Balancing Three Phase Loads

[encoreman]

We are installing a new communications system for a customer. They specified that the system should be three phase. We are getting 480 volts from the customer's system which turns out to be three phase. We have already ordered the "shelter" that will house the communications equipment. I see our choices as these:
1. Connect a single phase transformer to the three phase system.
2. Replace the internals of the breaker panel with three phase guts and balance our load as best we can (the single phase air conditioning is already installed).

The transformer is already on site which means we may have to operate 120/208 on the single phase panel with reduced capacity (I'll have to see if there is enough power to handle it).

The customer may be able to balance their load if we run in single phase mode.

My question is:
Will this cause the customer any problems? Especially if he doesn't make any attempt to balance the rest of the load?

Thanks!
Jack

 

[charlie b]

Did you explain that correctly? You have me confused:

They specified that the system should be three phase. We are getting 480 volts from the customer's system which turns out to be three phase.

 

[augie47]

May just be a bad hair day, but after 3 reads I am still confused. You stated they spec'd 3 phase and the supply is 3 phase 480. Did you wire the "building" for 3 phase 208Y/120 or 240/120 ? Did you share neutrals ? What is the secondary voltage on the transformer ?

 

[the blur]

2. Replace the internals of the breaker panel with three phase guts and balance our load as best we can (the single phase air conditioning is already installed).
Thanks!Jack

That is not an acceptable UL practice.

 

[kwired]

That is not an acceptable UL practice.

Maybe. If you are buying commercial 20 inch wide equipment the cabinet and panelboard (guts) are separate items. You purchase the cabinet that fits the panelboard and any accessories ordered with it.

The so called "loadcenters" that come assembled are not as easy to interchange items, as the cabinets are more specific to a particular interior, but there are some cabinets used for more than one interior.

 

[Gac66610]

I don't do a lot of 3 phase work, so here i go
if it specs 3 phase and your supply is 3 phase why not just hook it up 3 phase?
did i miss something? :?

 

[kwired]

Here is what I find confusing in the OP:

They specified that the system should be three phase. We are getting 480 volts from the customer's system which turns out to be three phase. We have already ordered the "shelter" that will house the communications equipment. I see our choices as these:
1. Connect a single phase transformer to the three phase system.
2. Replace the internals of the breaker panel with three phase guts and balance our load as best we can (the single phase air conditioning is already installed).

The transformer is already on site ...

It looks to me like they specified three phase. You have three phase available - even a 120/208 transformer. Then your choices seem to indicate they sent you single phase panel with the equipment - is that right?

 

[encoreman]

I agree, I didn't do a very good job of trying to explain the problem. I expected you to read my mind .

Let me start over. This has happened to us several times so I am explaining a general example not a specific one.

We actually have two customers. The end user and the company that hired us, the general contractor. For this example lets just say we are responsible for supplying a shelter (a prefabricated building purchased from a manufacturer) and connecting power to the shelter from the end user's panel using a transformer supplied by the contractor.

The contractor told us they would supply 120/240 so we ordered a complete "shelter" that is single phase with single phase air conditioning and other equipment. It is designed to operate on 120/240 single phase.

When we get to the site we find out the end user has three phase. Let's say they have 1200 amp 480 volt service and they will supply us with power for the shelter. The contractor is supplying the transformer and it is on site, a three phase 120/208 transformer.

In this case, the contractor made the error in telling us we would be provided with 120/240 power. We have to find the best solution to solve the problem. Here are the choices I have identified:

1. Replace the single phase panel (or the panel interior whichever meets code) with a three phase panel. The air conditioner is the only thing that operates on 240 and it will operate on 208 as well. We then try to balance our load as best between the three phases on this panel.

2. Since our load is around 100 amps, we could use two legs of the three phase transformer and retain the single phase panel. There would be no load balancing.

3. Get the contractor to replace the transformer with 120/240 volt single phase.

As I understand it, there will be no impact on the shelter power quality except choice 2 which will reduce the ampacity of the panel. Any harmonics generated will not appear on the downside of the transformer.

In any case we will end up "unbalancing" the end user's three phase load.

Questions:
1. What symptoms can the end user expect?
2. How can we reduce or mitigate the effects to the end user (which choice should we choose)?
3. Can the end user balance the load on their side to minimize any effects?
4. Do I need to even worry about it, it is a problem for the contractor and the end user?

It is important to maintain positive relationships with the contractor and the end user.

Thanks!

Jack

 

[Smart $]

... (a prefabricated building purchased from a manufacturer) ...

IMO, if you decide to change over to 3?, you are going to have to work with the shelter manufacturer. Else you will likely void their warranty.

Operating single phase will place an unbalanced load on the service, regardless of transformer used. Without details of the service and its loading, it is difficult to say whether it is balanced as is or the level of ease or difficulty to rebalance with the new load.

As for transformer, I believe it proper to match to the usage.

 

[kwired]

You also need to make sure your total VA demand is acceptable to the transformer used. 25 KVA is 104 amps 240 single phase, but is only 69 amps per phase @ 120/208 three phase, if you feed single phase panel with three phase your transformer has some capacity available that is not usable. If it was sized by VA alone, based on what is needed for single phase, it could be undersized.

 

[encoreman]

What symptoms should the customer look for.

What symptoms should the customer look for.

I appreciate your willingness to help and the information you are providing in interesting and helpful. However, I am looking to answer these questions:

Questions:
1. What symptoms can the end user expect? (from the unbalanced load, for example, motor heating, computer problems, other equipment problems)
2. How can we reduce or mitigate the effects to the end user (which choice should we choose)?
3. Can the end user balance the load on their side to minimize any effects?
4. Do I need to even worry about it, it is a problem for the contractor and the end user?

I guess I also need to know how to measure the problems. Do I put an oscilloscope on the line and look for an imperfect sine wave? If I see an imperfect sine wave how bad does it have to be before the customer experiences problems?

Has anyone experienced problems with an unbalanced load? What did the customer complain about? How did you solve it?

Thanks everyone for your answers so far!

 

[kwired]

I appreciate your willingness to help and the information you are providing in interesting and helpful. However, I am looking to answer these questions:

Questions:
1. What symptoms can the end user expect? (from the unbalanced load, for example, motor heating, computer problems, other equipment problems)
2. How can we reduce or mitigate the effects to the end user (which choice should we choose)?
3. Can the end user balance the load on their side to minimize any effects?
4. Do I need to even worry about it, it is a problem for the contractor and the end user?

I guess I also need to know how to measure the problems. Do I put an oscilloscope on the line and look for an imperfect sine wave? If I see an imperfect sine wave how bad does it have to be before the customer experiences problems?

Has anyone experienced problems with an unbalanced load? What did the customer complain about? How did you solve it?

Thanks everyone for your answers so far!

Depends on how severe the imbalance is. There often is enough diversity in loads that you can not guarantee balanced loading anyway. If you know you have a fairly continuous load you should try to balance it. Doesn't mean you have to balance your addition across three phases - maybe if there is obvious enough imbalance on existing system you connect to two phases in a way that helps balance the load some. The biggest problem will be whether or not the transformer supplying you can handle the imbalance. With utility service this is usually not too big of a problem as long as you are not severely imbalanced, if you are using on site generation you often need to watch out so you don't imbalance too much.

You can have for example a 100 kVA three phase generator or transformer just don't put all 100 kVA on two lines.

Heard a story once about a new school that burned up a POCO transformer in fairly short time. They wired all rooms with three banks of lights on three switches. Each room had phase A supplying front row, phase B supplying middle row, phase C supplying back row. In mornings or evenings when building was in light use often times just the front row in everyroom would be on putting all the load at the time on phase A. They figured this eventually contributed to enough imbalance and overloading of that one phase to take out the transformer.

 

[jim dungar]

You may be solving a problem that does not exist.

You say the service could be 1200A 480 3PH and your unbalanced load might be 100A @ 240V this would reflect to the 480V as only a 50A load. 50 A unbalance on 1200A is only 4%, how much closer do you want to get?

 

[Strathead]

I get the impression that you are predominately concerned that the single phase HVAC load will unbalance the 3 phase system. If I am not correct ignore the rest. I so, your concern should be that it is good work practice to balance your loads, but Chances are really good that the "hut" manufacturer supplies single phase HVAC units even with the 3 phase preassembled buildings. Even in commercial applications, little 3-5 ton heat pumps are usually single phase anyway. They are quite a bit cheaper in my understanding.

 

[encoreman]

Single phase loads on three phase systems, issues.

Single phase loads on three phase systems, issues.

I talked with a power quality engineer at OGE Electric Services, an electric utility in Oklahoma City. Together we put this together. Comments?

Single Phase Service on a Three Phase Supply Circuit

In the event where we are adding a large single phase load to a three phase system, there may be some issues. Each case will have to be looked at individually. A solution to the problem will depend on the load, the supply voltage and possibly other factors.

Here are some different scenarios.

Supply Circuit: 480 volt, Three Phase
No transformer on site, circuit breaker two pole 100 amp minimum
Load: Continuous load less than 160 amps per leg at 120 volts (240 volts available).

Use a 480-120/240 50 kVA single phase transformer. Connect the 480 volts side of the transformer to the two phases with the lightest load. If less load, a smaller transformer can be used.

Supply Circuit: 480 volt, Three Phase
480- 120/208 volt 75 kVA transformer already on site
Load: Continuous load less than 160 amps per leg at 120 volts (208 volts available).

Connect the three phase transformer to load the using the two phases with the lightest load.

Verify all 240 volt loads will operate on 208 volts.

Supply Circuit: 480 volt, Three Phase
480- 120/208 volt 45 kVA transformer already on site
Load: Continuous load less than 100 amps per leg at 120 volts (208 volts available).

Connect three phase transformer to load using the two legs of the transformer that have the lowest phase load of the system supplying power.

Verify all 240 volt loads will operate on 208 volts.

Supply Circuit: 120/208 volt, Three Phase
No transformer needed, circuit breaker two pole 200 amp minimum
Load: Continuous load less than 160 amps per leg at 120 volts (208 volts available).

Connect single phase load to the two phases that have the lowest load.

Verify all 240 volt loads will operate on 208 volts.

The only time you would change out a single phase panel for a three phase panel would be when the load exceeds the capacity of two phases of the supply source (for example, 45 kVA three phase transformer on the supply side, needing more than 30 kVA on the load side).

For the most part the disadvantage of supplying single phase power from a three phase source is that there is not maximum utilization of resources. You could get more power from three phase equipment. For example if you want to supply the same amount of power to a single phase circuit you have to use a 75 kVA three phase transformer instead of a 50 kVA single phase transformer. In that case, one of the phases of the three phase transformer is not used.

Very few of the harmonics on the supply circuit will pass through the transformer to cause problems to the connected load. However, having an unbalanced load (different currents on the different phases) tend to amplify harmonics on the supply side of the transformer.

On older systems it was allowable to undersize the neutral or share neutrals on three phase systems. This has not be allowed for some time but it still occurs on older systems. When additional load is added to these systems especially significant unbalanced load the harmonics will amplify and there may be voltage issues on these system.

 

[Smart $]

Comments?

A lot of contingency planning there... :happyyes:

As I said earlier, "... I believe it proper to match to the usage."

Should there be compelling reason to not heed my advice, consider the following: In your 3? trannie scenarios, a 480-208/120 trannie is typically configured delta-wye. With only two lines of the secondary used as 120/208 1? 3W source, there will still be current on all three primary lines. The primary line common to the two loaded secondary windings will have 1.732 times more current on it for balanced loading of only two secondary lines. For example, if you have 100A on each of secondary lines "a" and "b" (and 0A on "c"), primary A will have 43.3A, while B and C will have 25A each. You will have to "rotate" the connected windings such that the primary line with the most current (A in the example) is connected to the 480V supply line with the lightest loading.

 

[encoreman]

I concur

I concur

I concur, you should always match load to the equipment. I shall do as you recommend!