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Discussion Starter · #1 ·
I am building a new home 500 ft. from the nearest transformer. The light company set a pedeastal 200 ft. into my property. At the time they did not tell me they were allowed a 3% voltage drop to the pedastal. If I have a max load of 278 amps and run 300 mcm cable I will have another voltage drop of 3%. Now the light company is saying every time a motor kicks on, the lights will dim. They suggest I should have put a transformer on my property and not a pedastal, even though they suggested it. Would I be better off to run double wire in parallel series, or just let the light company install the transformer?
 

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I would go with the transformer. Voltage drops will tax everything that's electronic, today that is almost everything including the toaster.
 

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DGR,IABD
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There's a couple of things that stick out to me right away. The pedestal is 300 feet from the house, at which you have a nominal voltage available of around 223 volts after the voltage drop. If you run 300 MCM from the pedestal, you'll have another 6.1 volts of drop, or ending up with about 218 volts at the house panel. That's a tad low, but we're assuming that you'll actually have a 278 amp load at the time a motor kicks on. This may or may not be true. Most residential HVAC equipment is rated 200-230 volts on the data plate, since it is also installed in light commercial applications where the nominal voltage is 208 volts.

Are you running the conductors from the pedestal to the house? 300 MCM cable is only rated for 240 amps per the NEC, and 350MCM is only rated at 260 amps. If you ran 500MCM for that last 300 foot run, you'd only have another 3 volts drop, which would put you right at 220, and you'd be fine.

Be advised that if your load at the time that a motor kicks on in the house is only around 150 amps, you'd only have around 2.4% drop for the entire 500 foot run, which would put you at about 225 volts. The load at your house is dynamic, and the 278 amp calculated demand load is a worst case scenario.

You may still have a little wink from time to time when an air conditioner kicks on, but lots of people do. It will cost you AT LEAST several thousand dollars for the power company to extend their primary and set the transformer closer to your home. I'm not trying to talk you out of having the transformer put closer to the house, but it might not be the doomsday scenario that you might think. Another thing that the power company isn't telling you is that they have taps on their transformer that they can set for +2-1/2%,+5%, and +10% to offset voltage drop. If they set it for even just +2-1/2% from the get-go, you'll be fine.
 

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Discussion Starter · #4 ·
Voltage drop

Thanks so much for your response. I wll talk to the power company tomorrow. If they will compensate for the voltage drop to the pedastal that would be great. If they don't, the 500 mcm cable may cost me as much as the transformer. The power company said ballpark figure of $ 5k.
 

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DGR,IABD
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Yeah, what you need to ask them is if the can "change the transformer tap to plus 2-1/2%" to offset part of the VD. That would be the proper nomenclature to use. With the way copper prices are going, it's hard to tell if buying 500mcm or paying to have the transformer moved would be cheaper. I'm sure you'll find all that out.
 

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Pay more attention to md. It's been 21 yrs. since I worked for Duke.
 

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Discussion Starter · #8 ·
Voltage drop

Well, I thank both of you guys. There are always cost over runs it seems and anyway to save a few dollars helps. I just wasn't too crazy about my lights dimming everytime the ac or water well motor kicks on and I sure don't want premature motor failure because of low power. I'll let you know what the power company tells us.
 

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Discussion Starter · #9 ·
Texstar said:
Well, I thank both of you guys. There are always cost over runs it seems and anyway to save a few dollars helps. I just wasn't too crazy about my lights dimming everytime the ac or water well motor kicks on and I sure don't want premature motor failure because of low power. I'll let you know what the power company tells us.
Im sure not to savy with electricity or computers. The power company says the transformer is not adjustable but can guarantee no more than 3.3% voltage drop. If the transformer is out of my budget is there a wire I can run underground the last 300 feet and not lose any more voltage than recquired?
 

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DGR,IABD
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I'm not sure why kind of goofy transformer they're using that doesn't have adjustable taps, but if that's what they say, that's whay you're stuck with.

Texstar said:
If the transformer is out of my budget is there a wire I can run underground the last 300 feet and not lose any more voltage than recquired?
To quote myself:

mdshunk said:
If you ran 500MCM for that last 300 foot run, you'd only have another 3 volts drop, which would put you right at 220, and you'd be fine.
 

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Discussion Starter · #11 ·
mdshunk said:
I'm not sure why kind of goofy transformer they're using that doesn't have adjustable taps, but if that's what they say, that's whay you're stuck with.



To quote myself:
I spoke to the power company again today. They want $ 5700 to run a transformer all the way to the house. He said I should not have more than 3.3% drop going into my house. That means any wire I would run for the last 300 ft. could not have any voltage loss. I thought I saw on a voltage drop calculator you could 4 or 5% and still be okay without premature motor failure or dimming lights.
 

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DGR,IABD
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Texstar said:
I thought I saw on a voltage drop calculator you could 4 or 5% and still be okay without premature motor failure or dimming lights.
Yes, sort of. That 3 or 4% is CUSTOMER voltage drop. The max is 6 or 8% total. Half from utility, half from customer's premis wiring. Remember, you'll come out of the transformer at 240 volts. Even with 8% voltage drop total, you'll be at 220 volts which is just fine.
 
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