Unbalanced single phase input power

[RudyL54]

What is the easiest way to balance the load of each line, assuming a single phase two-pairs input?

Right now, the voltage of one is about 120V while the other is only 85V.

There is only one neutral line. What bad effect can it do other than lower voltage level measure and cannot supply the required power.

This is for computer usage of servers.

 

[weh3]

You do not have a load balance problem, you have a grounding problem. The load refers to how much current is being supplied by each phase. The voltage from the utility should not change unless you have either a ground fault or an open neutral.

Either is bad and increases the risk of shock or worse. The grounded service conductor (neutral) and the equipment ground are the same conductor in a 120/240 V service from the utility. Call your electrician and verify the connection from the transformer neutral to your service entrance.

Regards,
William

 

[RudyL54]

What the contractor told us was that,one line is drawing more current because of too many loads compared to the other line. This according to him brings down the voltage.
I was skeptical about his comment.

Now, you told me something else which seems logical. So, what you are saying is, there could be a bad grounding or an open neutral? Could you elaborate on this?

These two lines supplied power to various servers in the computer room thru a UPS with battery backup. Could it be a problem between a UPS and the junction box connections?

 

[rbulsara]

please post the currents in each phase and the rating (nameplate data) of the UPS...

and what else is connected to this system?

It is still not clear to me, if the lines you are talking about are on load side of the UPS or the line side..

 

[buzzp]

Don't rule out having one phase overloaded. This could also be the issue depending on the transformers (single phase or three phase transformers). It could also be a grounding issue as suggested but there is only one neutral so it sounds like this may not be it. Much more info is needed on the transformer configurations, types, and measurements.
Low voltage causes lots of problems with different equipment. Don't assume that things just shut off on a low voltage situation. Some may use more power (motors in fridgerators, etc) while others will act strange. Definately something you want to follow up on to see where the problem lies.

 

[electricpete]

It sounds to me like the same power that is supplied to homes.

One possibility is an open neutral problem. If you are solidly connected on all 3 lines back to transformer you should not normally see this type of difference. An open connection at the neutral causes neutral voltage to float according to voltage divider determined by the connected loads.

Unbalanced loads among the two lines could be a factor if you are very far from the transformer (voltage drop in the lines).

Least likely, there could be some problem with the transformer.

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[weh3]

Where is your 120/240 coming from? The way I see it, it will be from one of three places:

1. A single phase 120/240 V service. You will have only one transformer on the pole in this case.
2. A three-phase 240 V open-delta service. You will have two unevenly sized transformers on the pole in this case, one of which is tapped in the center for your 120/240.
3. A separately derived system in your plant/house. You will have a 480 V three-phase service supplying a low-voltage transformer, probably a dry type, in this case.

The voltage supplied by the utility does not change according to your load (unless you have a very low resistance short circuit). The utility provides what is known as a stiff bus, also as a zero-impedance or infinite bus. The only way to change the voltage is to change the voltage reference (neutral or ground).

Regards,
William

 

[ccjersey]

What would it do to the voltages if the two lines are from the same phase and mistakenly share a neutral? This may be an attempt at a multiwire branch circuit (shared neutral) that went wrong in the implementation.

Also, switching power supplies are infamous for creating harmonics which increase the current on the neutral so that separate neutrals are specified for computer circuits.

 

[RudyL54]

Thanks for all your comments.

The lower voltage was measure after the UPS with battery back up. One line was 120Vac while the other was only 85Vac. The UPS KVA rating is 16KVA and the meter measured a load about 80% of its rating.

It could also be that the max has been reached because the power factor is only about 70%. What do you think?

I was thinking that if there's really overload after the UPS, it should not decrease the voltage because the rating hasn't been exceeded.

The technician measured the input voltages from the junction box and found the input voltage on one line is lower.

 

[weh3]

You're driving a 14 kVA 70%pf load with a UPS? It sounds as though you need a backup generator. The UPS voltage output can and will be dragged down by such a large load.

How much lower is the input phase?

William

 

[davidbeach]

I was thinking that if there's really overload after the UPS, it should not decrease the voltage because the rating hasn't been exceeded.

But the rating has been exceeded. A 16kVA UPS rated for 80% PF is capable of producing 9.6kvar. A 14kVA load at 70% PF requires 10kvar. The UPS is overloaded by 400var.