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#86921 12/23/03 11:44 AM
Joined: Dec 2003
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I see that most of us (including Mike Holt) use the old formula for computing voltage drop. VD = I*K*2*L/CM (change 2 to 1.732 for 3 phase). "K" has been defined equal to from 10 to 12 for copper and from 17 to 21 for aluminum.
Wouldn't it make sense for us to change to the charts used in the NEC for these computations? Chapter 9, Tables 8 & 9 give a hundred different values for "K" (you have to do the math). Could it be there is a more accurate method of doing our computations?
VD = I*2*L*R/1,000 the value of "R" would be determined from the Table. Use Table 8 for DC, and Table 9 for AC, (using Effective Z at 0.85 PF for Uncoated Copper Wires as "R")
BTW, coated wires are varnished wires used in manufactured transformers and coils and the like.


Earl
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Doesn’t ‘coated’ refer to tin plating over copper? I thought it was applied to prevent a chemical reaction with vulcanized rubber and bare copper.

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I always thought that coated wires were the tinned ones too. But, then I saw it in a book one day, so I figured I was wrong. I guess I don't really know. Anybody?


Earl
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My understanding of 'coated' is also those conductors coated with varnish for use in coils.
The 'tinning' is a process required for copper wire used with rubber-insulated cable.
Just my understanding; some of this information is in the 'American Electrician Handbook', but there is no definite statement saying what exactly 'coating' refers to in the tables.

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Getting back to the title of this thread:

With a little algebra, I have re-written the formula for voltage drop so we can determine what size wire would be required for a long run, rather than trial and error. For single phase:

R = 15*E/I*L, where

R = the resistance per 1,000 feet of wire found in Table 8 or 9. Choose the next larger size of wire from the calculated value.
E = the voltage
I = the expected current draw
L = the distance from source to load in feet

For three phase:

R = 17.32*E/I*L


Earl
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Quote
"K" has been defined equal to from 10 to 12 for copper

A engineer on one of the other sites has measured the value of "K" in the real world after his installations have been put to use.

He found an even wider range of figures for "K" I believe he said he made a spread sheet based on about 11.

For me it is just easier to use a VD calculator program on my PC.

It has settings for the type of conduit, AC, DC, single phase, 3 phase, voltage etc.

You can figure for Voltage drop, minimum wire size or maximum length.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: Apr 2002
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Bob:
I have to agree with you, the 'EelectriCalc'
saves a lot of time. It's the small calculator size one. What program do you use in your PC?

Earlydean:

I'll give your method a shot ASAP.

John


John
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I guess I gotta break down and buy a code-calc.....


Earl
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Does anyone figure out the VD for each section of a circuit or does everyone just add up the total load and the total distance.

An example would be outdoor lighting. Say you have 10 120v lights 100w each. From the panel thru the switch to the first lamp is 100'. Then each light after that is 25' apart.

Do you figure 1000w at 325'? With only 1 size wire start to finish. That's how I've done it.

Or do you do 1000w at 100' for the feed. Then 900w at 125', 800w at 150', 700w at 175', etc..? Then the wire size changes as requiredn thru the different sections.

Tom

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Tom I did exactly that for someone on a forum.

They had an existing long outdoor lighting circuit with undersized wire.

Using the VD program I gave him a few different options on the least he would have to change.

It turned out if a he significantly up sized the wire from the feed to the first two loads from 14 AWG to 8 AWG he did not have to change the rest of circuit. [Linked Image]

Hotline, the program I have is made by Zolltek

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

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