Neutral calc.
- Thread starter Sharpie
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
What do you mean by "full size"? If you mean the neutral must be the same size as phse conductors, then you are not reading it right.
The neutral must be large enough to have enough ampacity to handle the calculated neutral load. That is not always the same as the calculated phase load. So a neutral can be smaller than the phase conductors, even without taking the 70% factor into account. For example, if you calculate the phase load at 500 amps, and the neutral load at 300 amps, you can pick conductors with those ampacity values. But if you meet the exception in the rule you cited, you could pick a neutral conductor with an ampacity of 270 (i.e., 200 plus 70% of the remaining 100).
The neutral must be large enough to have enough ampacity to handle the calculated neutral load. That is not always the same as the calculated phase load. So a neutral can be smaller than the phase conductors, even without taking the 70% factor into account. For example, if you calculate the phase load at 500 amps, and the neutral load at 300 amps, you can pick conductors with those ampacity values. But if you meet the exception in the rule you cited, you could pick a neutral conductor with an ampacity of 270 (i.e., 200 plus 70% of the remaining 100).
iMuse97
Senior Member
- Location
- Chicagoland
Per 220.61(C)(2), I think that you shall not reduce the neutral on any 3P,
4Wire system--on the portions where there are non-linear loads.
Since many, if not most, 3P commercial/industrial loads are non-linear, I've never reduced the neutral in such a situation. Many times in recent years the engineering specs. an increased neutral sizing.
4Wire system--on the portions where there are non-linear loads.
Since many, if not most, 3P commercial/industrial loads are non-linear, I've never reduced the neutral in such a situation. Many times in recent years the engineering specs. an increased neutral sizing.
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
What I am saying is that you size any wire for the calculated load. That includes the neutral wire. When you do that calculation, you sometimes get a result that lets you use a smaller neutral wire than the phase wires.
That said, however, in most cases I will not bother calculating a separate value for neutral load. I will simply use the same size neutral that I use for the phase conductors. I think Karl is saying essentially the same thing. In addition, in special cases (that I, for one, seldom encounter), it is appropriate to use a larger neutral, to account for a high percentage of harmonic loads.
That said, however, in most cases I will not bother calculating a separate value for neutral load. I will simply use the same size neutral that I use for the phase conductors. I think Karl is saying essentially the same thing. In addition, in special cases (that I, for one, seldom encounter), it is appropriate to use a larger neutral, to account for a high percentage of harmonic loads.
Some computer and receptacle loads may be non-linear, by I don't think there are enough to worry about in a typical office building. I think manufacturers have blown this issue out of proportion in an attempt to sell more expensive stuff. And many engineers have jumped on the bandwagon.
I'm not saying non-linear loads are never a problem, but its just not common enough to apply to an average service or feeder.
Even electronic lights ballasts usually only have 10% or 20% THD, so I wouldn't even consider those nonlinear.
Steve
iMuse97
Senior Member
- Location
- Chicagoland
You are probably right in nearly all circumstances--I was just spouting the line from the ongoing sales pitches we all hear. I will be more careful who I listen to. It helps me to try to formulate a sensible answer to questions such as this and hear the responses because you find out where the "real facts" lie.
iMuse97
Senior Member
- Location
- Chicagoland
Yes.
And yes, who am I to argue with Charlie?
My experience, and the talk around the shop, and almost every installation from resi. to very large industrial leads me to the conclusion that SOP in this area of the country is to never downsize the neutral. Of course, this is the region where MC and AC cable are looked upon with disdain, and many homes are still done in EMT.
I've had discussions and even arguments about doing this and the appropriateness of it, but never actually installed a neutral smaller than the phase conductors, even in resi.
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I've had to deal with some neutral overloading problems due to non-linear loads. One of them was in a very up-market hotel in the Middle East where a substantial proportion of the load was non-linear single-phase.
In many parts of the world, LV distribution is from a wye connected distribution transformer which provides 400V 3-phase and 230V single phase (to neutral) or similar. Generally, the system will be designed to try to ensure reasonable balance on each of the three phases,
Non-linear single-phase loads generally have high third harmonic content. At three times fundamental, this harmonic (and others) add in the neutral. And that can be a problem.
Would this translate to a typical office?
Probably.
I looked at our current waveform at home today and I tried to make it look like a typical small office.
A couple of computers, some electronic ballasts, some halogens, a couple of LCD displays, some ceiling fans, printers..
A bigger office would just have more of everything but that doesn't invalidate my point.
THD was more than 40%.
PetrosA
Senior Member
- Location
- Lancaster, Pennsylvania
I just read this article yesterday and this thread is probably a good place to ask what your opinions are about upsizing the neutral in non-linear load situations. The article contends that in some (increasingly common) conditions, neutral sizes should be doubled or even tripled. Thoughts?
article here
article here
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