Voltage drop
- Thread starter kfenn
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Re: Voltage drop
If you go to the Mikeholt.com Home Page and click on the Free Stuff link it will (eventually) take you to a free software based voltage drop calculator. There is a spreadhsheet based one, and also a software based one. The good one is is a little hard to find. I'd give you a link, but I think it's PHP and not linked directly. Post back if you cannot find it.
If you go to the Mikeholt.com Home Page and click on the Free Stuff link it will (eventually) take you to a free software based voltage drop calculator. There is a spreadhsheet based one, and also a software based one. The good one is is a little hard to find. I'd give you a link, but I think it's PHP and not linked directly. Post back if you cannot find it.
Re: Voltage drop
Kfenn, since you may not be running around with a PC in your pocket, the formulas are
Single phase VD = 2 x K x I x L / CM
Three phase VD = 1.732 x K x I x L / CM
approximate K = 12.9 copper 21.2 AL assuming 75 deg
I = amps
L = length
CM = circular mills
Exact K = at 75 deg
R x CM / 1000 feet.
R = resistance
CM = circular mills
Roger
Kfenn, since you may not be running around with a PC in your pocket, the formulas are
Single phase VD = 2 x K x I x L / CM
Three phase VD = 1.732 x K x I x L / CM
approximate K = 12.9 copper 21.2 AL assuming 75 deg
I = amps
L = length
CM = circular mills
Exact K = at 75 deg
R x CM / 1000 feet.
R = resistance
CM = circular mills
Roger
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Re: Voltage drop
Roger,
Thanks for posting that. Now I need some help.
Single phase VD = 2 x K x I x L / 1000
Three-phase VD = 1.732 x K x I x L / 1000
(both for cables up to 1000 feet)
Doesn't the K already factor in the CM?
Please correct me if I'm wrong. Thanks!
../Wayne C.
[ October 20, 2003, 12:42 PM: Message edited by: awwt ]
Roger,
Thanks for posting that. Now I need some help.
I thought it was:
Single phase VD = 2 x K x I x L / 1000
Three-phase VD = 1.732 x K x I x L / 1000
(both for cables up to 1000 feet)
Doesn't the K already factor in the CM?
Please correct me if I'm wrong. Thanks!
../Wayne C.
[ October 20, 2003, 12:42 PM: Message edited by: awwt ]
Re: Voltage drop
Wayne, let's run thru an example and then you can check it with an online calculator.
We are going to use approximate K =12.9
30 amps at 360' using #10 = 10380 CM table 8
2 x K x I x L
2 x 12.9 x 30 x 360 = 278640 / 10380 = 26.84
Using exact K, # 10 sol = 1.26 ohms 1000 feet
R x CM / 1000
1.26 x 10380 = 13079 / 1000 = 13.07
2 x 13.07 x 30 x 360 = 282312 / 10380 = 27.19
Roger
Wayne, let's run thru an example and then you can check it with an online calculator.
We are going to use approximate K =12.9
30 amps at 360' using #10 = 10380 CM table 8
2 x K x I x L
2 x 12.9 x 30 x 360 = 278640 / 10380 = 26.84
Using exact K, # 10 sol = 1.26 ohms 1000 feet
R x CM / 1000
1.26 x 10380 = 13079 / 1000 = 13.07
2 x 13.07 x 30 x 360 = 282312 / 10380 = 27.19
Roger
G
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Re: Voltage drop
I can't argue with your resulting answer
How does your formula jibe with Bob's (iWire's) graphic? Or, better yet, how does Bob's graphic jibe with your formula. I find the graphic to be confusing (your posted formula is cinchy):
I can't argue with your resulting answer
How does your formula jibe with Bob's (iWire's) graphic? Or, better yet, how does Bob's graphic jibe with your formula. I find the graphic to be confusing (your posted formula is cinchy):
stevebrown
Member
Re: Voltage drop
Your formulas as given in the table are correct. To me, the biggest variable is in the value used for resistance of the conductor. Both values in this table are from NEC Table 8, which gives the DC resistance at 75 degrees C. The values are for uncoated copper, #8 and #2 conductors.
Just out of curiousity, is this the table that the rest of y'all use for voltage drop calculations?
By the way - if there is ever any more discussion about the NEC being a design manual, the topic of voltage drop should be given as an example. The NEC gives correct, but very superficial, guidance in this area.
Your formulas as given in the table are correct. To me, the biggest variable is in the value used for resistance of the conductor. Both values in this table are from NEC Table 8, which gives the DC resistance at 75 degrees C. The values are for uncoated copper, #8 and #2 conductors.
Just out of curiousity, is this the table that the rest of y'all use for voltage drop calculations?
By the way - if there is ever any more discussion about the NEC being a design manual, the topic of voltage drop should be given as an example. The NEC gives correct, but very superficial, guidance in this area.
Re: Voltage drop
Personally I reverse engineer the problem. I use the same formula Roger uses except I simplify the equation, use 11.1 for K, and solve for the size of wire rather than the VD. So it looks like this:
CM=(22.2 * L * I)/VD
Where
CM = Circular Mills of cable needed
22.2 = 2 * K = 2 * 11.1
L = One way length
I = Load current
[ October 22, 2003, 02:03 PM: Message edited by: dereckbc ]
Simple, There is more than one way to "skin a cat". Bob's method uses the DC resistance of copper wire, whereas Roger's uses the "K" of copper.
Personally I reverse engineer the problem. I use the same formula Roger uses except I simplify the equation, use 11.1 for K, and solve for the size of wire rather than the VD. So it looks like this:
CM=(22.2 * L * I)/VD
Where
CM = Circular Mills of cable needed
22.2 = 2 * K = 2 * 11.1
L = One way length
I = Load current
[ October 22, 2003, 02:03 PM: Message edited by: dereckbc ]
- Location
- Massachusetts
Re: Voltage drop
Wayne as I mentioned it is not my graphic, I got it from a post by Scott Thompson.
IMO if Scott posted this it is correct, Scott knows his electricity.
As Dereck has said there are many roads to get where you are going.
Dereck in the past has posted some interesting real world info from testing in regards to the value of "K", it seems it is not really a definite number and you just do your best.
Wayne as I mentioned it is not my graphic, I got it from a post by Scott Thompson.
IMO if Scott posted this it is correct, Scott knows his electricity.
As Dereck has said there are many roads to get where you are going.
Dereck in the past has posted some interesting real world info from testing in regards to the value of "K", it seems it is not really a definite number and you just do your best.
G
Guest
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Re: Voltage drop
It's interesting that voltage drop is a bit like Trigonometry. You need tables to pull it off. Unless you have the table values internalized. It's not as simple as WEIR.
The software solution(s) may not be as portable, but software solutions can store (or calculate) more precise table values.
I know you didn't draw the table. Sorry to call it your table. I just wanted it known that it's not my table Thank you for bringing that table to the table.
It's interesting that voltage drop is a bit like Trigonometry. You need tables to pull it off. Unless you have the table values internalized. It's not as simple as WEIR.
The software solution(s) may not be as portable, but software solutions can store (or calculate) more precise table values.
I know you didn't draw the table. Sorry to call it your table. I just wanted it known that it's not my table
Thank you for bringing that table to the table.lady sparks lover
Senior Member
Re: Voltage drop
you know, there's a program on EDR.com
Or Electricaldesignersreference.com I can't remember which one, but the program is very good, and will do the calcs for you. I find the hand calcs to be a long process, but good to use.
Also, it's free... or at least it used to be..
Lady
[ October 23, 2003, 09:51 AM: Message edited by: lady sparks lover ]
you know, there's a program on EDR.com
Or Electricaldesignersreference.com I can't remember which one, but the program is very good, and will do the calcs for you. I find the hand calcs to be a long process, but good to use.
Also, it's free... or at least it used to be..
Lady
[ October 23, 2003, 09:51 AM: Message edited by: lady sparks lover ]
- Location
- Wisconsin
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- PE (Retired) - Power Systems
Re: Voltage drop
I like the manual calculations found in the "Ferraz Shawmut Book of Electrical Information". This book has five different voltage drop methods. The one I prefer is a "look-up" table which takes into account the load power factor and the magnetic properties of the raceway.
http://www.ferrazshawmut.com/resources/electricalhandbook.html
I like the manual calculations found in the "Ferraz Shawmut Book of Electrical Information". This book has five different voltage drop methods. The one I prefer is a "look-up" table which takes into account the load power factor and the magnetic properties of the raceway.
http://www.ferrazshawmut.com/resources/electricalhandbook.html
lady sparks lover
Senior Member
Re: Voltage drop
[ October 23, 2003, 02:01 PM: Message edited by: lady sparks lover ]
Actually it does take into account conduit, power factor, parallel conductors, type of conductors, etc. I used to do mine by hand as well, but when I found the program of the internet I guess laziness set in. However, I still remember how to do them by hand.
[ October 23, 2003, 02:01 PM: Message edited by: lady sparks lover ]
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Re: Voltage drop
I already stated this, but mikeholt.com has the free calculator too in the FREE STUFF link. There are two version:
1. Spreadsheet style.
2. Software driven.
Here is a link to the Excel spreadsheet voltage drop calculator.
The link to the software version is buried deeper. Does anybody have the link handy?
../Wayne C.
[ October 23, 2003, 02:16 PM: Message edited by: awwt ]
I already stated this, but mikeholt.com has the free calculator too in the FREE STUFF link. There are two version:
1. Spreadsheet style.
2. Software driven.
Here is a link to the Excel spreadsheet voltage drop calculator.
The link to the software version is buried deeper. Does anybody have the link handy?
../Wayne C.
[ October 23, 2003, 02:16 PM: Message edited by: awwt ]
Re: Voltage drop
Wayne, the electronic or online calculators don't do anything for a student learning theory.
Look at all the others posts, they are bringing the hands on work to the table.
Patty said
[ October 23, 2003, 08:54 PM: Message edited by: roger ]
Wayne, the electronic or online calculators don't do anything for a student learning theory.
Look at all the others posts, they are bringing the hands on work to the table.
Patty said
Roger
[ October 23, 2003, 08:54 PM: Message edited by: roger ]
G
Guest
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Re: Voltage drop
Point well taken. I apologize. I did not realize we were tailoring this thread to a student. Hopefully it's all good & useful
Point well taken. I apologize. I did not realize we were tailoring this thread to a student. Hopefully it's all good & useful
Re: Voltage drop
16a 24a 32a
12 100ft N/A N/A
10 180ft 120ft N/A
8 350ft 200ft 150ft
Oh well I cannot get the table to line up, but you get the idea.
[ October 23, 2003, 10:32 PM: Message edited by: dereckbc ]
Kfenn, by now you know there are several ways to determine voltage drop. From one train driver to another, here is the easiest method I know of. You only have to do it one time, and you are set for life until some joker at the NEC changes 310. Make two or three tables based on voltage drop percentages like 3% and 5%. Start the first column with wire gauges, and the first row with 80% of common OCPD. Fill in the data with the maximum footage. Something like this. Call it 5% voltage drop limit.
16a 24a 32a
12 100ft N/A N/A
10 180ft 120ft N/A
8 350ft 200ft 150ft
Oh well I cannot get the table to line up, but you get the idea.
[ October 23, 2003, 10:32 PM: Message edited by: dereckbc ]
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