Wire size for a motor feeder

[elmundogrande]

You have 5 - 5HP 3 phase 240 volt line with a total ampacity rating of 79.8 amps what size THWN feeder would be used to feed these motors? What is the correct size as per NEC. My past studies/teachings tells me to use a size #3, your thoughts please.

 

[david luchini]

You have 5 - 5HP 3 phase 240 volt line with a total ampacity rating of 79.8 amps what size THWN feeder would be used to feed these motors? What is the correct size as per NEC. My past studies/teachings tells me to use a size #3, your thoughts please.

#4 has a 75deg ampacity of 85.

 

[Dennis Alwon]

You have 5 - 5HP 3 phase 240 volt line with a total ampacity rating of 79.8 amps what size THWN feeder would be used to feed these motors? What is the correct size as per NEC. My past studies/teachings tells me to use a size #3, your thoughts please.

Does that include 125% of the largest motor?

 

[iceworm]

T430.250
240V, 3ph, 5hp >> 15.2A
4 each 5hp = 60.8A
1 each at 1.25 x 15.2 = 19A
Ampacity = 19 + 60.8 = 79.8, Matches OP

#4THWN, 75C column = 85A. Matches David

And I'm thinking you knew all this. Is there something else you are looking for?

 

[elmundogrande]

Does anyone use 110.14

Does anyone use 110.14

How do you know terminations are 75 degrees with no NEMA code letter or termination type

T430.250
240V, 3ph, 5hp >> 15.2A
4 each 5hp = 60.8A
1 each at 1.25 x 15.2 = 19A
Ampacity = 19 + 60.8 = 79.8, Matches OP

#4THWN, 75C column = 85A. Matches David

And I'm thinking you knew all this. Is there something else you are looking for?[/QUOTE

 

[Dennis Alwon]

Almost all terminations are rated 75C or higher. Older panels etc may not be but there should be something on the motor stating 75C. To be honest I wouldn't even think to check for 75C on a new piece of equipment.

 

[kwired]

How do you know terminations are 75 degrees with no NEMA code letter or termination type

Almost all terminations are rated 75C or higher. Older panels etc may not be but there should be something on the motor stating 75C. To be honest I wouldn't even think to check for 75C on a new piece of equipment.

Equipment made in last 30 years or so will most likely be 75C rated, older than that maybe not.

Motor leads likely are rated more than 90C, but your actual termination rating will depend on termination devices you use.

 

[iceworm]

How do you know terminations are 75 degrees with no NEMA code letter or termination type

I don't. The best I can do is believe that the OP is knowledgeable would tell us if the installation were out of the ordinary.

60C: definitely out of the ordinary for anything in the last 30 years.
90C : Never seen one.

 

[kwired]

How do you know terminations are 75 degrees with no NEMA code letter or termination type

I don't. The best I can do is believe that the OP is knowledgeable would tell us if the installation were out of the ordinary.

60C: definitely out of the ordinary for anything in the last 30 years.
90C : Never seen one.

Sure you have. Most were still used with conductors size per 60 or 75C ratings though. You very well have not seen a breaker terminal, motor starter, etc with 90C terminals though.

A lot of "wire nuts" are 105C rated, I think but often are used for conductors already sized per 60 or 75C ampacity

 

[iceworm]

Sure you have. Most were still used with conductors size per 60 or 75C ratings though. You very well have not seen a breaker terminal, motor starter, etc with 90C terminals though.

A lot of "wire nuts" are 105C rated, I think but often are used for conductors already sized per 60 or 75C ampacity

You are correct. I did not list as exceptions, MV-90, MV-105, alien containment fields, NASA ISS, custom built MCC, custom built combination starters ...

However, I'm pretty sure the context was connecting the motor(s) to something that would provide power - say like, "breaker terminal, motor starter, etc"

 

[elmundogrande]

In test conditions the answer is #3

In test conditions the answer is #3

You have 5 - 5HP 3 phase 240 volt line with a total ampacity rating of 79.8 amps what size THWN feeder would be used to feed these motors? What is the correct size as per NEC. My past studies/teachings tells me to use a size #3, your thoughts please.

This question came up in a test and 90% got it wrong as per the codes not trade practices always assuming cause.

 

[iceworm]

Aha -- Test
So the examination authority wanted you to use 110.14.C.1. And since they didn't tell you the equipment was marked with a temperature rating, you were supposed to use 60C column.

If true, real sweethearts, they are.

I wonder how many of them are still using TW. (No, I have not seen any of that either in the last 30 years either)

But they did make their point. You're supposed to check, "Does it have a temperature rating marked on the equipment?" They would have failed me as well.

 

[kwired]

You are correct. I did not list as exceptions, MV-90, MV-105, alien containment fields, NASA ISS, custom built MCC, custom built combination starters ...

However, I'm pretty sure the context was connecting the motor(s) to something that would provide power - say like, "breaker terminal, motor starter, etc"

My assumption also. Breaker terminal, motor starter, etc. likely 75C or 60C if old equipment under 100 amps. Motor itself likely has leads that are rated more than 90C.

That said typical neutral bus in a panelboard is rated 90C. Most have conductors sized per 75C ampacity connected to them. Some other terminal blocks or other splicing methods that are not also connected directly to an overcurrent device or other equipment can be rated and used at 90C.

 

[Dennis Alwon]

If this was a test answer then I would have been able to give you the correct answer because I am helping someone study and that type of question came up. Unfortunately, in the real world it is basically a non-issue as all things new are at 75C and I don't believe my supplier even carries 60C wire. NM would be at 60C but generally that is not used on 3 phase motors.

I have mixed emotions about that type of question but you can see what they are trying to do.

I have seen many test questions for motors and I don't believe I have ever seen the 60C rating ever show up. All answers assumed 75C for motors.-- So maybe I would have missed it also..

 

[kwired]

This kind of question (somewhat a trick question) is acceptable for educational situations, it makes the student think and learn about things, but shouldn't be tolerated for license exam situations IMO. Is a little unfortunate if a person fails test by one missed question and such a question was one of the missed ones but they did everything else right on that particular question. If multiple choice answers and both possible outcomes are not in the answer list, that is a little more fair.