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Line Design

In order to design the line there are few important elements to define: voltage drop, the load, sizing the cable, ampacity, and cable temperature. The load is based on the motor. Ampacity is that which is adequate for the rms value of the motor current profile plus the fast transient spike.

Electrical Calculations For Industrial Plants
Electrical Calculations For Industrial Plants (photo credit: Grundfos)

Choosing cable temperature includes the choice of ambient temperature for the line design.


The Load

The load is the current profile summarized in the motor electrical analysis. It is repeated here. The peak current is 1062 A, which lasts for 6 min. This is followed by 24 min with a current of 220 A. This cycle is repeated for 10 h/day.
The rms value of this current profile is:

(1,062)2 × (6 min) = 6,767,064

(220)2 × (24 min) = 1,161,600 / 7,928,664

Dividing this sum by 30 min yields the mean square, or 264,289 A2. The rms value is the square root, or 514 A. This is an rms load of:

(480) × (514) × (1.732) × 10-3 = 427kVA

Note that the 514 A value slightly exceeds the motor’s current, 421 A, when operating at 400 Hp and 460 V. But it doesn’t exceed the motor current, 547 A, when operating at 90% voltage and 80°C rise.

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