Voltage drop

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Dustin Foelber

Senior Member
I have a property where there is a 100 amp sub panel fed from a 200 amp main for about a 100'. The sub panel is located in a garage coverted to a work shop. The voltage is 240. When we run heavy load on the main or the sub the voltage drops from the utility by 22 volts at the main and about 28 volts at the sub. THe utility has said the will not address this issue until after we upgrade the panel to 400 which is schedule for 3 months from now. The problem I am having is many of the saws and sanders we are using will not latch there running contactors because of the voltage drop. Is there a problem with using a buck boost so the voltage is starting at 264 and gives it some room to drop? Is there a better solution?????
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
100' sounds like you may have other issues. How about wire sizes and existing loads on service and sub?
 

kbsparky

Senior Member
Location
Delmarva, USA
You are experiencing a voltage drop of 6 volts. Well within tolerances mentioned but not enforceable by the Code.

Sounds like the utility transformer is undersized. I don't believe that a buck/boost transformer will help much when the supply transformer is maxed out like that ... :mad:
 

HotConductor

Senior Member
Location
Philadelphia
Is the service fed from pole mounted pots? If so, what is the distance of the tri/quadplex run to the service tap? What size is the tri/quadplex?

How many customers are on the same transformer?

Are you at the end of the run? If so you're getting the lowest voltage.

What is the voltage at the POCO transformer? You'll need the POCO to get that.

What is the distance of the service drop and what size and type of conductors are being used?

Like kb said your problem is not the 6 volt drop. Your problem is the 22 volt drop at the service entrance.

Do not use an autotransformer for this type of problem.
 

StephenSDH

Senior Member
Location
Allentown, PA
It is both your utility transformer and the feeder size to the subpanel. You really need to correct the issue, instead of boosting the voltage. Sounds like utility transformer is the primary culprit. You might have a hard time having them upsize their transformer. Changing your panel to 400A will not do anything for you unless they upsize their transformer.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100228-2028 EST

Dustin:

Your question lacks a lot of necessary detail. Thus, I will make some assumptions.

Assumptions:

At the main panel with no load you read 240 V. Loaded with 100 A steady state, max for your shop, (a continuous 100 A on a 100 A would be too high), this drops to 218 V. If the source and load were resistive, then the source impedance as seen at the main panel from and including the transformer and wire is about 22/100 = about 0.22 ohms. Power company is probably using small wires over a long distance from their transformer to your meter, and if it is a 25 KVA transformer then it has substantial internal impedance relative to your load. What is the distance from the transformer to your main panel?

Do you know what is the transformer size? Sometimes it is marked on the outside. What is the distance from the transformer to your main panel?

The additional drop from the main to the shop, a 100 ft distance, is 6 V at an assumed 100 A produces a line impedance of 6/100 = 0.06 ohms. That might be #8 copper.

At the main and the subpanel you need to define the no load voltages, and the voltage changes for a known load current. I suggest that to run the tests everything is turned off at the main except the breaker to the subpanel.

Would a boost transformer be useful, probably. But you really need to define your load currents, wire sizes, and material.

What are your large motors, size? Is there a lot of inertia load? How long to get up to full speed?

Is inrush your major problem?

Your present result may be what you should expect.

.
 

Dustin Foelber

Senior Member
Thank you for your input gentlemen.
The voltage no load is 240 volts steady. All conductors are properly sized and terminated from main to sub panel. THe problem I have is the utility in are area is not very accommodating to customers on these types of issues. Their stance is that this is designed for basic residential loads and we are running multiple motor loads so the voltage drop is not their issue.
I am trying to offer the customer an alternative. My main problem is with the units that are using the motor feed incorporated with a control circuit holding a run contactor coil. The voltage drop on itself and when other load are turned on keep opening the contactors. Also when other loads cycle I would assume the voltage drop cause amperage spikes to the motors running which could burn windings out. Any possible solutions not involving the utility would be greatly appreciated.
 

kwired

Electron manager
Location
NE Nebraska
on site power source is an option, but anything you do on your location to try to compensate for inadequate supply will also have other undesired results. total power need from the utility does not change no matter what you do, the most you can possibly do is use soft start equipment if starting current is the main problem, running load will still be the same.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100301-1133 EST

Dustin:

You need to provide us with what is the real problem. Is it just inrush at the start-up of a motor that causes the dropout of the relays? Or under steady-state conditions is there a problem?

If it is simply a start-up transient problem, then with all other loads off is it possible to successfully start the largest motor with the greatest inertia load?

If you start the largest first, and work down to the smallest last, then can you get all running? Under these conditions with no load in the house, and all machinery running and under moderate load, what is the shop subpanel voltage and load current?

.
 

mivey

Senior Member
Thank you for your input gentlemen.
The voltage no load is 240 volts steady. All conductors are properly sized and terminated from main to sub panel. THe problem I have is the utility in are area is not very accommodating to customers on these types of issues. Their stance is that this is designed for basic residential loads and we are running multiple motor loads so the voltage drop is not their issue.
It may not totally be their issue. The voltage at the service is definitely below what I would consider acceptable and I probalby would recommend an upsized service. But, most utilities would not be expected to serve a commercial load from a standard residential service. If you get a commercial type service, then the POCO should handle the voltage issue at the service without complaining about it.

As for what you do in the mean time: the transformer might get you by, but answer gar's questions first.
 

__dan

Senior Member
Voltage drop

The power company is heavily regulated by the state, so sometimes you have to use the right word or phrase corresponding to their regulated legal requirements. In this state, calling the utility and filing a "voltage complaint" gets the utility apparatus moving. Making the same call and reporting low voltage or imbalance does not have the same effect. Use the words that the utility legally responds to. Chances are, dropping 20 volts at the service at less than rated load is a violation of their responsibility to the DPUC or whoever their regulatory body is.

Given enough incentive, the utility should come out and put on a voltage recorder as a first step towards resolution.

Second: Use Sola ferro resonant constant voltage transformers sized and applied only to the control circuits for the starters. Check this load but I am guessing one Sola protected circuit for all the machine control circuits is 200 to 300 VA. Probably, put in a switch to power off the Sola when not in use. It has a small idling loss.

The Sola constant voltage transformer should stop the starter contactor chattering and clear the immediate problem. If you use one larger unit you will have to run separate 'line conditioned' control power circuits to each machine. Alternately, you could put one small Sola at each machine for the control circuit sized for the coil load. Those units look like big doorbell transformers.

Running the motors at steady low voltage ~ 220 volts, there will be some loss of torque compared to running at rated 240 volts. That may be OK depending on the capacity and working load applied. So, stopping the contactor chattering with ferro resonant transformers on the control circuits may be enough for your application.
 
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