MCC and Motor overcurrent protection question

[Learner]

i'm studying NEC for my project. and i have many questions.

i want to be checked if my understand is correct.

first. conductor, if i have 100 hp 480V 3phase motor, FLC is 124A by table 430.250.

and conductor size must be not less than 125% of FLC. by 430.22

124A * 1.25 = 155A.

So I can choose conductor THHN 3 awg by Table 310.17

second, breaker
single motor OCPD : 430.52(C) and Table 430.52(C)(1)
FLC * 2.5 ≤ Size of Breaker ≤ FLC * 4(less than 100A)
FLC * 2.5 ≤ Size of Breaker ≤ FLC * 3(greater than 100A)
(FLC * 1.5 when motor type is wound-rotor or DC)


124A * 2.5 = 310A
124A * 3 = 372A

So, 100HP motor's Breaker size is between 310A and 372A.

Following Table 240.6(A) Breaker Size is 350A.

Third, MCC bus and OCPD.
If i have 480V 3Phase motors : 10 X 50hp and 1 100hp
and the other Loads 15kW
FLC
50hp : 65A
100hp : 124A

the other load : 15kW/(480V * 1.73) = 19A

by section 430.24
conductor size ≥ 1*124A*1.25 + 10*65A + 19A
-> conductor ampacity is bigger than 824 A
and i choose busbar rating 1000A(following by quick bus bar size selector table in internet)

MCC OCPD must not exceed busbar rating. Section430.94
So I can choose MCC OCPD rating 1000A.
(Normal Feeder breaker size < largest rating ocpd + other motors + other loads) -Section 430.62


This calculation is based on my understanding of NEC and it can be wrong. Please check and let me know where i'm misunderstanding.

Thank you for reading and have a nice day.

PS.
From my former experience in my country, conductor ampacity must be larger than breaker ratings.
But in this calculation i choose 350A breaker and THHN 3 AWG Cable which has allowable ampacity 165A, for motor.
Is this okay? or is there any exceptions? or code?

 

[augie47]

Few comments:
Since you referenced NEC I assume Korea is using NEC
Table 310,.17 is for "conductors in free air", 310.16 is more common here (conductors in a raceway)
In either event, you need to take 110.14 into account as terminal temperature limitations must be taken into account. In most cases the termination limits for your motor and your MCC would require you to use the75° rating of the conductor so for most US installs it would be a 2/0 conductor (in raceway. (If it was a "free air" install, a #2 would be required based on the 75° 110.14 termination rules)
The 300% rating on the breaker is only allowed when the 250% is inadequate. As a PE you could show that it was needed, but, more commonly, the 250% rule is used as the max breaker.

 

[don_resqcapt19]

...

PS.
From my former experience in my country, conductor ampacity must be larger than breaker ratings.
But in this calculation i choose 350A breaker and THHN 3 AWG Cable which has allowable ampacity 165A, for motor.
Is this okay? or is there any exceptions? or code?

Motors are a special case. The breaker is only providing short-circuit and ground-fault protection for the conductors. The motor itself must also have overload protection. Since the motor is the only load on a motor branch circuit, the motor overload protective device also provides the overload protection for the conductors.

 

[Learner]

Few comments:
Since you referenced NEC I assume Korea is using NEC
Table 310,.17 is for "conductors in free air", 310.16 is more common here (conductors in a raceway)
In either event, you need to take 110.14 into account as terminal temperature limitations must be taken into account. In most cases the termination limits for your motor and your MCC would require you to use the75° rating of the conductor so for most US installs it would be a 2/0 conductor (in raceway. (If it was a "free air" install, a #2 would be required based on the 75° 110.14 termination rules)
The 300% rating on the breaker is only allowed when the 250% is inadequate. As a PE you could show that it was needed, but, more commonly, the 250% rule is used as the max breaker.

Thank you for your answer. i'll study and requestion about what i don't understand.