# Voltage drop



## Beden469 (Oct 11, 2009)

Have a 100 amp panel fed with wrong wire. 1/o aluminum 600 feet from meter. I know it's wrong wire and have discussed changing it with home owner. The problem I am asking about is when testing at meter socket voltage from electric company is 115 volts on feed side at each leg. 230 volts total. When air compressor is turned on drops to 210 volts. Seems to me problem is also at incoming power. Why the voltage drop at incoming side of meter? Reading 173 volts in shop after goes thru wrong wire and 600'. Just curious if I should also contact elec. company and complain as incoming is to low or if they have range that they are responsible for and is good.


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## Black Dog (Oct 16, 2011)

Beden469 said:


> Have a 100 amp panel fed with wrong wire. 1/o aluminum 600 feet from meter. I know it's wrong wire and have discussed changing it with home owner. The problem I am asking about is when testing at meter socket voltage from electric company is 115 volts on feed side at each leg. 230 volts total. When air compressor is turned on drops to 210 volts. Seems to me problem is also at incoming power. Why the voltage drop at incoming side of meter? Reading 173 volts in shop after goes thru wrong wire and 600'. Just curious if I should also contact elec. company and complain as incoming is to low or if they have range that they are responsible for and is good.


Probably a old transformer and near the end on the line so the voltage is low. it was 220/110 at my house for a long time till they put in a new transformer now it is high 250/125 volts...

Welcome to the forum.


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## Beden469 (Oct 11, 2009)

Sorry I screwed up instead of 210 volts on line side of meter after compressor is on its 190volts. Just curious why voltage drop is also present at meter side


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## chicken steve (Mar 22, 2011)

F it

~CS~


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## Big John (May 23, 2010)

If they follow a tolerance of plus or minus 5%, then it sounds like you have a case, because with the compressor on your service voltage is low by 12.5%. 

That said, even if they gave you a rock stead 240V, you'd still have a 15% drop to your remote shop, which is obviously no good.


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## Beden469 (Oct 11, 2009)

Thanks black dog. Is this something can call com Ed about?


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## chicken steve (Mar 22, 2011)

Beden469 said:


> Thanks black dog. Is this something can call com Ed about?


I'm sure they'll do the math for you....~CS~


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## Beden469 (Oct 11, 2009)

Thanks big john. You helped. As for you chicken Steve I walked into the situation and if you could read you would see I said I know the feed is wrong and am changing it. Didn't put it in. Just curious why the drop before meter.


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## Big John (May 23, 2010)

I didn't see your updated numbers before I posted, so am I to understand that when the loads are on in the remote garage you've got 190V at the service and 173V at the shop?

If that's true you definitely have a case with the utility. This is the standard that many of them follow:








And your voltage drop right now is an absurd 21% to your service, but only 9% to your remote building.

If you can get them to fix their end of things quickly, you will technically be inside the NEMA voltage drop range for that compressor. As long as you can confirm that the customer's equipment runs properly then, and confirm there's not gonna be a load increase in the future, you may not have to replace that wiring.


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## Beden469 (Oct 11, 2009)

Thanks big john. You been such a help. We are replacing the service anyway because it's convient. Customer has wire and a backhoe. But was afraid after doing that, problem would still exist. Chart is great. I appreciate the help.


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## georgemcfly (Apr 10, 2010)

*can someone tell me what i'm doing wrong here!? VD calculation*

First off, I apologize if I'm breaking protocol but I'm a newbie and can't post yet so I figured this might be a good place to post this question as a reply. I'm studying for my Masters Exam and just when I though I had it licked I was humbled. Just to be clear, I'm using my Ugly's book as a refrence for this formula but keep getting an alternate and incorrect answer! Appriciate the help in advance!

Question: The voltage drop on a #10 AWG solid THHN uncoated copper conductor on a 120v circuit is 85' long and connected to a 2HP single phase motor is ?

When I use the 2K x L x I / Cm= VD formula I come up with 5.06v (INCORRECT)

The testing software has it simply at VD= I x R which gives me the correct answer of 4.94v 

This simple formula (not listed in Uglys) is working and the other isn't. WHat am I not seeing here?!


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## McClary’s Electrical (Feb 21, 2009)

georgemcfly said:


> First off, I apologize if I'm breaking protocol but I'm a newbie and can't post yet so I figured this might be a good place to post this question as a reply. I'm studying for my Masters Exam and just when I though I had it licked I was humbled. Just to be clear, I'm using my Ugly's book as a refrence for this formula but keep getting an alternate and incorrect answer! Appriciate the help in advance!
> 
> Question: The voltage drop on a #10 AWG solid THHN uncoated copper conductor on a 120v circuit is 85' long and connected to a 2HP single phase motor is ?
> 
> ...


Why would we answer this? Have you bumped your head? You cant just memorize uglys to pass your test.


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## Bad Electrician (May 20, 2014)

Why is the voltage low at the meter?

Voltage drop on the utility line, under sized transformer, long utility distribution, high resistance connection.


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## georgemcfly (Apr 10, 2010)

I realize that, believe me. My question is simply why does the prescribed formula produce an incorrect answer? The incorrect answer is listed among the possible options so obviously there's some truth in the math. I'm just humbly requesting some insight to my quandary.


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## georgemcfly (Apr 10, 2010)

The question is purely hypothetical for test preparation purposes.


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## amigi968 (May 24, 2008)

Ignore mcclary's. He's the second biggest asshole on the forum.


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## RIVETER (Sep 26, 2009)

amigi968 said:


> Ignore mcclary's. He's the second biggest asshole on the forum.


I resent that. He is not the biggest.


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## Barjack (Mar 28, 2010)

georgemcfly said:


> First off, I apologize if I'm breaking protocol but I'm a newbie and can't post yet so I figured this might be a good place to post this question as a reply. I'm studying for my Masters Exam and just when I though I had it licked I was humbled. Just to be clear, I'm using my Ugly's book as a refrence for this formula but keep getting an alternate and incorrect answer! Appriciate the help in advance!
> 
> Question: The voltage drop on a #10 AWG solid THHN uncoated copper conductor on a 120v circuit is 85' long and connected to a 2HP single phase motor is ?
> 
> ...


Where are you getting your value for K?


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## Carultch (May 14, 2013)

Barjack said:


> Where are you getting your value for K?


It could be a matter of using K at different temperatures. Resistivity of a metal increases with increasing temperature.

For other reasons, it also increases due to the current being AC instead of DC. This effect is only significant enough to matter, when the wire is larger than 1/0.

The standard practice is to use a K value that is applicable to DC current in copper at 75 C, for applications where the size is 1/0 and smaller. In which case K for copper = 12.9 Ohm-kcmil/kft, and K for aluminum is 21.1 Ohm-kcmil/kft.

A strategic practice I use, is to use resistances at 60C, when we know that the wire is upsized beyond the minimum local size, such that using 60C rated wire allows it to remains code compliant. This is something that works in practice, but it will not work for taking an exam. The NEC doesn't contain a strict requirement on voltage drop, so it is up to you to determine what the limit should be, and what the calculation method should be. I usually go here, to find the AC and DC resistances at the various temperatures accordingly.
https://www.anixter.com/content/dam/Anixter/Guide/7H0011X0_W&C_Tech_Handbook_Sec_07.pdf


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## Barjack (Mar 28, 2010)

I was trying to help point Mcfly in the right direction. He was using the wrong value for K. 


Sent from my iPhone using electriciantalk.com


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## Max10 (Sep 29, 2020)

What size wire are you going to install to replace the 1/0 Al?


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## oldsparky52 (Feb 25, 2020)

Max10 said:


> What size wire are you going to install to replace the 1/0 Al?


Ugh Max, the last post on this thread was over 5 years ago. You probably won't get an answer.


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## Max10 (Sep 29, 2020)

My bad. I saw the topic and is intrigued me. Thanks. I'll keep an eye on that in the future.


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