Phase balancing

[steve66]

I am wondering what would typically be considered an acceptable limit on the difference on phase currents. This is a 208V 2000A service that has about a 13% difference in phase currents. The power company says we need to balance the loads better. But 13% doesn't sound that bad to me. The acutal load is only about half the service size.

Steve

 

[charlie]

Re: Phase balancing

Steve, take a look at the way electric utilities load their transformers. We will estimate the demand on the information you give us as to the type of equipment and its usage to get a number. We will look at the square footage of a building to get a number. We will take 50% of a service and then multiply by 80% to get a number. We then will compare numbers and make a guess as to the size of transformer we need. If you are loading a service to 1/2 of its capacity, you are likely loading a three phase bank or PMT to its full capacity. With that in mind, 13% is too much but 1/2 of that is probably acceptable.

Now you have a choice. Either take care of the problem or go the utility commission with a complaint before you are disconnected. It is my guess that the electric utility will prevail and you will still be required to balance your loads or pay for the electric utility to replace the bank or PMT with the next size. This would be the same as reflecting harmonics into a transformer and we would charge a customer for a larger transformer since the customer caused the problem.

 

[iwire]

Re: Phase balancing

So the POCO undersizes the transformer and it is the customers fault?

 

[steve66]

Re: Phase balancing

The transformer is 750KVA. The load on the transformer is only about half of that. So, I don't see what the big deal is. 13% sounds about as balanced as possible to me. It's a medical office building, and most of the load is probably lights and receptacles.


Steve

 

[iwire]

Re: Phase balancing

Steve you are saying you are 260 amps out of balance, getting it closer than that should be pretty straight forward.

I would make a list of the loads on each feeder something may and probably will jump out at you as being the major culprit.

Once you identify which feeder(s) it will be a simple matter of pushing some branch circuits around.

If there are no 3 phase loads of the affected feeders you may just have to swap the phases of one or more feeders.

 

[pierre]

Re: Phase balancing

Assuming no 3 phase motor load on the lighting panels, how about using a clampon and seeing if you can swap a couple of the feeders to the panel(s) to help alleviate the situation.

Pierre

 

[steve66]

Re: Phase balancing

Bob:

The transformer has 2 service laterals. Each lateral had about 400-500 amps of load on it. If there is a 13% unbalance, then there is about 130 amps total unbalance. This would only be about 65 amps per lateral. Each lateral feeds 10 or 20 3 phase subpanels. That gives 6 amps or less unbalance on each panel.

I don't think there is going to be one big load that is causing the unbalance. I think it is divided throughout. One reason may be that in circuting each suite, I always started with lights at the top of the panel. A small suite may only have two lighting circuits, and they would both end up on the A phase. The loads would balance better for larger suites if they had 6 or more lighting circuits.

If the opinion seems to be that 13% is too much unbalance, I think the solution would be to take some of the lighting circuits on the A phase and move them to the B or C phase.

Steve

 

[iwire]

Re: Phase balancing

Steve it could be as bad as your figures show, 6 amps per panel.

But if it is out of balance that "perfectly" you are a very unlucky guy.

I must not know how to figure out of balance, 13% of 2000 is 260.

 

[charlie]

Re: Phase balancing

Originally posted by Bob:
So the POCO under sizes the transformer and it is the customers fault?

Yes, the electric utility sizes the transformer for the load, not the service. If we miss, we are responsible for replacing the transformer now or 30 years in the future. The EC will set the service based on Article 220 and even sell a larger service if the customer is willing to size it for the future. Article 220 doesn't allow for very much diversity that we know will be there.

Additionally, if load is added a couple of years from now, the customer will be charged for his service upgrade. The electric utility will replace the transformer at not cost. We don't go away and we have a true lifetime guarantee.

Now if a transformer is unbalanced, why wouldn't we require it to be balanced? We can not make full use of our facilities if it is unbalanced.

 

[steve66]

Re: Phase balancing

Bob:

The unbalance was 13% of the actual load. Not 13% of the total service size. So 13% of 1000 amps is only 130 amps.

Steve

 

[lady sparks lover]

Re: Phase balancing

I would say, 10% betwen the phase. In other words balance the phases within 10% of each other. I believe that's "good practice" if you want to be realistic. That's JMO!!!

[ December 17, 2003, 10:12 AM: Message edited by: lady sparks lover ]

 

[lady sparks lover]

Re: Phase balancing

 

[brian john]

Re: Phase balancing

I just reviewed the last ten load survey I have completed, these include hospitals, high schools, office buildings, news organizations, manufacturing facilities, stores, malls, with loads from from 600 amps through 3000 amps imbalance ranged from 2% to 9%, the average of the 10 surveys was in the 5%-6% amp range.

I think these load surveys are pretty representative of services I have seen over the years.

Another byproduct of current imbalance is voltage imbalance and that can cause motor damage and other PQ issues.