cheetah476
Member
- Location
- Louisville, KY, USA
I have searched this thread and have not found a clear answer to what I believe is a simple question.
I am installing a 2300V, 1167Hp, 251 FLA motor with a soft starter fed from an existing fused switchgear lineup. I am looking for clarification with sizing the conductors from the fuses to the starter and the starter to the motor.
Starter to Motor:
The way I read the code since it is over 600V is not to use 430.22(A) (see FPN No. 3) but use 430.224 to set the conductor size. 430.224 states "Conductors supplying motors shall have an ampacity not less than the current at which the motor overload protective device is selected to trip." The motor is 251A FLA so if the overload in the soft starter is set at <285A (max current for MV-105 cable from table 310.75) then the cable size can be 4/0 MV-105. This requires 105 degree C connections at both the motor and starter.
(Using 430.22 & table 430.250 gives 275*0.8*1.25% = 344A and a cable size of 350 MCM MV-105 which I don't believe is correct).
Fuses to Motor:
Conductors based on Fuse Sizing. Therefore having fuses less than 285A would allow the use of 4/0 MV-105 cables.
I am trying to obtain thoughts to compare with the engineered design. I think it is correct but I'm not 100% sure.
I am installing a 2300V, 1167Hp, 251 FLA motor with a soft starter fed from an existing fused switchgear lineup. I am looking for clarification with sizing the conductors from the fuses to the starter and the starter to the motor.
Starter to Motor:
The way I read the code since it is over 600V is not to use 430.22(A) (see FPN No. 3) but use 430.224 to set the conductor size. 430.224 states "Conductors supplying motors shall have an ampacity not less than the current at which the motor overload protective device is selected to trip." The motor is 251A FLA so if the overload in the soft starter is set at <285A (max current for MV-105 cable from table 310.75) then the cable size can be 4/0 MV-105. This requires 105 degree C connections at both the motor and starter.
(Using 430.22 & table 430.250 gives 275*0.8*1.25% = 344A and a cable size of 350 MCM MV-105 which I don't believe is correct).
Fuses to Motor:
Conductors based on Fuse Sizing. Therefore having fuses less than 285A would allow the use of 4/0 MV-105 cables.
I am trying to obtain thoughts to compare with the engineered design. I think it is correct but I'm not 100% sure.