K constants

Status
Not open for further replies.
S

Sparky Joe

Member
Location
Salt Lake City
I seem to have forgot where I wrote down my 'K's for copper and aluminum, can someone help me out with the 75degree C ones.

If you don't know what I'm talking about; 2KIL/Cmil=voltage drop

Thanks
 
Don't use the approximate "K" values for that voltage drop calculation. Use the actual "K" formula of K = R X CM / 1000.

Example: Table 8 resistance of #12 solid copper is 1.93 Ohms/kFT.

K = 1.93 x 6530 / 1000 = 12.6029 - Use the entire number (do not round)

This would be the most accurate method using NEC direct-current resistance at 75?C.

Some persons, including myself, feel this calculation is a little conservative, however it is widely accepted as an accurate value. I have seen K factors as low as 10.5 for CU.
 
Thanks for the info.

And thanks Gravity, I've never seen it dont that way, but I guess all it took for me was adding 2+2, now that I know its impossible to lose them again.
 
bphgravity said:
K = 1.93 x 6530 / 1000 = 12.6029 - Use the entire number (do not round)
Click to expand...

Every math teacher I ever had would disagree. At best your precision is 3 places. What justification is there to not round?

Martin
 
The NEC permits rounding for certain calculations. 220.2(B) and Chapter 9 Note (7).

For actual K - the whole number should be used for complete accuracy. If you want to round up or down, go ahead. It's not that big of deal.

I personally use VD = I X R for voltage drop calcs. That way I don't have to use "K". When I do use K, I use the whole unrounded number.
 
bphgravity said:
I personally use VD = I X R for voltage drop calcs. That way I don't have to use "K". When I do use K, I use the whole unrounded number.
Click to expand...

Isn't the R in that equation based off of a K value? The resistance/1000ft values in Chapter 8 Table 9 are based off of a K value for 75C.

I don't see any way around doing a voltage drop calculation without involving K in some form.
 
The K # is actually based on the R value from table 8. If you do the calculation for 1000 kcmil copper you will find that the K value is very close to 12.9 which is the figure most use for K. I have yet to do a voltage drop where the exact value gave a different wire size than the approximate value. I am sure it is possible, but as most people don't consider voltage drop at all, using the approximate K-value will put you lightyears ahead of the competition.
 
bphgravity said:
K = 1.93 x 6530 / 1000 = 12.6029 - Use the entire number (do not round)
Click to expand...

You know, I think it's a matter of perspective. As an EE, you are at you desk, have to do all the math before putting your stamp on it - and your math better be right.

As an EC, I too, need to be right, but I might be troubleshooting why a 2 hp motor branch circuit with 150' of #12 could be the problem. Or, I might have to bid a job at my desk, and being right enough to make it work is as important as being cheep enough to get the job.

So in the field I use 12.9 and at my desk... okay I still use 12.9... but I might start using 12.6029
 
Michael,
If you start using 12.6029 you will only be exactly correct for #12 CU wire, just as you are only exactly correct for 1000 kcmil when you use 12.9. Either one should probably be close enough for most applications, especially since many people don't consider voltage drop at all! Maybe an avaerage of 12.75 would be good. Back years ago I was taught to use a K of 12.0. I think that may have been based on the resistance at 60 degrees Celsius.
 
haskindm said:
...If you start using 12.6029 you will only be exactly correct for #12 CU wire, just as you are only exactly correct for 1000 kcmil when you use 12.9. Either one should probably be close enough for most applications, especially since many people don't consider voltage drop at all! Maybe an avaerage of 12.75 would be good. Back years ago I was taught to use a K of 12.0. I think that may have been based on the resistance at 60 degrees Celsius.
Click to expand...

You guys are using the wrong number, unless you happen to be running bare copper. Uncoated does equate with bare, does it not? Shift your focus two columns to the right...

If you are not running DC circuitry, also have a look at Table 9: Alternating-Current Resistance... Note it provides only uncoated copper values. I'd like to see how they verified that in metal conduit!

Additionally, the basic voltage drop formula does not take into account an increase in resistance as the current approaches the ampacity of the conductor. General rule: Use K + 1 for circuits loaded over 50%.

VD = 2 x R x L x I

...or...

VD = 2 x K x L x I ? cmil,

therefore...

K = R x cmil
 
No, we are using the correct column. Uncoated and bare are not the same thing. Coated indicates a tinned or plated copper conductor. Not a very common material.

I do agree that Table 9 is a more accurate table for calculating voltage drop of an ac circuit. Most ac systems are not purely resistive.
 
Uncoated copper is what we install most often. Uncoated does NOT mean bare. Coated copper is the old wire that has a silver (usually tin) coating on the copper wire. My understanding is that this was done to avoid a chemical reaction between the copper conductor and the rubber insulation. Since we no longer have rubber insulation on conductors, this coating is not needed.
 
Uncoated is NOT Bare... OK, my bad, and thanks!

haskindm said:
Since we no longer have rubber insulation on conductors...
Click to expand...
Then what is the insulation on RHH and RHW (-2)? The last time I used some (what I recall as being) RHW, which was fairly recent, the strands were not tinned.
 
The insulation on RHW is a material called "Thermoset". I believe this is different from the "natural rubber" that required the tinned coating. Very little "natural rubber" is used in anything these days, almost everything (including tires) is made from a synthetic material.
 
haskindm said:
The insulation on RHW is a material called "Thermoset". I believe this is different from the "natural rubber" that required the tinned coating. Very little "natural rubber" is used in anything these days, almost everything (including tires) is made from a synthetic material.
Click to expand...
You are probably correct regarding natural vs. man-made rubber being used. However, "Thermoset" is not a material per se... it refers to a physical property of the material. A thermoset material is one which when shaped is not reshapeable (at the molecular level) after heated. Regarding rubber the process is generally referred to as vulcanization. Materials of a similar nature that can be shaped and reshaped by heating are referred to as thermoplastic. Some synthetic rubbers fall into this category.
 
...and you'll also notice trade name listings for and with "thermoplastic". Do you know how many different types of thermoplastic there are in existance? I certainly don't... and I used to R&D processing equipment for a plastics (and occasionally TPR... that's short for thermoplastic rubber) manufacturer in my younger years :) Vinyl (PVC) itself has seemingly countless variations.
 
Status
Not open for further replies.
Top