Voltage drop...OK?

Old 07-22-02, 10:40 PM
Steve Mclean
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Voltage drop...OK?

I live on a 50 acre farm. A couple electricians helped me get power to my mobile home some 800 feet away from Edison pole about 20 years ago. The system has worked nearly flawlessly all these years. Occassionaly a breaker will trip on the AC or clothes dryer if too many appliances are on. I did some checking: Voltage at 100 amp service near Edison pole is 118volts. Voltage some 800 feet away at trailer is 118 volts. When I run 9000 BTU 110 volt plug in type AC, voltage drops to 112. Microwave drops voltage to 110 from 118. One and half HP 220 volt pump goes from 118 to 115 volts. My question is, are these voltage drops within reason?

The basic setup is Edison has a 10KVA transformer serving this property. From this transformer an approximately 50 foot run of wire (I"m guessing #2 copper) go to 100 Amp panel. From there power runs about 700 to 800 feet via #2 Aluminum bundled cable in a PVC conduit. It's 4 wires: two hot, one neutral and one ground.

Old 07-23-02, 08:10 AM
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Depends on who you ask! The code outlines that 3% voltage drop on a branch circuit is acceptable, 5% from service to final overcurrent device. Most electrical equipment will specify approx 10% as a nominal voltage.
The main thing to consider is this, the larger the drop the harder the appliance has to work consequently, theoretically speaking, the shorter it's lifespan. The only way to lower voltage drops is to increase wire size to the load.
I think the ultimate answer to your question is to look at the individual laods and find out what the manufacturers limits are on voltage and operation and let that be your guide.
I am trying to give the pollitically correct answer here and I think I have done so. There are voltage drop calculations and I have brought them to light here before and basically been told I was getting too technical, so we wont go there again.
Old 07-23-02, 09:01 AM
Steve Mclean
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voltage drop attn sparksone

THank you for your reply re voltage drop. Some information I read was confusing. I.e. I got the impression voltage drop referred to the difference in voltage between household circuits that were not in use. In any case, it appears my situation is marginal, if voltage drop refers to the drop that happens when an appliance is in use. The microwave drawing down voltage by just under 6%. If I run the toaster oven and the microwave the voltage drops from 119 to 105. Guess that's over 10% and I shouldn't operate both at once. One day I hope to get larger wiring up the hill, but for now my budget says no. Any additional feedback is welcome.

Old 07-23-02, 09:22 AM
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To answer your question there is voltage drop that is inherent in the conductors themselves without any load. The conductors have resistance and will constitute a very small load that will cause voltage drop without the addition of any load.
As for Voltage drop the drop is dependent on the load. For example the higher the load with a given conductor and distance the the larger the voltage drop that will occur when the load is in use. Hence, if you limit the load you will limit the voltage drop. Change any of the variables and the voltage drop will change. The variables are the wire size, the distance, the load and the voltage. Change any of these and you will affect the voltage drop!!
Hope I answered your questions.
Old 07-23-02, 11:24 AM
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As sparks said that is marginal. I have done long runs too and get a little paranoid and use 4/oo alum. Like he said you need to fig out what is minimum. Maybe 105 is ok on some stuff,, the toaster it wont hurt but the micro is going to be more sensitive. Keeping load balance at the panel when running multiple equipment will help a lot. Gas dryer would be a big help as well as any major appliances being gas. I have converted migrant housing from hodge podge of stuff to all gas jut to keep load off elecrtic lines. Your well is on 220 and that helps.
Old 07-23-02, 12:10 PM
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If you normal operation in your home drops voltage to exceed 5 % you would be in a situation of concern considering this to be a house where the load diversity can change to a larger degree.

Considering that fact that you have been operating so long without adverse reaction it sounds like while marginal it is holding. Also considering the cost factor of increasing that feeder to your home 800' away you tend to want to avoid that option if there is any other options.

The normal method of correcting a voltaged drop problem that exists in a wiring design is to over size the wire to compensate for the distance thus lessening the voltage drop concern. In your case this would be a costly undertaking.

A couple of alternative thoughts that may be done in your case if to possibly install a boost transformer at your home and increasing the voltage by use of that boost transformer. This also could be rather expensive considering the 100 amp load factor involved.

A second option that would be great if available is if your Utility company's transformer has + or - adjustment to their voltage as an option within their transformer. Some older transformers had a series of lugs that the Utlity company could move the line connections to your home to raise or lower the voltage approximately 3% + or - for each lug the hot line is moved over. Uually there was to lugs to the + side that could increase you voltage output on the load side of their transformer that could compensate for you voltage drop concern. The voltage would read high on the beginning of you service conductor but by the time it gets to your home the voltage is more compatible to where it should be on the end of the service conductor. Check with your Utility company and see if your transformer has this buck/boost option on their transformer output. This may be an inexpensive approach to solve your problem if you are lucky enough that the Utility company can do this buck/boost adjustment within their existing transformer or possibly switch to a different transformer for you at a reasonable cost that does have this buck/boost option in their transfromer.

May be worth investigating that power company buck/boost transformer output option with your serving power company as an inexpensive option to solve you voltage drop concerns. They may have that option available to you.

Let us know how you come out

Good Luck


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