Voltage Drop Ques.

[vramp7]

Im installing a underground 200amp service at a residential house. Using 4/0 alum urd, the length of wire is 250 feet. Am I doing this correctly? 2 x 250' x .10 x 200a / 1000 = 10 volts of drop, which is 5% of 200amps.

 

[charlie b]

I used a calculator spreadsheet, and my answer was 6.1 volts dropped. That is about 3% of 240 volts. But my calculator is not smart enough to know that aluminum is sometimes used. It only knows the formula for copper conductors. That will probably account for my answer being lower than yours, and confirms that you are at least in the right ball park. Where did you get the value of 0.1 for your approach? I don't have the K numbers handy for aluminum conductors.

Please note that your 10 volts is not 5% of 200 amps. You need to relate the VD to the system voltage, which you did not give, but which I presume is 240 volts.

 

[vramp7]

The .1 is from the code book table on voltage drop in the back of the book per 1000 feet. Im just wondering if I should up the wire size for the length? Ive seen many contractors put in up to 300 feet of 4/0 alum underground for a 200amp 1 Phase 240 volt service, but is this ok?

 

[Dennis Alwon]

It really depends on what your calculated load is. If the calculated load is 80 amps there wouldn't be a problem, however I am getting a problem if the load you have is more than 150 amps. Check out this online calculator
http://www.elec-toolbox.com/calculators/voltdrop.htm. It is based on a 3% drop.

I used 150 amp load 250 feet on a 240 volt single phase service and I got 250mcm aluminum.

 

[vramp7]

The ho will proprably never us a total of 200amps at one time. the house is running on 100amps now. But its getting an 3 car garage addition, and mabe a hot tub along w/the other existing loads like dryer, range, and lighting. The toolbox calc uses 3% for calculations but I thought code was 5%.

 

[charlie b]

The NEC does not give a limit for voltage drop. In a couple places, all in non-enforceable “Fine Print Notes,” it says that a VD of 3% for feeders and a total of 5% for the combination of feeder plus branch circuit should give satisfactory operation. But if the VD is higher, you are not in violation of any code article.

 

[vramp7]

So using 250 alum is what I see is the best for VD.

 

[benmin]

Im installing a underground 200amp service at a residential house. Using 4/0 alum urd, the length of wire is 250 feet.

The power company in my area will only allow 220' of 4/0 AL for that service.

 

[1793]

... Where did you get the value of 0.1 for your approach? I don't have the K numbers handy for aluminum conductors...

Table 8, 2005, 4/0 AL ohm/kFT =0.100

 

[nafis]

this might help....

this might help....

this is what i came up with

vd = IR
R = .11 ohms per /1000 ft ( ch9 table 9 )
R =0.11/1000 * 250 = .0275 ohms
now Vd = 200 * 0.0275
= 5.5 v this is ur voltage drop ( ohms law )

asuming ur services 240 single phase at 3% = 7.2 v and 12 v at 5%

I =cmil* vd / 2kd
= 211600 ( chp 9 table 8 )*12v /2*21.20hms*250= 2539200/10600 = 144 A
hope you see the big picture now

 

[Snorks]

I just ran your senerio through Volts with the following assumptions and results:

Ambient Temp @ 68?F
36" or less depth
Aluminium Conductors

@4/0 - 2.25% VD
@250 kcmils - 1.92% VD

This is using IEEE Std 141 Exact Voltage Drop Formula.

Hope this helps

 

[vramp7]

From the calculations im seeing the vd on 4/0 wire @ 250' is less than 3%. Ya get 11 volts div by 240 gets 5.5 volts per leg.

 

[George Stolz]

Vramp, in your original post, you used the formula I normally use, and I came to the same results.

I see 4.35% @ 230.0v with 4/0.
I see 3.66% @ 231.5v with 250.
I see 3.04% @ 232.9v with 300.

I'd question whether a volt and a half is worth the price difference between 4/0 and 250, myself, unless my math is way off. Two jumps is only giving you three volts, line-to-line, at a full 200A. I don't know if I'd worry about it.

JMO,

 

[S'mise]

Got VD?

Got VD?

Some of you may have forgot about absolute K. When using large wire it makes a difference. the ohms/kf *cm/1000=a.K or .1*211600/1000=21.16 (for 4/0AL) for this single phaze problem it would be: 2*21.16*200*250/211600=10volts or 4.2% at the full 200amps. play with the numbers and see what you get.

 

[vramp7]

V D

V D

I will use 4/0 because most houses have a 200amp service but probably use under 100amps or 100 peak. If it gets close to 200amps for everyday use its time to go to 400amp or 325 as the meter socket says it handles.