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Author Topic: Electrical Wire Size, Voltage and Amp Drop?  (Read 737 times)

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Offline Corley5

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Electrical Wire Size, Voltage and Amp Drop?
« on: November 24, 2016, 09:12:56 PM »
  So I've got a couple trenches dug to bury electric wire in.  One is from the house to the milk house.  It's 250'.  The house has a 200 amp service and I'd like to put a 200 amp breaker in the box and a 200 amp panel in the milk house.  Someday I'll put a new service at the milk house if we expand and need more juice but for now this is what I want to do.  From the online calculators 4/0 with input of 220v would give me 209 volts at the end of that run.  Does this sound right?  That's a 5% drop.  I've read that the NEC requires no more than a 3% drop but no more than 5% is recommended by someone?  I'm not pulling permits for this.  It's agricultural and none are required  ;) ;D  The purpose of this is to run a reverse osmosis machine, vacuum pump, blowers on the evaporator, lights etc in the milk house which will be the new maple sugar shack.  Total possible load will be less than 100 amps with everything running.   
  The other trench is 900'+ but less than a 1,000' from the milk house/sugar shack to the woods where the vacuum releaser and electric pump will be in a 4'X8' pump house.  The pump is rated at 6 amps 115 volts but I want enough juice for lights and light power tools as needed.  According to the calculator 4/0 wire will get me 115 volts and 25 amps at the end of that run with an input of 120 volts.  A 5% drop. It would 3%, 116 volts at 15 amps.  I think that would be sufficient for what I need?
  Does this sound right?  I need to get this wire purchased and buried before the ground freezes up or we get a bunch of snow.  I'm pushing my luck.  Will 4/0 direct burial wire work for what I want to do? 
  I'm also burying 2 1 1/2" lines for sap and vacuum and a 3/4" inch water line to the pump house.  I've already got an old water line to the milk house and next season want to upgrade it while adding lines to the OWB to heat the building.       
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Offline Corley5

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Re: Electrical Wire Size, Voltage and Amp Drop?
« Reply #1 on: November 24, 2016, 09:14:56 PM »
I should add that I know enough about electricity to be a real danger to myself and others  ;) ;D
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Offline Larry

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Re: Electrical Wire Size, Voltage and Amp Drop?
« Reply #2 on: November 24, 2016, 09:51:00 PM »
Voltage drop is figured from the power company transformer to the end of the circuit at the customer.  How far from the transformer to your house and what gauge wire was installed?  Than voltage drop can be figured.

Also 220 volt went out years ago.  Your probably on a 240 volt system.  You might be able to get professional help from the utility.  Your looking at way too much money to make a mistake at this point.

Larry, making useful and beautiful things out of the most environmental friendly material on the planet.

We need to insure our customers understand the importance of our craft.

Offline Corley5

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Re: Electrical Wire Size, Voltage and Amp Drop?
« Reply #3 on: November 25, 2016, 04:48:25 AM »
It's about 130' from the transformer to the house.  I'm unsure of the wire size.  I'll check it later today.
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Offline jdonovan

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Re: Electrical Wire Size, Voltage and Amp Drop?
« Reply #4 on: November 25, 2016, 08:07:21 AM »
The house has a 200 amp service and I'd like to put a 200 amp breaker in the box and a 200 amp panel in the milk house. 

Not quite practical, the house is going to be drawing some load all the time, so you really can't have 200 in from the utility, something to the house, and another 200 going to the milk house.


Quote
Someday I'll put a new service at the milk house if we expand and need more juice but for now this is what I want to do.  From the online calculators 4/0 with input of 220v would give me 209 volts at the end of that run.  Does this sound right?  That's a 5% drop.  I've read that the NEC requires no more than a 3% drop but no more than 5% is recommended by someone?

NEC is what most places use as a reference when permits/inspections are involved. Many of their recommendations are based on almost a century of learning from mistakes. I would not casually discount their guidance.

I would not put in a new feeder, that was going to start life at a 5% drop. Now if we assume the house is going to use 50 of your 200A, and only 150 is going to the milk house, then you can get less than 3% drop, on 150A draw on 4/0.

As you are discovering, it takes BIG wire to run loads at long distances.   

That voltage drop is resistance in the wire, and it all gets lost as heat. If you are drawing 150A@240v in the milk house, the difference in loss of 2.5% vs 5% is almost 1000 watts, lost to heating the ground, wire, connectors etc...

Offline Rural

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Re: Electrical Wire Size, Voltage and Amp Drop?
« Reply #5 on: November 25, 2016, 09:29:41 AM »
Since voltage drop on a line depends upon the current being drawn which is not necessarily the total of all the connected equipment. As it is most improbable that everything will be on and drawing full power at the same time the statement that " From the online calculators 4/0 with input of 220v would give me 209 volts at the end of that run." requires further information. What will the maximum actual load at any given time, this is called the "Demand Factor" which depends in part upon the type of load connected. Large motors require a demand factor of 1:1 whereas lights and the like can be calculated at 2:1 or less. Motor loads are the most problematic as the momentary high start up current tends to 'bump' the voltage if the incoming lines are already loaded near their capacity.

Offline muggs

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Re: Electrical Wire Size, Voltage and Amp Drop?
« Reply #6 on: November 25, 2016, 10:15:44 AM »
Consider using PVC conduit instead of direct burial wire. It is not very much more work.   Bob

Offline Corley5

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Re: Electrical Wire Size, Voltage and Amp Drop?
« Reply #7 on: November 25, 2016, 10:29:51 AM »
  The house isn't lived in.  Grandma passed away a year ago and we have no plans to have it inhabited.  There's a freezer running in the basement that's it other than lights.  The well pump is powered from the house
  Loads for the sugar shack are 20 amps for the vacuum pump, 4 amps for the evaporator, 22 amps for the RO, and a 20 amp breaker for the remote pump but running amps are 6.  That's 66 amps not including lights.  I need to call Leader and get starting amps for the vac pump and RO.
   
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Offline Den Socling

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Re: Electrical Wire Size, Voltage and Amp Drop?
« Reply #8 on: November 25, 2016, 11:00:32 AM »
I asked for and got a pole and separate service for my shop which is less than 100' from my house.

Offline TimRB

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Re: Electrical Wire Size, Voltage and Amp Drop?
« Reply #9 on: November 25, 2016, 07:01:00 PM »
Doing some back-of-the-envelope calculations it looks to me like you'd be okay.  Here's my take:

First situation--250 feet of 4/0 wire at 200A.  Info on the web says 4/0 wire is 161uohms per meter.  250 feet is about 76.2 meters for a total resistance (per wire) of 12.27mohms.  (200A)(12.27mohms)=1.5V drop (per wire).  So you lose 1.5V in the supply side and 1.5V in the return side for a total of 3V dropped in the wiring, giving you 220-3=217V at the load.  3/200 is 1.5 percent.

Second situation--1000 feet of 4/0 wire at 25A.  1000 feet is about 304 meters, for a total wire resistance of 49mohms.  (25A)(49mohms)=1.22V drop per wire, or 2.45V total dropped in the wire.  120-2.45=117V at the load.  2.45/120 is about 2 percent.

Back-of-the-envelope.  YMMV

Tim



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