Where is the value for 'K' found for voltage drop calculations?

[Stevareno]

I have gone through Mike Holt's 2011 master electrician prep exam and on the answer sheet for the voltage drop questions, he gives an ohm value for 'K' but does not show any reference to how that number is derived.
This is not given in the question, so I have to assume it's found somewhere in the code book.

The question asks: What size conductor is required for a single phase motor where the FLA = 52 @ 115 volts? Apply the NEC recommended voltage drop limits.

 

[kwired]

I have gone through Mike Holt's 2011 master electrician prep exam and on the answer sheet for the voltage drop questions, he gives an ohm value for 'K' but does not show any reference to how that number is derived.
This is not given in the question, so I have to assume it's found somewhere in the code book.

The question asks: What size conductor is required for a single phase motor where the FLA = 52 @ 115 volts? Apply the NEC recommended voltage drop limits.

I can't tell you exactly where K comes from without a little searching but can tell you it depends on type of conductor and temperature which were not in the question you gave us.

 

[resistance]

I have gone through Mike Holt's 2011 master electrician prep exam and on the answer sheet for the voltage drop questions, he gives an ohm value for 'K' but does not show any reference to how that number is derived.
This is not given in the question, so I have to assume it's found somewhere in the code book.

The question asks: What size conductor is required for a single phase motor where the FLA = 52 @ 115 volts? Apply the NEC recommended voltage drop limits.

Note: Motor nameplate is used to size overload protection, and not conductors. So, the question is mute. Are you sure this is a mike Holt question? What was the answers?

 

[resistance]

Note: The value for K is 12.9 ohms for cu, and 21.2 for Alum—where “K” represents the dc resistance for a 1K Cmil conductor that is 1K feet long @ a 75 degree operating temp..

 

[Hv&Lv]

you can figure them on your own, or look them up...

k = R (resistance of the conductor based on size from the NEC) times circular mils
divided by 1000.

so for say a 1/0 cu coated wire, from table 8, go to 1/0, copper, in the coated column, and under ohm/kFT, look at .127. now multiply that times the circular mil (105600) from the same table, then divide by 1000= 12.88

Of course, you could just look them up, but it never hurts to know how do figure them by the book.
You could also use the standby values, but it is exact? this way

The number is derived from•K=Direct-current constant. K represents the DC resistance for a 1,000-circular mils conductor that is 1,000 feet long, at an operating temperature of 75?C

 

[Stevareno]

Oops, in trying to shorten the question, I forgot to show distance involved.

Full question:
A single phase, 5 HP motor is located 110' from panelboard. The nameplate indicates that the voltage is 115/230V and the FLA is 52/26A.
What size conductor is required if the motor windings are connected to operate at 115V? Apply the NEC recommended VD limits.

Answer on the answer sheet:

Cmil = (2 * K * I * D) / VD
K = 12.9 ohms, copper
I = 52A
D = 110'
VD = (115 * 0.03) = 3.45V
Cmil = (2 * 12.9 ohms * 52A * 110') / 3.45V
Cmil = 42,776
Chapter 9, Table 8 = 8 AWG

 

[Stevareno]

you can figure them on your own, or look them up...

k = R (resistance of the conductor based on size from the NEC) times circular mils
divided by 1000.

so for say a 1/0 cu coated wire, from table 8, go to 1/0, copper, in the coated column, and under ohm/kFT, look at .127. now multiply that times the circular mil (105600) from the same table, then divide by 1000= 12.88

Question is asking for wire size. If I don't initially know the wire size, how can I possibly find 'K'? :blink:

 

[Howard Burger]

Sourse of K

Sourse of K

Steve, the FLA issue aside, K is derived from table 8 of the NEC: for K multiply the area of a wire in circular mills by the resistance in ohms per 1000 feet as found in Table 8. I don't think K is mentioned anywere in the NEC, just as voltage drop is only mentioned in an Informational Note #4 on 210.19.A.1

As to the why these numbers were chosen for K, maybe one of the PEs on the forum will chime in, but you're pretty safe remembering K for Cu is 12.9 and Al is 21.2 (or 13 and 21 for simplicity).

 

[Stevareno]

As to the why these numbers were chosen for K, maybe one of the PEs on the forum will chime in, but you're pretty safe remembering K for Cu is 12.9 and Al is 21.2 (or 13 and 21 for simplicity).

So, 'K' is a constant, no matter what size the conductor is?

Edit: OK, I see. I calculated several wire sizes. No matter the wire size, K seems to figure out the same.
Now I feel like a :dunce:.


Thanks for the help.

 

[kwired]

So, 'K' is a constant, no matter what size the conductor is?

Edit: OK, I see. I calculated several wire sizes. No matter the wire size, K seems to figure out the same.
Now I feel like a :dunce:.


Thanks for the help.

If you want to be really accurate I believe you need to figure out what K is in manners described in other posts.

If you just want to be close enough (and probably on the conservative side just in case) it is common to use 12.9 and 21.2 (or at least very close to these values) How often does your conductor actually operate at 75?C? Conductor operating lower temp than that will have less drop than what you calculated so you will be on the safe side in general.

 

[resistance]

Still lost on why your study sheet uses the FLA instead of the required amps in T. 430.248?

 

[kwired]

Still lost on why your study sheet uses the FLA instead of the required amps in T. 430.248?

The real question is why does the answer use FLA instead of T. 430.248 values?

In a real application you will have the nameplate available also but need to know that you use the tables for just about everything except overload protection.

 

[Stevareno]

Still lost on why your study sheet uses the FLA instead of the required amps in T. 430.248?

I'd guess that is because the value is given in the question.
If the question had omitted that info, then you would have to use the table.

 

[resistance]

I'd guess that is because the value is given in the question.
If the question had omitted that info, then you would have to use the table.

That doesn?t make since. Sure you don?t have a correction sheet with your study guide?

 

[resistance]

The real question is why does the answer use FLA instead of T. 430.248 values?

whatever:roll:

 

[Stevareno]

That doesn?t make since. Sure you don?t have a correction sheet with your study guide?

Nope. I only downloaded the practice exam that comes with an answer sheet. No study guide.
The answer sheet clearly shows that the value given in the question is used in the calculation.
If I had used the table to find FLA my answer might have come out different than that given on the answer sheet.

 

[George Stolz]

...he gives an ohm value for 'K' but does not show any reference to how that number is derived.

May I take this opportunity to sell you on the following formula instead?

VD = ?RID

Which is
VD = 2RID for a 2-wire circuit
VD = √3RID for a 3-phase circuit

It gives almost identical results, just a little quicker - and you don't have to remember the K constant, the resistance values in Table 8 are all you need.

 

[jumper]

May I take this opportunity to sell you on the following formula instead?

VD = ?RID

Which is
VD = 2RID for a 2-wire circuit
VD = √3RID for a 3-phase circuit

It gives almost identical results, just a little quicker - and you don't have to remember the K constant, the resistance values in Table 8 are all you need.

I'll take it, can I get fries with that?

 

[resistance]

Nope. I only downloaded the practice exam that comes with an answer sheet. No study guide.
The answer sheet clearly shows that the value given in the question is used in the calculation.
If I had used the table to find FLA my answer might have come out different than that given on the answer sheet.

Mike Holt has a page (or pages) on this site, that offer corrections to their study materials. Maybe someone will post the link. I believe the question is in error, unless I?m missing something?which wouldn?t surprise me, seeing that I?m not using this formula, or any math formulas on a regular basis.

 

[resistance]

Maybe this is it: http://www.mikeholt.com/catalog.php?id=catalogbookcorrections