1. Junior Member
Join Date
Oct 2014
Location
Utah
Posts
13

## Test Questions

I have a lot of questions as I prepare for journeyman test. All help is greatly appreciated!

What is the minimum size copper conductor needed for a 208 V three-phase machine located 140 feet from the power source? ( Machine draws 27 amperes, 3% voltage drop, use 12.9 K)

2. Originally Posted by sclement
I have a lot of questions as I prepare for journeyman test. All help is greatly appreciated!

What is the minimum size copper conductor needed for a 208 V three-phase machine located 140 feet from the power source? ( Machine draws 27 amperes, 3% voltage drop, use 12.9 K)
I have taken several journeyman tests and I do not think that I have ever seen a voltage drop calculation on any of them. It has been a while though. As is the practice, show your method and the folks here will chime in to help.

3. Junior Member
Join Date
Oct 2014
Location
Utah
Posts
13
I have taken several journeyman tests and I do not think that I have ever seen a voltage drop calculation on any of them. It has been a while though. As is the practice, show your method and the folks here will chime in to help.
I've heard the same thing, not sure if Utah has a couple voltage drop questions or not? But anyway I'm not sure exactly how to do this problem? That's kinda why I posted it here for some help with it?

4. Originally Posted by sclement
I have a lot of questions as I prepare for journeyman test. All help is greatly appreciated!

What is the minimum size copper conductor needed for a 208 V three-phase machine located 140 feet from the power source? ( Machine draws 27 amperes, 3% voltage drop, use 12.9 K)
#10 THWN Solid= 10380 CM
Using Vd= 1.73*12.9*27*420 / 10380
You get 2.72 V
Way less than 3% of 208V which is 6.24V

12.9 = K
420= Length of conductors (3X140)
1.73=Sq. Root of 3

Ampacity per 430.22 must be 125% of the FLC = 33.75 A

#10 THW Ampacity @ 75 Degrees = 35 A
Last edited by Unbridled; 08-13-17 at 12:47 AM.

5. Junior Member
Join Date
Mar 2017
Location
US
Posts
12

## Test question

Originally Posted by sclement
I've heard the same thing, not sure if Utah has a couple voltage drop questions or not? But anyway I'm not sure exactly how to do this problem? That's kinda why I posted it here for some help with it?
Have taken 3 journeymen test in the last yr for diffrent states. Haven't seen any vd questions like it neither. ..I agree with unbridles' formula. But would uses 6.24 as my voltage drop in the formula since 208 is the applied voltage. 1.732*12.9×140*27/6.24=13,534cma which is larger than a number 10awg so we must go with #8..
430.22 wouldn't apply in this case because its not stated that the motor is continous. I would take inconsideration to use what the question gives you when taken a test.They are tricky in the way the questions are asked.I'm open for corrections and learning. Thanks... formula...single phase. Cma=2×k×i×l /vd....3phase ...cma=1.732×k×i×l/vd..
2 is the lenth back and forth,k is the costant,i is applied amperage,l is the actual length.,vd is percentage of the applied voltage. We switch (2) to(1.732) in the single to 3phase. When using this formula.

6. My Southwire VD calculator app says #8.

7. agree with post #5 and #6

8. Originally Posted by sclement
I have a lot of questions as I prepare for journeyman test. All help is greatly appreciated!

What is the minimum size copper conductor needed for a 208 V three-phase machine located 140 feet from the power source? ( Machine draws 27 amperes, 3% voltage drop, use 12.9 K)
Post #5 and 6 are correct. The way you had your question worded through me of.
I used the voltage drop of #10 CU.(2.32 V)
Your question states to use 3% Vd(6.24 V)

3% Vd is the max. per NEC. In practical use, however, I don't calculate per wire size based on the possibility of a 3% Vd, I size it per the actual Vd of the conductor I'm contemplating on using.
If you remove the wording "3%" from your question, wouldn't you calculate you conductor size just not to exceed 3%, such as #10??

9. Senior Member
Join Date
Dec 2013
Location
salt lake city, UT USA
Posts
119
I have taken several journeyman tests and I do not think that I have ever seen a voltage drop calculation on any of them. It has been a while though. As is the practice, show your method and the folks here will chime in to help.
FYI- The Utah journeyman exam is actually 3 separate exams- code examination (80 questions), theory examination (50 questions), and a practical examination (7 practical exercises). There are 10 voltage drop calculation questions on the theory exam.

10. Senior Member
Join Date
Oct 2009
Location
Austin, TX, USA
Posts
9,573
Originally Posted by Unbridled
3% Vd is the max. per NEC.
The NEC does not dictate a 3% max Vd.

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