MV Motor Cable Size

[Electriman]

Good morning,

I am working on a project that involves sizing cable for a MV voltage motor (600 hp, 2300v). When you are working on LV motors you can always find the current on the NEC table 430.50 and based on that you can select the cable size.

But there is no such a table for 2300 V and 4000 V motors. You can find vendor data for these motors but I am not sure if we can use these data for selecting cable size.

What do you guys recommend? Is there a full load amp table for MV motors to use for sizing cable similar to that of LV motors?

Thanks,

 

[jumper]

I know absolutely nothing about MV motors, but I can tell you that they are covered in part XI of 430, which is 430.221-430.227.

 

[don_resqcapt19]

It appears that you either have to select the motor and its overload protector rating at the time of design or take the worst case from the vendor data and design to that.

430.224 Size of Conductors
Conductors supplying motors shall have an ampacity not less than the current at which the motor overload protective device(s) is selected to trip.

 

[Ingenieur]

Use the highest of a few mfgs ratings
use the right rpm though
FLA rises for 900 and 1200
otherwise 135-140 A range

a rough estimate 600 x 746 / (0.81 x 1.732 x 2300) = 139

but a motor like this is associated with a piece of equipment/machinery which is usually being designed around...ask the process or mech engineer if he has data

 

[topgone]

Good morning,

I am working on a project that involves sizing cable for a MV voltage motor (600 hp, 2300v). When you are working on LV motors you can always find the current on the NEC table 430.50 and based on that you can select the cable size.

But there is no such a table for 2300 V and 4000 V motors. You can find vendor data for these motors but I am not sure if we can use these data for selecting cable size.

What do you guys recommend? Is there a full load amp table for MV motors to use for sizing cable similar to that of LV motors?

Thanks,

For cabling design, I always use the cable manufacturer's data and start to fit my choices from the cable manufacturer's published figures. Here are the important design checks I usually do when doing things above:

1. Check for the voltage drop at the motor terminals if acceptable. The efficiency of the motor is greatly impacted by the supply voltage.
2. Check for the voltage dip at the motor terminals during starting. Remember that the motor torque is directly proportional to the square of the voltage it receives.
3. Check for the operating temperature your selected cable, given the number of conductors per phase, ambient temperature of the cable location, the cable design temperature, the grouping factor and temperature derating factor.
4. Check if the cable insulation will not get damaged if ever an accidental short-circuit occurs at the motor terminals. This has something to do with how fast your protections are set to isolate your cable before the insulations melt.

Hope that helps.

 

[Sahib]

Check if the cable insulation will not get damaged if ever an accidental short-circuit occurs at the motor terminals. This has something to do with how fast your protections are set to isolate your cable before the insulations melt.

The US electricians do not usually do it.

 

[Electriman]

Thank you all for your great inputs.

 

[Julius Right]

At first we have to state the current.
In NEC Table 430.250 Full-Load Current, Three-Phase Alternating-Current Motors shows the current for up to 500 HP. If we shall take the product of efficiency and power factor as per 500 HP and use it for 600 HP we’ll get 141.5 A .However, according to ABB Catalogue for NEMA MV Motors the maximum will be 149 A.
According to IEEE 242 ch. 10.5 Medium-voltage motor protection the protection setting has to be 1.25-1.40 times the rated. So, 1.4*149=209 A.
NEC 430.224 Size of Conductors:
“Conductors supplying motors shall have an ampacity not less than the current at which the motor overload protective device(s) is selected to trip.”
Using a suitable table from art.310.60 Tb.60-85 for 209 A we’ll get 1/0 to 3/0 copper.
Let’s say the distance from supply transformer to motor terminal is 330 ft.[100 m] and the impedance of a 3/c 3/0 copper conductor cable will be 0.081 + j 0.037 ohm/1000 ft. [Z=(0.0267+j0.0122)]
Let’s say Ishortcircuit at the motor terminal will be 20 kA and the circuit breaker will clear the fault in 0.2 sec this cable will withstand the short-circuit.
If the lock current will be 8*Irated at start D.O.L. the voltage drop will be:
sqrt(3)*(R*cos(fistart)+X*sin(fistart)) pf[cosfi] at start =0.3 then DV=2%