Using a transformer to reduce voltage drop

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J

Junior_EE

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New York City
Here in NYC, we have had many discussions with electrical contractors about voltage drop code requirements. It always gets interesting. The following is the latest discussion.

We have 208Y/120V 3-phase, 4-wire service. Let's say the energy code requires voltage drop for a feeder to be no greater than 2%. There is a long run where the voltage drop is considerable, i.e. greater than 2% without upsizing wiring or simlar. To help, the contractor installs a 208D-208Y transformer with 2.5% taps at the beginning of the run. If the voltage drop of the feeder without the transformer is, say 5%, is it legal to install a transformer at the beginning of the run and to adjust the taps using the design load so that the beginning of the run is a 5% higher voltage (i.e. 218.4Volts) and the end of the run sees a voltage of 208Volts? Perhaps this is legal if the drop is 5%, but what about if the drop is 7.5% or 10%? Is it still legal to boost the voltage at the beginning of the run that much?
 
ActionDave said:
If you push your voltage to max at max load it will be too high during times of light to medium load.
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I guess that's kinda the question: Having what % voltage above utilization voltage is a problem? If you boost the voltage by 5% at the front end under maximum demand, when the feeder is lightly loaded, is being 2-3% above utilization voltage on the back end a problem? What about if you are 5% or 7.5% over 208Volts? Is that a problem? 1.) Is doing this allowed and 2.) to what extent is it allowed?
 
The transformer to compensate for voltage drop also doesn't eliminate the energy lost in the drop.

Say you had some sort of fancy tap changing transformer which adjusted the voltage at the beginning of the feeder so that the voltage at the load was 'correct'. You are still heating the wires and wasting energy, exactly what the energy code is trying to prevent.

-Jon
 
It would be more expensive because you would need two transformers instead of one, but instead of 208, step up to 480, then back down at the end of the run.
 
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