Voltage Loss

[RAC1]

I have a 480 volt 3 phase service feeding a 150kva stepdown transformer. Without tranformer connected I have 478 volts, after load is applied (50amp per phase), the voltage drops to 442 volts. Using all voltage drop calculations , all conductors, and actual voltage drop calculation, this should not occur but is. Any ideas on what to check for?

 

[brian john]

What is the load?

I assume all 3 phases are dropping the 442

I would measure the voltage at every accessible point from the primary of the transformers all the way back to the source.

 

[RAC1]

voltage loss

voltage loss

yes all phases are dropping the voltage within a few volts. I have been told that the line was tapped into underground but not sure of the method used or if i can access it reasonably.

 

[kwired]

How much voltage drop occurs in the supply transformer vs drop in the conductors to the point where you are doing the measurements?

If the POCO transformer is undersized you could have a lot of drop in their transformer alone.

Remember most of the time basic voltage drop calculations assume an unlimited supply capability, but truth is even POCO primary abilities are sometimes an important factor.

 

[RAC1]

voltage loss

voltage loss

the transformer is not a POCO, but a 150 kva from a MDC. it feeds a 150 amp 480 volt service to a building 954' away.

 

[ptonsparky]

A lot of missing information. Can you supply wire size, type, conduit?

 

[brian john]

If he does what was suggested in post 2 he can determine the source of the VD,

I would think it is either a transformer issue or a conductor issue. The chances that there are connections issues that equally drop the voltage under load seems remote.

 

[Smart $]

I have a 480 volt 3 phase service feeding a 150kva stepdown transformer. ...

... I have been told that the line was tapped into underground ...

the transformer is not a POCO, but a 150 kva from a MDC. it feeds a 150 amp 480 volt service to a building 954' away.

Not only missing information, but also confusing.

Generally speaking, an MDC (assuming main distribution center) cannot feed a service. Typically, a service feeds an MDC and the service ends there. The MDC then supplies loads either by branch or feeder circuits.

What line was tapped into? What supplies the tapped line? Need wire sizes, distances, load current, etc.

Where are you measuring the voltages?