Voltage drop calculation spreadsheet

[dsewing]

I love the voltage drop calculation spread sheet that Tom Moss created. I was wondering if Tom could provide a reference as to where he got his formula. I am curious where he got the K value (conductor resistivity) for copper conductor with current<50% conductor ampacity. I haven't been able to find this anywhere.


David

 

[bob]

Re: Voltage drop calculation spreadsheet

I was not able to open the spreadsheet. What is the formula you are questioning.

 

[bphgravity]

Re: Voltage drop calculation spreadsheet

I have at least 5 different books that discuss voltage drop and at least three of them use a different value for "K".

The only sure fire method is to use the R x cm /1000 forumla using Table 8 values from the NEC.

(For example, 12-awg solid has a rsistance of 1.93-ohms at 1000 ft. It has an area of 6530 cm. 1.93 x 6530 = 12,603 / 1000 = 12.603)

Now if you are looking for conductor size, obviously you can't find the actual "K", but the most widely accepted value is 12.9 for copper and 21.2 for AL. This is based on the fact that a 1,000 cm conductor, 1000 ft long has this resistance at 75 degrees C. (1 cmft. = 12.9-ohms)

 

[dsewing]

Re: Voltage drop calculation spreadsheet

The spread sheet posted on Mike's web site uses the following formula

2k x length x current/circular mill
(K values listed are 12.9 for copper and 11 for copper where current <50% of ampacity, both values for 30C. Not sure where he got the 11 value, I have not seen a reference for it.

NEC table 8 gives you values for resistance/1000ft of conductor. If I apply these values in a formula I would use the following formula:

Vdrop = R value from table 8 NEC corrected for 30C * current * length/1000

I get two very different numbers via these two formulas

which is correct?

David

 

[steve66]

Re: Voltage drop calculation spreadsheet

I have not taken a close look at the spreadsheet or the other formulas listed, but if your formula doesn't consider reactance, the voltage drop you calculate will be low. Even the power factor of the load plays a small part in the voltage drop (Two identical runs of wire with the same magnitude of current, and different PF's on the loads give different voltage drops).

Steve

 

[steve66]

Re: Voltage drop calculation spreadsheet

The spreadsheet I use looks up the values of resistance and reactance in a table for the type of wire and conduit used. Then it calculates:

Z = R * PF + X * SIN ( ARCOS (PF))

Then it calculates:

Vdrop (line to neutral) = I * Z * L/1000

where Z = impedence, X = reactance, PF = Power Factor, SIN = Sine function, ARCOS = arcosine function, I = current, and L = circuit length.

Steve

 

[charlie b]

Re: Voltage drop calculation spreadsheet

Originally posted by dsewing: I get two very different numbers via these two formulas. Which is correct?

Is the one from the spreadsheet exactly, or very close to, double the other?

I tried to use the spreadsheet last night for my own purposes, and I suspected it had an error. Having read this thread this morning, I am certain the spreadsheet has an error. The error is not in the math or in the formulas, but rather in the instructions. Here is what I mean.

Originally posted by dsewing: The spread sheet posted on Mike's web site uses the following formula: 2k x length x current/circular mils.

I agree that this is the formula that the spreadsheet uses. I also agree that that is the correct formula. But I submit that the ?length? in that formula should represent the ?one-way distance.? You account for the fact that there are two conductors (i.e., the hot conductor going to the load and the neutral returning from the load) by use of the factor ?2? in the formula. But the instructions tell you to use, as the length of the run, ?Two times length of conductor in feet (length of 2 conductors).? To me, this looks like you are doubling the double. In other words, you should either enter the one-way distance into the formula, or if you use the two-way distance, you should drop the factor of 2.

May I ask the forum members for a comment?

 

[dereckbc]

Re: Voltage drop calculation spreadsheet

Charlie, I do not use the calculator mentioned, I use my own based on the formula

CM=(12.9 * I * 2 * L)/VD

Where
CM = Circular Mills
12.9 = The constant of copper
I = Load current in amps
L = One-way length in feet.
VD = Voltage Drop

I modify K to = 11.1 then multiply by 2 to give me 22.2 and simplified the formula to:

CM=(22.2 * I * L)/VD

I think where the confusion is in the distance. You can use loop distance or one-way distance. If you use one-way distance you need to multiply by 2 to get the loop distance. In my modified formula I simplified by using 22.2 (2 * 11.1) and used the one-way distance for simplicity.

Or you could just use the ohms per K/ft from Chapter 9. IMHO any method you use is like any other electrical formula. Use a micrometer to measure, mark it with chalk, and cut it with an axe. It's only in the "Ball Park", not exact.

Edited my math from 22.1 to 22.2
Secong edit for clarification of modified formula

[ May 25, 2004, 09:36 PM: Message edited by: dereckbc ]

 

[jschultz]

Re: Voltage drop calculation spreadsheet

www.edreference.com has the electrical designer's reference program. It has a great voltage drop tool and I especially like the series voltage drop tool for doing site lighting.