Such as multiwire conductors on Circuit Breakers 1,3,7. Then you have A?, B? and A?,
the neutral can be overloaded under these circumstances and while this is easy to avoid, it is an installation error that is more common then one would expect.
In this case the A? currents are additive, with 11 amps on each phase conductor, the neutral current would be 19 amps.
A common problem we see is multiwire circuits not properly installed. Such as multiwire conductors on Circuit Breakers 1,3,7. Then you have A?, B? and A?,
the neutral can be overloaded under these circumstances and while this is easy to avoid, it is an installation error that is more common then one would expect.
In this case the A? currents are additive, with 11 amps on each phase conductor, the neutral current would be 19 amps.
Last edited:
- Status