Constant K

[ZCBee]

In the formula

R=K*L/CM

where does the constant 10.4 derive?

I calculate the resistance of 2 #10 copper 30ft each utilizing NEC table 9 as

{1.21/1000 = x/60} = 0.0726 ohms

but, when I use the first formula, I get 0.060 ohms.

My guess is reactance being factored into the constant "K" ??

Thank you.

 

[K8MHZ]

In the formula

R=K*L/CM

where does the constant 10.4 derive?

I calculate the resistance of 2 #10 copper 30ft each utilizing NEC table 9 as

{1.21/1000 = x/60} = 0.0726 ohms

but, when I use the first formula, I get 0.060 ohms.

My guess is reactance being factored into the constant "K" ??

Thank you.

The table you used is for calculating the resistance at 75 degrees C. The formula is for 'standard temperature' or 20 degrees C.

I don't know for sure if that is the reason, but I do know the difference in temperature exists. Also, in the next table (for 3 phase) it specifically mentions reactance.

 

[Cold Fusion]

In the formula

R=K*L/CM

where does the constant 10.4 derive?

I calculate the resistance of 2 #10 copper 30ft each utilizing NEC table 9 as

{1.21/1000 = x/60} = 0.0726 ohms

but, when I use the first formula, I get 0.060 ohms.

My guess is reactance being factored into the constant "K" ??

Thank you.

As K8MHZ said:
T8 is resistance for direct current at 75C.
Copper changes resistance with temperature

The temperature coefficient is .00393 (per deg C)

So if the copper were 100 ohms at 75C then at 20C it would be:
R(25) = 100 - (100 x .00393) x (75-20) = 100 - 21.6

So a 55C change in temp will give a 21.6 percent change in resistance.

Again as K8MHZ says, it appears that the K factor you picked is for a temperature less than 75C.

I'd guess K = 10.4 is right for about 30C

cf

 

[Cold Fusion]

As K8MHZ said:
T8 is resistance for direct current at 75C.
Copper changes resistance with temperature

The temperature coefficient is .00393 (per deg C)

So if the copper were 100 ohms at 75C then at 20C it would be:
R(25) = 100 - (100 x .00393) x (75-20) = 100 - 21.6

So a 55C change in temp will give a 21.6 percent change in resistance.

Again as K8MHZ says, it appears that the K factor you picked is for a temperature less than 75C.

I'd guess K = 10.4 is right for about 30C

cf

Oh - oh. Error on my part. I didn't look at T8. I used a coeficient for a copper RTD. T8 uses a coeficient of .00323

So that would change my response
R(20) = 100 - (100 x .00323) x (75-20) = 100 - 17.8

So a 55C change in temp will give a 17.8 percent change in resistance.

Using your numbers, K = 10.4 calculates to a temperature of 21.3C. As K8 said close to 20 C. Probably the only difference is my calculation in-accuracies.

cf

 

[ZCBee]

Thanks for the help.