Is there a threshold for how much current should be on the neutral leg of a 3-phase system before taking steps to correct it?

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careful

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I know that the goal is zero current, but what would be a real-world ratio for neutral:hot amperage before taking steps to fix it. Assume a 3-phase 208V 4-wire system with single phase loads that have been balanced on paper, but real-world usage is not perfectly balanced.
 
I think the basic premise (goal of zero neutral current) is, at best, flawed- I would not be concerned if there was 100% of a phase's current on the neutral, that's what it's for and why it's usually sized for the full current. Granted one ought to take some steps to balance things out if this was a regular occurrence, but with varying single-phase loads its going to happen.

What's the context of the question?

(This is a problem with large (multi-MW) loads, like electric railways, but that's not the case here.)
 
careful said:
I know that the goal is zero current, but what would be a real-world ratio for neutral:hot amperage before taking steps to fix it. Assume a 3-phase 208V 4-wire system with single phase loads that have been balanced on paper, but real-world usage is not perfectly balanced.
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When your voltage imbalance becomes excessive, maybe >5%.
 
zbang said:
Amps are amps at any PF, so why the question? Is this more that a simple academic/code exercise?
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The question is about how much is too much, but in a real-world setting where there are multiple things to worry about. You wouldn't knowingly design an imbalance of 100%. Since I don't know of any value in code, i think its more of a standard practice question.
 
careful said:
That would be the limit of the conductor, yes, but power factor starts to worsen
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That is really not correct. Low power factor of the harmonic distortion variety may cause more neutral current, but in that case lowering the neutral current is simply an effect of achieving a certain power factor, not a goal in itself. In other cases, such as imbalanced load or displacement power factor, there is simply no such straightforward correlation between power factor and neutral current.
 
wwhitney said:
I think you could reasonably design a system that may occasionally or theoretically have an imbalance of 100%. E.g. any fully loaded feeder with only L-N loads. [This loading case would also control voltage drop considerations.]
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To clarify, you could design on the assumption that up to 100% of the occurent load might be unbalanced, but you wouldn't design so that 100% of the total connected load would be unbalanced.
 
wwhitney said:
Right, that wouldn't be "fully loading" a 3-phase feeder with L-N loads, that would be leaving 1/3 of the capacity on the table.

Cheers, Wayne
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I agree. It shouldn't overload anything either. In fact if it is three single phase transformers wye connected for the supply, you should be able to physically remove the one with no load on it and have no effects on what load is being utilized.
 
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