Motor Feeder and branch calcs- this look ok?

[Telaid]

I have the following motors all running at the same time:

(qty)

(1) 75 HP 460V 3ph, 87 FLA

(2) 50 HP 460V 3ph 57 FLA

(1) ?HP 480V 3ph 615 FLA ( data was given to me, no HP)

A utility transformer will feed a 1200 amp CT cabinet with a main breaker. This will feed a distribution panel with a main breaker and breakers for each of the motors. I calculated the branch load breakers as follows from 430.52, all to be instantaneous trip.

87A*125% = 108.8 A -> 110A breaker
57A*125% = 71.25 A -> 80A breaker (2)
615A*125% = 768.7A -> 800A breaker

The feeder from the CT cabinet to the distribution panel i calculated from 430.24A as
=(615*125%) + 57 + 57 + 87 = 969.75A

then from table 310.16 (60 degree column for worst case) i use 3 sets of 1250 kcmil.

Feeder protection

The necessary breaker i calculated from 430.62 as 800 + 87 + 57 + 57 = 1001 Amps. So ill put a 1000 amp breaker in the distribution cabinet.


Does this look correct? im unsure of what to do since the 3 smaller motors are being controlled with an ABB VFD installed in an ABB bypass cabinet which allows the VFD to be taken out of service. The large motor is controlled by its own VFD.

Any input would be helpful.

Thanks

Ron

 

[augie47]

calculations...

calculations...

I see problems...

to begin with, the calculations should be based on 430.252, not nameplated data.
Instantanheous breakers would have to be part of a listed motor control device and those numbers, IMHO, wouold be wayyy low for an IT breaker.
Unless I miscaluculated, if the 615 agrees with nameplate your feeder would need to be 994 amps.
Even with the VFD, I think your 100 amp breaker is too small.

my opinion

 

[S'mise]

big motor

big motor

Your using nameplate fla instead of tables; should be 96a for 75hp, 65a for 50hp and the last motor must be a 100hp locked rotor? Get more info from the nameplate (design B..ect) If you are using inst trip breakers you would multiply by 800% not 125%. (table 430.52 in 2002 nec) The 125% is for the branch condustors. 1awg for the 75hp, 4awg for 50hp, and the biggie motor would need something like (2) 600KCmil. Just guessing where to look on the table. It is more likely you will be using ITC breakers. Good luck.