jimmy_mcquade
Member
Hi Everyone,
I asked this same question last week on the NEC forum and got a lot of
Good answers, but I am still confused. The system design has changed
some according to dad and I am now trying to understand this on top of
everything else.
The system is 208 3 phase and is a continuous running production line.
There are (12) 3 phase motors associated with a proposed MCC. (2) 7 ? hp,
(2) 5 hp, (2) 3 hp, (3) 1 ? hp, and (3) 1 hp motors.
Now the system will also include another (2) 20 hp motor fans on equipment
being purchased and (4) 20 amp, 120 volt plc control panels. Dad told me
that no 3 phase Motors would be run with less than a #12 wire (plant spec).
Table 430.247 to 430.250 gives the motor amps. 430.22 tells us the motor
overload protection, 430.22 is for the branch conductor size, 430.24 is the
feeder conductor, and 430.46 is the feeder protection.
For the (12) motors, I get a continuous load of 136.6 amps.
For the 7 ? hp motor, 24.2 amps, Motor overload current 30.25 (430.32 at
125%), #8 wire (table 310.16), 60.5 amps (70 amps)branch protection
(430.52 at 250%), 30.25amps (40 amps) feeder conductor (430.24).
The MCC main breaker would be 182.4 amps (430.62), 175 amps next size
down. Since this is a continuous load, the 80% breaker rule would apply and
the size would be 225 amp (225 * 80%) = 180 amps
You would also do the same thing for the (2) 20 hp motors and I think the
plc panels also.
The main breaker for all of these devices would be 426 amps (400 next size
down).
Since this is a continuous load, the 80% breaker rule would apply and the
size would be 500 amp (500 * 80%) = 400 amps. As dad says, you always
forget something, so go for a 600 amp service and breaker.
My questions are.
#1. Is this correct, #2. does the 80% breaker rule apply for the breakers,
and #3 if the branch protection for the 7 ? hp motor is 70 amps, wouldn?t
the wire size also be rated for 70 amps?
I cannot find a way to attach my spreadsheet, or i would attach it.
Thanks in advance,
Jimmy
I asked this same question last week on the NEC forum and got a lot of
Good answers, but I am still confused. The system design has changed
some according to dad and I am now trying to understand this on top of
everything else.
The system is 208 3 phase and is a continuous running production line.
There are (12) 3 phase motors associated with a proposed MCC. (2) 7 ? hp,
(2) 5 hp, (2) 3 hp, (3) 1 ? hp, and (3) 1 hp motors.
Now the system will also include another (2) 20 hp motor fans on equipment
being purchased and (4) 20 amp, 120 volt plc control panels. Dad told me
that no 3 phase Motors would be run with less than a #12 wire (plant spec).
Table 430.247 to 430.250 gives the motor amps. 430.22 tells us the motor
overload protection, 430.22 is for the branch conductor size, 430.24 is the
feeder conductor, and 430.46 is the feeder protection.
For the (12) motors, I get a continuous load of 136.6 amps.
For the 7 ? hp motor, 24.2 amps, Motor overload current 30.25 (430.32 at
125%), #8 wire (table 310.16), 60.5 amps (70 amps)branch protection
(430.52 at 250%), 30.25amps (40 amps) feeder conductor (430.24).
The MCC main breaker would be 182.4 amps (430.62), 175 amps next size
down. Since this is a continuous load, the 80% breaker rule would apply and
the size would be 225 amp (225 * 80%) = 180 amps
You would also do the same thing for the (2) 20 hp motors and I think the
plc panels also.
The main breaker for all of these devices would be 426 amps (400 next size
down).
Since this is a continuous load, the 80% breaker rule would apply and the
size would be 500 amp (500 * 80%) = 400 amps. As dad says, you always
forget something, so go for a 600 amp service and breaker.
My questions are.
#1. Is this correct, #2. does the 80% breaker rule apply for the breakers,
and #3 if the branch protection for the 7 ? hp motor is 70 amps, wouldn?t
the wire size also be rated for 70 amps?
I cannot find a way to attach my spreadsheet, or i would attach it.
Thanks in advance,
Jimmy