CONDUCTOR/ C.B SIZING FOR WATER TRANSFER PUMP

[TMMB]

I HAVE 3PHASE WATER TRANSFER PUMP rating 93 KW , 380 , 60 HZ and the motor power consumption is 56 KW , my questions are :

1- the sizing to calculated for the pump rating (93kw) or for the consumption (56kw)
2- i belive that the design of the conductor/C.B must be based on the pum rating ; I= 93000/1.73*380*0.8 =176.6 A
THE CURRENT CARRYING CAPACITY OF THE CONDUCTOR MUST BE NOT LESS THAN 176.6 * 1.25 =220.75 A
and the C.B = 1.75 * 176.6 =309 A , THEN the C.B IS 325 A

PLEASE ADVICE FOR THE CORRECT WAY ACCORDING TO NEC

REGARDS FOR ALL

 

[don_resqcapt19]

The sizing of motor conductors in the NEC is based on 125% of the motor current as shown in tables 430.247 through 430.251. These tables only cover the typical voltages used in the US and there is no direct way to apply the NEC requirements to your motor.
That being said, the rule in the NEC would require conductors at 125% of the worst case full load motor current for the size of your motor. The sizing of the conductors is not based on the nameplate current of the motor that has been installed. The NEC only uses the nameplate current for the sizing of the motor overload protection. The table values are used for conductor sizing and for the sizing of the short circuit and ground fault protective device.

 

[TMMB]

thank you for reply , but i need ur advice reagrding the way i have been calcuate manually , it is correct?

 

[erickench]

First determine the horsepower and voltage for the pump motor. Then use NEC Table 430.250 to determine the FLC. I don't advise using the nameplate power rating since this is not in accordance with NEC 430.22. You must determine the FLC from the tables and multiply it by 125% to calculate the conductor size. The FLA on the nameplate is used for the overload calculation. As for the short circuit protection I personally would use an instantaneous device on the branch circuit which in this case would be 800% or %1100. By doing this you would achieve selectivity.

 

[Jraef]

First determine the horsepower and voltage for the pump motor. Then use NEC Table 430.250 to determine the FLC. I don't advise using the nameplate power rating since this is not in accordance with NEC 430.22. You must determine the FLC from the tables and multiply it by 125% to calculate the conductor size. The FLA on the nameplate is used for the overload calculation. As for the short circuit protection I personally would use an instantaneous device on the branch circuit which in this case would be 800% or %1100. By doing this you would achieve selectivity.

Despite not indicating where in the world he is at in his profile, I'd hazard a guess that with a 380V 60Hz supply (and motor), he is not in the US and therefore may not have access or need of our NEC. As far as I know, the only places in the world with that odd combination of V and Hz are Korea (ROK), Peru, Trinidad / Tobago and parts of Brazil.

I HAVE 3PHASE WATER TRANSFER PUMP rating 93 KW , 380 , 60 HZ and the motor power consumption is 56 KW , my questions are :

1- the sizing to calculated for the pump rating (93kw) or for the consumption (56kw)
2- i belive that the design of the conductor/C.B must be based on the pum rating ; I= 93000/1.73*380*0.8 =176.6 A
THE CURRENT CARRYING CAPACITY OF THE CONDUCTOR MUST BE NOT LESS THAN 176.6 * 1.25 =220.75 A
and the C.B = 1.75 * 176.6 =309 A , THEN the C.B IS 325 A

PLEASE ADVICE FOR THE CORRECT WAY ACCORDING TO NEC

REGARDS FOR ALL

You left out Efficiency in the equation, you need that when looking for input current, as you would for conductor sizing. If you are just looking at motor active current, you don't use the Eff.

I = kW / 1.732 * V * PF * Eff

It would not make a lot of difference, but it might be the difference between one size and the next.
I usually assume 95% eff if it's a new motor, could be as low as 80% if it's old, lower in all cases if it is slow (i.e. more than 4 poles).

This is, if you cannot just read the motor FLC from the nameplate, which is the best way.