Hello, I am trying to size the neutral on the output side of a 225 KVA UPS. The UPS is 480V in and 208V output. I would normally size it to 100% of the cable size of any one phase but we are connecting single phase loads onto the panels feed from this UPS. I hae ben told that sizing at 100% is fine, but I have also been told that we should upsize the Neutral to 2 times the size of any one phase. How do you size the neatral? Assumming that the loads are not completely balanced, is there a way to determine the load that the neutral will cover?
Sizing the Neutral on the output side of a UPS
- Thread starter Gentles
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Sizing the Neutral on the output side of a UPS
If at any given moment, the loads are perfectly balanced among the three phases, and if there are no harmonics present, the current in the neutral wire will be zero. So at those times, it wouldn't matter what size the neutral is. If the loads are not balanced, and again if there are no harmonics, the current in the neutral will be no higher than the current in the most heavily loaded of the phase conductors. So at those times, it is sufficient that the neutral be the same size as the phase conductors.
All things change if the loads are harmonic. The problem is that in a three-phase wye configuration, certain of the harmonic currents will add up in the neutral. The worst case, as-I-recall-but-could-not-prove-because-I-forgot-how-to-do-that-math, is that the harmonic current plus the imbalanced linear current can be as high as 170% or so of the rated current. That is why it is not uncommon to use a neutral wire that is double the size of the phase conductors.
If at any given moment, the loads are perfectly balanced among the three phases, and if there are no harmonics present, the current in the neutral wire will be zero. So at those times, it wouldn't matter what size the neutral is. If the loads are not balanced, and again if there are no harmonics, the current in the neutral will be no higher than the current in the most heavily loaded of the phase conductors. So at those times, it is sufficient that the neutral be the same size as the phase conductors.
All things change if the loads are harmonic. The problem is that in a three-phase wye configuration, certain of the harmonic currents will add up in the neutral. The worst case, as-I-recall-but-could-not-prove-because-I-forgot-how-to-do-that-math, is that the harmonic current plus the imbalanced linear current can be as high as 170% or so of the rated current. That is why it is not uncommon to use a neutral wire that is double the size of the phase conductors.
john m. caloggero
Senior Member
Re: Sizing the Neutral on the output side of a UPS
It appears that the UPS system is a feeder for the purpose of supplying computer loads. Where the feeder is supplied from a 3-phase, 4-wire WYE system, the neutral will be subjected to third harmonic current that can reach approx twice the phase current. The neutral size for the feeder should be at least double.
The single-phase branch circuit loads can be supplied by a neutral the same size as the ungrounded conductor, where each ungrounded conductor has its own neutral. The third harmonic currents are additive when a common neutral is used for the three ungrounded conductors.
It appears that the UPS system is a feeder for the purpose of supplying computer loads. Where the feeder is supplied from a 3-phase, 4-wire WYE system, the neutral will be subjected to third harmonic current that can reach approx twice the phase current. The neutral size for the feeder should be at least double.
The single-phase branch circuit loads can be supplied by a neutral the same size as the ungrounded conductor, where each ungrounded conductor has its own neutral. The third harmonic currents are additive when a common neutral is used for the three ungrounded conductors.
rick hart
Senior Member
- Location
- Dallas Texas
Re: Sizing the Neutral on the output side of a UPS
Agree on the 200% neutral. 100% is asking for trouble on electronic loads.
My question:
Why is the input and output different voltages?
How will you ever perform maintenance on the UPS, bypass or work around it in case of failure?
I can't see how the two systems can be synchronized outside of the UPS.
Agree on the 200% neutral. 100% is asking for trouble on electronic loads.
My question:
Why is the input and output different voltages?
How will you ever perform maintenance on the UPS, bypass or work around it in case of failure?
I can't see how the two systems can be synchronized outside of the UPS.
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