multiple questions

kimrichi

Member
i was reading in abook and many points i did not understand appreciate if somebody could help
1- when choosing larger motor should not we choose the motor with the larger amperege not the larger va because in that book a walk in cooler with 20 A 208 volt 3 phase was chosen over vent lhood with a 6850 va 208 volt single phase (32 A)
2-could we have 3 phase load 277 volt and if it was for lighting doe not need a neutral
3-what does it mean a 100 A fuse with 10000 A interupting rating
4-what size thwn conductor required for 279 A load parrallel in 3 sets do we choose from column 60 c (279 divided by 3 is 93 less than 100 A)
 

augie47

Moderator
Staff member
Location
Tennessee
Since your profile states "student", why don't you tell us what you think the answers should be rather than us simply fill in the blanks for you.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
i was reading in abook and many points i did not understand appreciate if somebody could help
1- when choosing larger motor should not we choose the motor with the larger amperege not the larger va because in that book a walk in cooler with 20 A 208 volt 3 phase was chosen over vent lhood with a 6850 va 208 volt single phase (32 A)
Do the math on this a 20 amp 3phase motor --Watts= volts * amps= 208*1.73*20 amps= 7200 watts

2-could we have 3 phase load 277 volt and if it was for lighting doe not need a neutral
I have never seen a 277v load that didn't need a neutral. I assume if a 277 v transformer existed with 2 hots at 277 that would work but I have never experienced that.

3-what does it mean a 100 A fuse with 10000 A interupting rating
I will let the more experience give a good definition on that one

4-what size thwn conductor required for 279 A load parrallel in 3 sets do we choose from column 60 c (279 divided by 3 is 93 less than 100 A)
You choose from the col that the insulation is rated THWN is rated 75C and almost all terminations are rated 75C so you use that col.

Here is the trick on that.. Art. 310.4 will not allow parallel runs less than 1/0. So 1/0 is the answer
 

kimrichi

Member
Since your profile states "student", why don't you tell us what you think the answers should be rather than us simply fill in the blanks for you.
ok for (1) i believe it is wrong to choose the walk in motor as the largest although it is the largest va
i do not know the answers for 3 and 4
for number (4) i believe we should choose from column 60c because amperage is less than 100 A according to 110 14 c 1 a
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
ok for (1) i believe it is wrong to choose the walk in motor as the largest although it is the largest va
i do not know the answers for 3 and 4
for number (4) i believe we should choose from column 60c because amperage is less than 100 A according to 110 14 c 1 a
110.14c1a refers to ampacity as you stated however if the terminals are rated 75C and the wire is rated 75C then you may use the 75C column.

I answered the other 2 above
 

kimrichi

Member
110.14c1a refers to ampacity as you stated however if the terminals are rated 75C and the wire is rated 75C then you may use the 75C column.

I answered the other 2 above
i am sorry i did not get it do you mean what i said is right and choosing the walk in cooler is wrong
 

Smart $

Esteemed Member
Location
Ohio
i am sorry i did not get it do you mean what i said is right and choosing the walk in cooler is wrong
I'm assuming your talking about a feeder or service load calculation, which for this application starts at 220.14(C), and points us to 430.24 for multimotor loads... and (1) therein states "125 percent of the full-load current rating of the highest rated motor, as determined by 430.6(A)"... which points us to Tables 430.247 thru 250... which all refer to motor rating by horsepower. So the motor with the highest horsepower rating is the "largest".
 

kimrichi

Member
I'm assuming your talking about a feeder or service load calculation, which for this application starts at 220.14(C), and points us to 430.24 for multimotor loads... and (1) therein states "125 percent of the full-load current rating of the highest rated motor, as determined by 430.6(A)"... which points us to Tables 430.247 thru 250... which all refer to motor rating by horsepower. So the motor with the highest horsepower rating is the "largest".
i am sorry but this contradicts what mike holt says in his book nec exam preparation 2008 edition page 195
 

Smart $

Esteemed Member
Location
Ohio
I don't have it. You'll have to quote what it says.
Nevermind... you are correct. :ashamed:

430.17 Highest Rated or Smallest Rated Motor. In determining
compliance with 430.24, 430.53(B), and 430.53(C),
the highest rated or smallest rated motor shall be based on
the rated full-load current as selected from Table 430.247,
Table 430.248, Table 430.249, and Table 430.250.
 
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