Acceptable unbalanced load current

[MFS-PDX-313]

working on a project for a customer. They are supplying three-phase 208 wye. I've got various loads in the circuit: some are single-phase line to neutral, some are three-phase, and a couple of them are single-phase line to line. I did my current calculations and I have 39.3A on L1, 27.0A on L2, and 26.3A on L3. Is there some regulation or standard that says what the limit is to unbalancing loads like this?

 

[ptonsparky]

Looks fairly balanced to me. What is your feeder size?

 

[MFS-PDX-313]

Don't know the feeder size. But that is about a 40% current unbalance.

 

[ptonsparky]

I would worry if that was a single load such as a motor. Not for multiple.

 

[ptonsparky]

Don't know the feeder size. But that is about a 40% current unbalance.

FWIW it's closer to 27%

 

[JGinIndy]

FWIW it's closer to 27%

I am having a senior moment how did you arrive at 27% ? I am sure I will look real stupid in a bit.

 

[MFS-PDX-313]

27% or 40%, both seem a bit high. Is it still acceptable to do?

 

[oldsparky52]

I'm interested in both, how the 40% and how the 27% are calculated .... please.

 

[MFS-PDX-313]

I've been told to average the currents, then take the largest difference in current over the average, then multiply by 100 to get the %

 

[ptonsparky]

I think this is right. I'm sure we will find out soon enough.



Your looking at current imbalance so 5th column over.

 

[ptonsparky]

As you all know, it won't be the first time I'm wrong.
This what I use for motor loads. It is not relevant to your mixed single and three phase with line to neutral loads.

 

[MFS-PDX-313]

I came across this: http://www.hvac.amickracing.com/Electrical Information/Current Unbalance.pdf

It also says that 10% is the maximum acceptable unbalance for current, but is that for a single load?

 

[ptonsparky]

I came across this: http://www.hvac.amickracing.com/Electrical Information/Current Unbalance.pdf

It also says that 10% is the maximum acceptable unbalance for current, but is that for a single load?

My spreadsheet uses 2%, for both voltage & amperage, and I use it for single motor loads. I would hazard a guess that it could be used for multiple motors to flag a problem.

 

[LarryFine]

In a largely-single-phase-load environment, constant large changes in load balance are unavoidable.

 

[infinity]

I did my current calculations and I have 39.3A on L1, 27.0A on L2, and 26.3A on L3. Is there some regulation or standard that says what the limit is to unbalancing loads like this?

There is nothing wrong with these numbers, they're just part of having 1Ø loads.

 

[don_resqcapt19]

I came across this: http://www.hvac.amickracing.com/Electrical Information/Current Unbalance.pdf

It also says that 10% is the maximum acceptable unbalance for current, but is that for a single load?

That is only for the current going into the actual load...has nothing to do with the balance on the electrical supply system.

There is no issue with your current balance on your panel. It is impossible to get the loads balanced when they are in use because they are not all on at the same time.