Neutral size at 173%

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relbas

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I am about to install 3phase 208 40 kw UPS system but the manufacture require the neutral wire to be sized at 1.75X phase conductor
it does not make sense to me.
Any ideas?
 
Expected harmonic currents in the neutral of a wye system are additive and will cause overloading of the neutral conductor. Possibly a little suprising they did not specify 200%.
 
They want to oversize the neutral on the output

Neutral output wires: rate for 173% of output phase current if feeding
all Switch Mode Power Supply loads without power factor
correction
 
relbas said:
They want to oversize the neutral on the output

Neutral output wires: rate for 173% of output phase current if feeding
all Switch Mode Power Supply loads without power factor
correction
Click to expand...


Yes, they are concerned with addtive harmonic currents on the neutral.
 
This is the same reason that a neutral is considered a current carrying conductor in art. 310.15(B)(4)(c). When there are major harmonics present , as there are in computer systems and fluorescent lighting, the neutral must be oversized to protect it from overheating.
 
relbas said:
They want to oversize the neutral on the output

Neutral output wires: rate for 173% of output phase current if feeding
all Switch Mode Power Supply loads without power factor
correction
Click to expand...
Single phase non-linear loads producing third harmonic.
These add arithmetically in the neutral in a three phase supply.
 
brian john said:
I should have been clearer, the input is 208/120 with a 1.73% over sized neutral or on the load?
Click to expand...
I have forgotten how to do the math on this one. But what I recall is that the maximum current that can flow in the neutral of a balanced system that has harmonics present is equal to the current in the phase conductors times the square root of three (approximately 1.732). The rest of this response will have to be supplied by others. :cool:
 
charlie b said:
I have forgotten how to do the math on this one. But what I recall is that the maximum current that can flow in the neutral of a balanced system that has harmonics present is equal to the current in the phase conductors times the square root of three (approximately 1.732). The rest of this response will have to be supplied by others. :cool:
Click to expand...
I think there is no one size fits all solution. The shape of the waveform determines harmonic content.
I have no difficulty in running a few calculations where the third harmonic is in excess or 70% of the fundamental.
 
I asked about this before and I think it was Dereck that told me that 1.73 was safe with the loads we would run into but on paper you could far exceed 1.73. (BTW, those are my words, not his and it was a while ago.)
 
infinity said:
In this white paper from APC they state that the theoretical highest harmonic current to be 1.7X the phase current.
Click to expand...
Not exactly what it states.
The theoretical maximum neutral current will be 1.7 of the rated phase current value, if
all circuits are loaded to max rating, no loads are above 675 W, and all loads are generating
third harmonic at the compliance limit.
Click to expand...
Quite a few caveats in there.
 
relbas said:
They want to oversize the neutral on the output

Neutral output wires: rate for 173% of output phase current if feeding
all Switch Mode Power Supply loads without power factor
correction
Click to expand...

Any compliance with Standards of your equipments, other than UPS, as mentioned in white paper in post #13 ?
 
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It has been a long time since I've seen neutral currents >100% of the phase current. Years ago, this was a problem, but not recently in loads I've seen, especially at the point of service entry (PCC).
 
Over loading of neutral is a problem in call centers of non-IEC compliant countries i/c U.S.However providing multi-wire for neutral,this problem may be solved.
 
ron said:
It has been a long time since I've seen neutral currents >100% of the phase current. Years ago, this was a problem, but not recently in loads I've seen, especially at the point of service entry (PCC).
Click to expand...

Almost all systems we see today are so over designed that neutral current issues are almost none existent. I have seen a few sites with harmonic issues in the past all were converted warehouses that became call centers, Race Tracks that became casinos, and a large data center converted from main frames to a call center.

Another area we use to see all the time were with generators not sized or equipped to handle the distortion.

One of the biggest issues I have seen with harmonics is software and hardware techs and engineers, blaming any equipment issue on harmonics, then when you prove there are no harmonic issues, they point to grounding.
 
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