Voltage Drop Calcs

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izak

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is anyone familiar with Tom Henry's Voltage Drop formulas?

Specifically the one thatyou use to find wire size needed, given the load, distance, voltage, and voltage drop permitted (in volts)

I have had some REAL issues with this formula and Tom Henry's company cannot even give a good answer...
 
Re: Voltage Drop Calcs

It's all Ohm's law. Why torture yourself?

Edit: I want to see it too. :confused:

[ April 15, 2005, 11:45 AM: Message edited by: physis ]
 
Re: Voltage Drop Calcs

Ok

Tom Henry gives several formulas to cover pretty much anything you need to do with Voltage Drop

with these, he also provides a chart that lists the EXACT 'K' values for All wires, solid, stranded, copper, aluminum, coated or not from #14 to 1000 kcmil

These values range anywhere from 12.5598 to 12.9

The Formulas are as follows:

BTW: K is Exact K according to the chart
D is Distance
I is Amperes
CM is Circular Mils
(sorry if everyone already knows that)
To find Voltage Drop - VD = 2 x K x D x I divided by circular mils


To find Wire Size - CM = 2 x 12.9 x D x I divided by Voltage Drop permitted in Volts

To find Distance - D = CM x VD permitted divided by 2 x K x I

To find Load - L = CM x VD permitted divided by 2 x K x D


when I say Wire size, distance, or load, it is just a way of breaking the VD formula into different parts using different KNOWN factors..

Ideally, if you want to know what size wire you will need to pull 23 amps at 240 volts with a circuit length of 225 feet, you would use the second formula

if you wanted to know how far you could run a #10 copper wire with a load of 12 amps at 480 volts you would use the distance formula...

has anyone seen these formulas? they seem like they could be useful and time saving...

EXCEPT

I have found what I feel to be a LARGE error in the second formula, the one for wire size, as applies to the code requirements for Ampacity per 310.16


The Problem i have is that if you use the Wire Size formula to figure the Circular Mils required to handle a Load of 100 Amperes at 480 volts 3phase on a circuit 150 ft long, it tells you to use #6 wire..... now according to 310.16, #3 is the MINIMUM required for 100 amp load, not considering circuit length, or derating...

i know this has been an extensive post, but this is something that REALLY irritates me.

Tom Henry's course costs QUITE a bit of money, and when you call them and say, HEY!! your formula is USELESS! All they say is.... well you have to look at table 310.16....

well hell, thats where I STARTED!! Before I THOUGHT about VOLTAGE DROP!!!

does this make any sense to anyone but me??

:)
 
Re: Voltage Drop Calcs

OOOOPS

One more thing...

on 3 phase circuits, on all formulas, substitute the '2' with 1.73

im sure that this is important ;)
 
Re: Voltage Drop Calcs

I suck at math, but I do have an opinion on what you're dealing with. I think the problem is linked to using a "K" value out of the book. Different wire sizes have different resistances, and the "K" factor seems to be too generalistic to do any good.

I use the ? x Rf x I x D method, it works good for me if I don't screw it up. :)
#3 CU THHN has .254 ohms/kFT
#2 CU THHN has .201 ohms/kFT
#1 CU THHN has .160 ohms/kFT
1/0 CU THHN has .127 ohms/kFT


Vd = 1.732 x Rf x 100 x 150
Vd = 25980 x Rf
I multiply everything so I don't have any other numbers to work with than the resistance, and my ?xIxD number. I can check several wires sizes quickly from this point.

25980 x .000254 = 6.60 v (#3)
25980 x .000201 = 5.22 v (#2)
25980 x .000160 = 4.16 v (#1)

6.6 v over 480 v = 1.4% drop. :( :D [/i]

[ April 16, 2005, 08:21 AM: Message edited by: georgestolz ]
 
Re: Voltage Drop Calcs

Originally posted by georgestolz:
I suck at math,
Click to expand...
So do I, math shmath who needs it when you have a PC. :D

I use a program on my PC to figure voltage drop.

It's sweet, you can figure voltage drop, maximum length or minimum wire size using AL or CU conductors, single or 3 phase and ferris and non ferris raceways all in just seconds.

I agree with you it is the "K" that makes the difference. I have seen "K" listed from 9 to 13.

Pick a number. :D
 
Re: Voltage Drop Calcs

Originally posted by iwire:
So do I, math shmath who needs it when you have a PC. :D
Click to expand...
Yeah but lugging this sucker in my bags all day gets so old. :D
title-imacg3-onhover082504.gif
 
Re: Voltage Drop Calcs

and assume you're using THHN, which has an outer coating
Click to expand...
George
Were you refering to the covering of the insulation or the coating of the strands? I noticed you used the coated colum of table 8 for your figures. I assumed most THHN was uncoated but I could be mistaken on this.

Thanks
Buck
 
Re: Voltage Drop Calcs

the idea behind those formulas is that instead of starting at 310.16, with say a #6 wire, figuring a VD calculation using it and then saying, oops, that one is too small, go up to the next one, do it again, and again, until you get it where you want it, you can supposedly get the correct wire size the first time...

and BTW, the K chart I have has different values for Every different wire, as i stated before - from #14 to 1000 kcmil, solid or stranded, alum or copper, tinned or not tinned.
 
Re: Voltage Drop Calcs

with these, he also provides a chart that lists the EXACT 'K' values for All wires, solid, stranded, copper, aluminum, coated or not from #14 to 1000 kcmil
Click to expand...
The problem with this is that the "EXACT 'K' values" are only true for a one foot long piece of conductor with a one circular mil diameter at a given temperature. I have 12.88384 for copper at 75? C. but that value doesn't accurately represent the resistance of say a 10 AWG copper conductor.

I checked the formulas you posted and they are acurrate. At least as far as they can be using "K" for resistance. I have no idea why this industry insists on using these formulas with "K" instead of accurate math, but this is the standard method.

This is better:
_____________________________________________________________________________

Vd = 1.732 x I x D x Rf [for three phase]
Vd = 2 x I x D x Rf [for single phase]

Rf = Resistance per foot (From chapter 9, table 8, ohms/kFT devided by 1000)
D = distance
I = Current
Vd = Voltage drop

_____________________________________________________________________________


It all comes from these three formulas, Ohm's law:

V = Voltage
R = Resistance
I = Current

V=IR
R=V/I
I=V/R

All three actually say the same thing but the first one is where voltage drop comes from.

V=IR or (voltage drop) = (current) X (resistance)

In other words, the resistance of a piece of wire times the current through it equals the voltage drop across it.

=============================================================================

I checked your 100 amp 150 feet 480 volt 3 phase issue and I think the reason they're telling you to look at Table 310.16 is because you don't use voltage drop to determine the minimum conductor size. You use Table 310.16 for that.

Then you work from there to adjust for voltage drop.

Edit: It seems like I've been writing this one all morning. This post isn't addressing your last post.

[ April 16, 2005, 02:59 PM: Message edited by: physis ]
 
Re: Voltage Drop Calcs

Bob and George,

When I know I'm going to have to do voltage drop calculations in the field I don't want to take any chances, so I use ENIAC, mostly because I don't feel comfortable with Apple products.

eniac.jpg


Edit: Oh, and I think Steve Jobbs is an ego maniac. Times at least 20 or 30.

[ April 16, 2005, 01:29 PM: Message edited by: physis ]
 
Re: Voltage Drop Calcs

Originally posted by buck33k:
and assume you're using THHN, which has an outer coating
Click to expand...
George
Were you refering to the covering of the insulation or the coating of the strands? I noticed you used the coated colum of table 8 for your figures. I assumed most THHN was uncoated but I could be mistaken on this.

Thanks
Buck
Click to expand...
Look at Table 310.13, and compare THW with THHN. :D

[ April 16, 2005, 01:48 PM: Message edited by: georgestolz ]
 
Re: Voltage Drop Calcs

Originally posted by georgestolz:
Look at Table 310.13, and compare THW with THHN.
Click to expand...
George I do not believe the 'plastic wrap' on THHN is the coating Table 8 is referencing.

I have read that the coating Table 8 refers to is varnish type coatings.

Look at Note 1 to table 8

1. These resistance values are valid only for the parameters as given. Using conductors having coated strands, different stranding type, and, especially, other temperatures changes the resistance.
Click to expand...
I do not believe it is possible to have a conductor with strands coated in that plastic.

If the strands are covered in that plastic you would have separate conductors.
 
Re: Voltage Drop Calcs

I'm not 100% sure, Bob. That's doesn't seem right. A nylon jacket would contribute to the heat retained in a conductor. I remember using a study guide prior to taking my test that made the distinction between THW and THHN. It drove me crazy until I saw the last column of table 310.13.

The study guide specifically referenced table 310.13 in the answers.

The "coated strands" could be referencing some type of coating under the insulation?

I honestly don't know. :(
 
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