Contractor Exam Prep - Unit 8 Challange question 2

[S-117]

Hello everyone.

I've caught many errors in this book, but I don't want to bother Mike Holt with every issue I see.

I am really not getting how they came up with the answer for this question.
The solution shows a table for all the motors, (3 15hp three-phase motor, 3 3hp one-phase motor, 3 1hp one-phase motor)
the table makes no sense, because for the 3hp single phase, on the table it kinda looks like to me as if it were a 2-phase motor. It makes each motor take up two of the electric lines.

also, for the 1hp motor, it assumes that each motor is on a different line. Not sure if the question has an error, the answer has an error, or the question simply had assumptions made but not stated.

thanks

 

[don_resqcapt19]

Most of us do not have the book, so it is hard to comment.

If the 3hp single phase motors are using two breaker spaces, it just means it is single phase, line to line. It it only used one breaker space, it would be single phase line to neutral. While two phase does exist, it is very rare and I doubt if you would find in in any of Mike's books.

 

[Dennis Alwon]

If this is the question about the size of the feeder for those motors then Mike Holt's answer is correct, IMO.

3 motors 15 hp, 3phase = 46.20 amps
3 motors 3 hp, single phase 208 = 18.70 amps
3 motors 1 hp 115V = 16 amps

The 3 hp single phase motor at 208V does connect to 2 phases but it is still considered single phase. The clue is the 208v. On a 3 phase system we can have many different voltages but the ones we normally see on exams are 3 phase 208V. So for a single phase 208V that is measured across any 2 phases. For single phase 115V it would be from one phase to neutral

 

[Dennis Alwon]

Most of us do not have the book, so it is hard to comment.

If the 3hp single phase motors are using two breaker spaces, it just means it is single phase, line to line. It it only used one breaker space, it would be single phase line to neutral. While two phase does exist, it is very rare and I doubt if you would find in in any of Mike's books.

I think the op is calling 208 single phase--- 2 phase as I mentioned above

 

[mgookin]

I think the op is calling 208 single phase--- 2 phase as I mentioned above

There are too many 3's in that question for it to be used as a teaching aid.

I always tried to get professors to use prime numbers in calculus questions because it makes it clearer what's happening with the math.
Here in this question we have 1p and 3p loads and there are 3 of everything!

Example:
Teach a student that f' of x² = 2x and he's got several possibilities of why that's true running through his head, with the chances being none of them are correct.
Teach that same student that f' of x⁷ = 7x⁶ and he can see that f' of xⁿ = nx^(n-1).

 

[david luchini]

If this is the question about the size of the feeder for those motors then Mike Holt's answer is correct, IMO.

I don't believe the answer is correct. I come up with 198.55A, and #3/0 would be acceptable.

They should have converted the loads to VA rather than adding amps. The three 3HP, 208V single phase motors don't contribute 18.7*2 amps per phase,
they contribute 18.7*1.732 amps per phase.

 

[don_resqcapt19]

I don't believe the answer is correct. I come up with 198.55A, and #3/0 would be acceptable.

They should have converted the loads to VA rather than adding amps. The three 3HP, 208V single phase motors don't contribute 18.7*2 amps per phase,
they contribute 18.7*1.732 amps per phase.

I don't see anything in 430.24 that lets you use VA. The rule is all based on full load currents.

 

[S-117]

There are too many 3's in that question for it to be used as a teaching aid.

I always tried to get professors to use prime numbers in calculus questions because it makes it clearer what's happening with the math.
Here in this question we have 1p and 3p loads and there are 3 of everything!

Example:
Teach a student that f' of x² = 2x and he's got several possibilities of why that's true running through his head, with the chances being none of them are correct.
Teach that same student that f' of x⁷ = 7x⁶ and he can see that f' of xⁿ = nx^(n-1).

Exactly! perfect comment

 

[S-117]

Most of us do not have the book, so it is hard to comment.

If the 3hp single phase motors are using two breaker spaces, it just means it is single phase, line to line. It it only used one breaker space, it would be single phase line to neutral. While two phase does exist, it is very rare and I doubt if you would find in in any of Mike's books.

wait so what is the different between 2-phase motor and single phase motor wired to two phases?

also, are these motors in the problem the ones that are normally listed with two voltages? (eg. 120/208.... *i'm guessing on numbers)

 

[S-117]

Most of us do not have the book, so it is hard to comment.

If the 3hp single phase motors are using two breaker spaces, it just means it is single phase, line to line. It it only used one breaker space, it would be single phase line to neutral. While two phase does exist, it is very rare and I doubt if you would find in in any of Mike's books.

also, why is it assumed that each 1hp motor is wired to different lines? is that a code requirement or an arbitrary assumption?

 

[S-117]

Most of us do not have the book, so it is hard to comment.

If the 3hp single phase motors are using two breaker spaces, it just means it is single phase, line to line. It it only used one breaker space, it would be single phase line to neutral. While two phase does exist, it is very rare and I doubt if you would find in in any of Mike's books.

wait, what book would you have if not the EXAM PREP ?

older editions? :blink:

 

[Dennis Alwon]

wait, what book would you have if not the EXAM PREP ?

older editions? :blink:

Mike Holt does not post here and we are just members scattered across the globe. Most of us have no need for the exam prep book. I happened t have one I bought for my worker so he could get his license. My book is the contractor exam book so it seems mike uses the same questions in many different books.

 

[david luchini]

I don't see anything in 430.24 that lets you use VA. The rule is all based on full load currents.

I don't see anything that doesn't let you use VA. If you wanted to take 430.24 that literally, a feeder supplying a 25HP, 460V motor and a 25HP, 200V motor would require an ampacity not less than
131.75. Add 5400VA of non-continuous receptacle load, and all of a sudden the feeder needs an ampacity not less than 5531.75.

 

[Dennis Alwon]

also, why is it assumed that each 1hp motor is wired to different lines? is that a code requirement or an arbitrary assumption?

It is not arbitrary but you would always try to balance the loads to find the minimum load. If everything were on one the same phases then the feeder would have to be larger...

 

[K8MHZ]

wait so what is the different between 2-phase motor and single phase motor wired to two phases?

also, are these motors in the problem the ones that are normally listed with two voltages? (eg. 120/208.... *i'm guessing on numbers)

A single wire is not a phase. It's better called a 'leg'. Any load connected to two 'legs', no matter if one is neutral or not, is single phase.

A two phase motor will have (usually) four conductors, or 'legs'. Two legs for each phase.

Two phase systems are very rare.

 

[S-117]

It is not arbitrary but you would always try to balance the loads to find the minimum load. If everything were on one the same phases then the feeder would have to be larger...

that's why I was asking. Because if it is NOT a requirement, then the loads won't be balanced and conductors would be undersized to what is calculated vs. what the installer arbitrarily does, no?

Unless there is a requirement to balance out the loads. (NEC noob here)

 

[S-117]

A single wire is not a phase. It's better called a 'leg'. Any load connected to two 'legs', no matter if one is neutral or not, is single phase.

A two phase motor will have (usually) four conductors, or 'legs'. Two legs for each phase.

Two phase systems are very rare.

does that mean that 3 phase has six legs?


also, the table in the answer key only shows 3 lines (which I assumed are 3 legs)

I guess it is tough to understand since the book doesn't explain 2-phase I think at all

 

[david luchini]

does that mean that 3 phase has six legs?

No, it has 3 legs.

 

[mgookin]

wait so what is the different between 2-phase motor and single phase motor wired to two phases?

also, are these motors in the problem the ones that are normally listed with two voltages? (eg. 120/208.... *i'm guessing on numbers)

You need to learn about a 3 phase 208/120 Y service. It has 3 phase conductors and a center tapped neutral.

Get two phase out of your head. There is none on this test question and there won't be (won't be expected to be) any 2 phase on the test. 2 phase is very rare, so rare, that for this purpose you'd be better off pretending it does not exist.

On the 3 phase 208/ 120 you measure 208 between legs and you measure 120 from neutral to any one leg. Use all 3 legs for 3 phase and you still have 208V.

Now does the math in post #3 make sense?

Again, having 3 of each motor on 3 phase questions was not the best they could have come up with.

 

[S-117]

You need to learn about a 3 phase 208/120 Y service. It has 3 phase conductors and a center tapped neutral.

Get two phase out of your head. There is none on this test question and there won't be (won't be expected to be) any 2 phase on the test. 2 phase is very rare, so rare, that for this purpose you'd be better off pretending it does not exist.

On the 3 phase 208/ 120 you measure 208 between legs and you measure 120 from neutral to any one leg. Use all 3 legs for 3 phase and you still have 208V.

Now does the math in post #3 make sense?

Again, having 3 of each motor on 3 phase questions was not the best they could have come up with.

3-phase isn't the issue for me

I think knowing the difference between the 2phase motor vs. 1phase motor wired to two phases will help me understand how the table in the book was created.