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Why does voltage drop as circuits are energized

  1. Jun 10, 2012 #1
    Electrical question:

    I live in Mexico ( retired ex-pat ) and my electrical supply ( 240 volt single phase ) comes through a transformer that is almost 1 km away from my house. The supply lines are 2-0 aluminum.
    Because of the distance and from usage by others on the line, the voltage is often low when it reaches me. To try and rectify this problem I installed a custom built step up transformer at my house prior to the power entering the distribution center. The transformer has taps to allow for incoming voltage as low as 175 volts and up through 230 volts .... The secondary side boosts the voltage to 240 volts ( or sometimes higher ) regardless of the incoming primary voltage.

    Problem is that although I initially have 240 volts +/-, as soon as I draw power .... especially from pumps/motors/compressors, the voltage begins to drop drastically. The worst culprit is the well pump ... when we flush the toilet and the pump activates, and the voltage drops down low enough to kick in my generator when activates automatically when the voltage drops below 175 volts. Sometimes the pump will start and stop several times and the power surges each time, then it settles down and voltage returns to normal and the generator kicks out again. Other times it just bogs down at 175 volts and I have to manually open and close it's circuit breaker to get the power to flow again.

    We have had this problem, on and off for several years, but to a lesser degree. Recently the power lines came down in a storm and the power company repaired the line and re-hooked my wires to the supply lines. The draw down in power seems to be worse since then
    Several possibilities/questions"

    1) If by chance the power company accidentally hooked my neutral wire to a hot leg on the supply line and vica-versa with a hot leg, what would happen ? Would this just kick a breaker somewhere, or would the system still work, but be the cause of my problems ?

    2) In searching for the cause of the problem I notice that in my step up transformer, I have amperage draw ( 6 amps ) on the neutral output leg on the secondary side ...is this normal ?
    ..... I have two hot lines ( Primary coming in ) and three secondary output lines ...2 hots and a neutral. The original Primary neutral goes directly to the distribution panel where it is then tied to the secondary neutral produced by the transformer.

    3) Where should I set the taps on the transformer ? To match the incoming power supply line voltage, ( when all power is turned off, the incoming supply voltage reads 230 volts ).... or to match the voltage when all the equipment is activated. When there is heavy voltage draw, a meter reading of the incoming supply line often shows as low as 175 volts.
    Currently I have the taps set at 175 volts.

    On a heavy load day with a.c. units on and swimming pool pump running and fans in the house etc. the individual units together are tag rated at approx 45 amps. When the well pump kicks in to flush the toilet, the pump theoretically adds another 10 amps to the load
    .... total 55 amps.

    I have a 200 Amp service panel .... the step-up transformer is rated at 25 Kva ... and the breaker panel down the line ( 1 km away next to the main transformer ) is rated at 150 amps and the main transformer itself is 75 Kva.

    Other than the three possibilities I have posed does anybody have any ideas what is causing
    the huge voltage drop as heavy circuits are energized.

    Sorry for the long post, but better to explain the problem in detail
    Last edited by a moderator: Jun 10, 2012
  2. jcsd
  3. Jun 10, 2012 #2


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    My initial thought would be that your transformer cannot handle the load
    and in trying to do so that is why the voltage is dropping

  4. Jun 10, 2012 #3
    I am not familiar with power line stuff. I have a question.......Are you using regulating electronics to stablize the output voltage from the power line? ie input 175V to 240V, output 240V.

    I am referring to the conditions before the new power line was installed. You already had problem that the output voltage AFTER your step up circuit drooping down before the new power lines. If you truly have a regulated output, it should provide you with a stable output voltage regardless of the input voltage droop AND if your current drawn is under the max allowable. If you see output voltage drop when you put in heavy load, it's your regulation circuit that is not regulating or you over drawn......Isn't it?

    You said the output from the line can go down to 175V, is your regulation circuit able to provide 240V even when the line input is down to 175V?

    BUT if you are just putting a step up transformer, that will be dangerous. What if all of a sudden people upstream turn their stuffs off and the line goes to 240V, you use a step up transformer running open loop, you're going to get much higher voltage and might burn all your stuffs.
  5. Jun 10, 2012 #4


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    A drawing would be good.
  6. Jun 11, 2012 #5
    To Davenn ...... the transformer is 25Kva .... I would think that is more than sufficient. At 240 volts it's supposed to handle 104 Amps.

    To Youngman .... Not sure what you mean by regulating electronics, but if you mean is the transformer automatic, then no it;s not. I have to manually change the tap connections to compensate for the powerline incoming voltage.
    Yes, when I am using a lot of equipment and the draw is heavy... say around 40 amps the incoming powerline reads 175 - 185 volts but the transformer boosts the secondary ( output line ) up to +/- 240 volts. When I flush the toilet and the well pump draws another 10 amps, the incoming voltage voltage drops below 175 volts and that's when the generator kicks in.
    Sometimes the voltage comes back on it's own and the generator shuts off, but when it doesn't I have to manually flip the well pump breaker on and off a few times to restore enough voltage for the generator to kick off and the system to stabilize.
    Yes I understand about usage upstream being turned off and my incoming power increasing.... that's why I asked if I should set my taps at normal incoming voltage or at voltage under full load draw. The problem with the fluctuating line voltage is not really other people drawing from the line but rather power company line usage WAY upstream before the 75Kva subdivision transformer. My transformer manufacturer says I can go above or below tap set voltage by 20% either way and not hurt anything but they don't ( won't ) say for HOW LONG I can go without causing damage.

    To Digoff: .... I'm new to this forum and haven't figured out how to add attachments to posts but to try and make it clearer, there are three incoming lines from the 240 volt power line. L1 and L2 are routed to terminals H1 and H2 on the incoming lugs of the transformer and the neutral wire is routed directly to the neutral lug in the distribution panel.
    The transformer coils produce 240 volts ( from any tap setting ) and power is sent out on the secondary side through lugs X1 and X3 The transformer also produces a new neutral and it is sent out via lug X2. The leads from X1 and X3 go to the hot lugs in the distribution center and the new neutral from X2 also goes to the distribution center and is connected to the same neutral lug as the power line neutral that by-passed the transformer. The neutral lug in the distribution center is also tied to a physical copper grounding rod.

    Hope that clarifies a few issues ... does this alter your responses ?

  7. Jun 11, 2012 #6


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    The basic reason for the voltage 'sagging' under load is that there is always a finite amount of series resistance in the connecting wires. When a transformer "can't handle" a big load is because there is an equivalent resistance (a source resistance) due to the resistance of windings and also the magnetic properties of the core.
    For every Ohm of this total series resistance (for whatever reason - and one Ohm doesn't sound much, does it?) and for every Amp of current supplied, there will be a One Volt drop. So, with only One Ohm of resistance in all that cable / transformer etc. and 30A being drawn, you can expect to lose 30V!

    This is why they always try to deliver power at high volts to as near as possible to the consumer; the current can be kept to a minimum for a given power.

    Someone mentioned "regulation", earlier. This can be achieved by varying the 'taps' on a transformer to keep the delivered voltage more constant but, by the time you need to do that, you are scraping the bottom of the barrel, rather. But there may be no option, other than to completely change the feed arrangement so that high volts are delivered nearer to the consumer (costly!). Often you just grin and bear it and use regulated power supplies for the low power local equipment that is fussy about its supply volts (computers and measuring equipment, for instance)
  8. Jun 11, 2012 #7
    I see, that answer my question that you don't have a regulated voltage converter. You just have a step up transformer that has multiple taps to get the correct output voltage. The output varies with the input line voltage.

    As I said I am not a power person. I cannot answer any of the power hook up question. Sounds like you main concern is the generator kicking on when the line drop below 175V and that cause your main inconvenience. If you get rid of this problem, you can live with everything else.

    If so, you should look at the generator instead. I don't know how much you know electronics and how handy you are, but there got to be some sort of voltage divider and voltage sensing circuit in the generator that kick the generator on. IF you can get to that part and lower the threshold further to say 165V, then it won't kick on, and you'll be happy.

    Sorry I bud in if I am wrong.
  9. Jun 11, 2012 #8
    Hi Sophiecentaur,
    Your terminology is slightly above my comprehension of electronics but I think I get the general idea about resistance etc. Problem is, there is nothing I can do about getting the power company to install another transformer closer to my house. The power company's line ends at the main 75Kva transformer about 1 km from my house. From there the feeder line is privately owned by the owner of the sub-division, ( not uncommon in Mexico ) and the owner will not install a booster transformer at say the 1/2 Km point. The cost of a booster transformer is very high in Mexico and I can't afford to do it myself and no one else is prepared to chip in because most Mexicans just run a few light bulbs and a t.v. in their houses and they just learn to live with low voltage. As you said, .... changing tap positions from time to time to match the incoming line voltage may be the only alternative but it's not very efficient and it still doesn't solve the problem of the tremendous draw when the well pump kicks in.
    Hi Yungman:
    I'm not so concerned about the generator kicking in when the voltage drops below 175 ... it's more a case of how to prevent the draw that causes the voltage to drop below 175 in the first place.
    I'm not an expert in electronics but I don't think the generator can be set to kick in at a lower pre-set voltage and even if it could and the voltage could drop down to 165, that wouldn't help anything because the lowest tap position on my transformer is 175 volts.
    Leaving the tap setting at 175 volts just to accommodate the times when we flush the toilet might cause heat build up in the transformer windings and premature burnout.
    I have a request in to the transformer manufacturer to clarify this point but so far no reply.

    Currently the main issue seems to be how to stop the well pump from drawing such high amperage when it starts up ( I have a hard start capacitor wired into the pump but that doesn't seem to help ). The pump uses 9 amps to run but it draws 20 amps to start and that is what causes the massive voltage drop.

    Any suggestions on what I can do to the pump ????
  10. Jun 11, 2012 #9


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    If your generator is able to run the well pump, you could get a small water tank and fill this as needed using only the generator. Fill it when it gets nearly empty.
    Then you would not need to start the generator every time someone flushes the toilet.

    However, it seems that the system is overloaded and there isn't much you can do about it, short of spending money yourself to upgrade the electrical line.

    It may be possible to run a high voltage line using step-up, step-down transformers but this would be very expensive for one customer. This would cut down on voltage drops in the line.

    I'm new to this forum and haven't figured out how to add attachments to posts
    On the screen where you type your posts, at the top, there is a picture of a paper-clip. This is where you attach a drawing or photo.
    If the image is on a website, you can use the thing that looks like an orange rectangle with a circle at one corner. You can put an image on a website using various hosting services. Dropbox is a good one, and free.
  11. Jun 12, 2012 #10
    Hi VK6KRO,
    Thanks for your input. My generator can run my entire house with power to spare but it runs on propane and is quite costly to run....therefore it is only used for emergencies when we lose " power company " power ... which in Mexico is quite often.
    We also have a water storage tank on the roof called a " tinaco " ( 3000 liters ) which we also use during long power outages. The only problem is that it's gravity flow and does not produce much pressure. However unless we can find a way to reduce draw from the well pump we may well have to start using the "tinaco" permanently and refill it as needed.
    We don't " need " to start the generator every time someone flushes the toilet .... it starts automatically because of the voltage drop.
    You are right .... the system appears to be overloaded but mainly because of the distance the power has to travel, rather than actual usage overload. You are also right that I can't afford the in-line transformers to solve the voltage drop/draw problem.
    If I could solve the excessive draw of the well pump, we could live with the occasional inconvenience of having to switch things on and off to avoid overload.
  12. Jun 12, 2012 #11


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    You probably can't change the startup current of the pump, or the voltage drop in the power lines, so you are left with the generator.

    You can get the generator rewired so that you use the water tank to refill the toilet.

    Your pump must be causing lots of annoyance at present as lights go dim every time you flush a toilet.

    You could fill the water tank every morning at 5 am, for example and you would then have lots of water for the toilet and maybe a garden during the day. Gravity feed is fine to refill a toilet cistern.

    Does your drinking water come from this well too, or is it piped into the house after being treated?
  13. Jun 12, 2012 #12


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    I think you need a holistic approach to this. Your mean supply volts seem to be quite adequate, so there must be a solution. If the noise is acceptable then you should use your large water tank and run the pump at night (or at other programmed times) - except in emergencies. That would solve your most annoying problem, I think. If you are not happy with the gravity flow for showers etc. then buy a power shower pump which will draw less current than your main supply pump. Your toilet refill can be sped up for gravity feed by using the appropriate control valve and large bore pipes.
    You are lucky (?) in that you don't have the problem of large heating loads.
  14. Jun 12, 2012 #13
    Hi Sophiecentaur,
    Interesting concept about the power shower pump
    When I google it, all I get is info from England .... can't seem to find any suppliers/info/prices from the U.S.
    Can you offer any sources on the subject for the U.Sp.s. we use bottled drinking water and slow toilet refills are not a problem .... just the shower
  15. Jun 12, 2012 #14


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    I work with voltage drop at work a lot. To confirm Sophie's findings..........

    Pluggin your numbers in my fancy voltage drop calculator.......

    Assuming 3,000 feet is approximately 1 Km.

    At 240 volt single phase......here are your drops due to resistance in wire assuming you meant you have 2/0 feeders aluminum.

    10 amps you have a 10 volt drop....already marginal.
    20 amps you have a 20 volt drop.....
    50 amps you have a 50 volt drop....
    100 amps you have a 100 amp drop.

    The numbers are real....just a coincedence the amps equal the volt drops.

    Even if you install new feeders at 500 mcm which are capaple of 400 amps.....you will still have a 36 volt drop at 3,000 feet of distance!

    So it's no wonder you are having problems!!!!

    So ya, to agree with sophie again.....having high voltage delivered to a transformer in your yard is the true solution!
    But we clearly understand your need to find alternate solutions.
    Last edited: Jun 12, 2012
  16. Jun 12, 2012 #15
    Thanks psparky,
    Yes that sounds about right .... when I draw 50 amps, the voltmeter drops 50 volts
    However ..... that's without going through my step-up transformer.
    Going through my transformer with the taps set at 180 volts ( incoming ) to allow for this 50 volt drop, the secondary output power goes up to 230 volts and then drops to 208 with 35 to 40 amps of draw but the problem remains that when my water pump kicks in, the additional starting amps ( wow ! up to 18 amps ) sucks the voltage below 175 and my generator kicks in automatically.
    Running amps on the pump are 7.1 but the startup is whats killing me.
    Either there's something wrong with my transformer ..... it can't produce power fast enough to keep up with the sudden demand or there is something wrong with my pump that makes it draw so much starting amperage.

    Any additional thoughts ??

    Sophiecentaur had a good thought about using my water storage tank in combination with a power shower pump ..... waiting for his reply for more info on this concept.
  17. Jun 12, 2012 #16


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    Re. Power Shower pump. I live in the UK and it is a common thing in many houses here. All plumbing suppliers stock them in the UK. What is your hot water supply system? If you use an oil heater and hot water storage tank then, if you use wide bore piping from main tank to shower head, then you should be able to get a convincing flow of shower water without a fancy pump. Just an alternative head needed with larger holes (or drill out the existing one). Is your waer tank as high in the roof space as possible? Could you put it up on a tower? haha. I presume you don't use an electric shower? They really are a bit unconvincing compared with loads of hot water from a tank.
    A fascinating thread. We movde from Ohm's Law in electric wires to a similar situation in water pipes.
  18. Jun 12, 2012 #17
    Didn't realize you liven in the U.K. .... so you probably don't know where to steer me in the U.S. to find one of these pumps

    Does anybody else in the U.S. reading this thread know anything about these pumps ?? suppliers/prices etc.

    Our storage tank is on a concrete roof and hot water is produced by going through an instantaneous LPG gas fired boiler which can work either under pressure ( from the well pump ) or by gravity flow from the storage tank.
    Unfortunately the supply lines from the boiler and from the cold water tank are 1/2 inch, which works well under pressure but not so good under gravity flow.
  19. Jun 12, 2012 #18


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    It might be a cheaper solution to replace the 1/2 inch piping with 3/4 or 1inch if you do it with plastic pipe (not copper!!£££ or $$$!!!). The good thing about that solution is that there are no on-going costs. Plastic pipe is very good and easy to work with, too. Choose a short route with minimal bends.
    You could always raise the tank a metre above its present position too ??
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