GENERATOR SIZING FOR PUMP STARTING

GENERATOR SIZING FOR PUMP STARTING
DETAILS OF LOAD
Consider  4 Pump motors of 60 kW , 400 V, 50 Hz each is connected to a  Panel fed from a Generator. Pump motors are started using soft starters. Running conditions are ;a) All pumps could be running one time b) Pumps could start one by one c) 3 Pumps running and one could be starting.
SIZING CRITERIA
1. Running kW or kVA should be less than the selected Generator capacity 2.Wosrt case starting scenario:  Total kVA or kW required should be within the sKVA rating of the Generating set.( sKVA rating shall be available in the Generator catalog) otherwise consider 1.5 times the Generator kVA rating.
RUNNING
Maximum running load= 4 pumps running
Pump rating=60kW, efficiency=91.8%
running  kW=60/0.918=65.35 kW
running  kVA=65.35/0.8=81.68 kVA
Total kW( 4 pumps running)	261.4		kW
Running pF=0.8 ( lowest valve from pump catalog)
Total kVA required while running= 245 kVA	326.75		kVA
STARTING
Worst case could be 3 pumps running and 4th starting
1) 3 Pumps running kVA= 81.68 * 3	245.04		kVA
2) 4th Pump starting kVA
Full load current of One pump	107		A
Starting current with Soft starter=2.5 FLC	267.5		A
Starting kVA of One pump	185.324		kVA
Total starting kVA = 3 pumps running kVA + One pump Starting kVA=245+185.324	430.364		kVA
Generator selection:
Running  kVA required	326.75		kVA
Starting kVA required	430		kVA
Generator required ,considering 80%  loading of Generator	408.4375
Nearest available size	400		kVA
400 KVA PRIME RATED GENERATOR SUITS THE REQUIREMENTS

Cable Sizing Calculations

In electrical system design, cable sizing is one of the major activity. Lets see how to size the cable for a load.


CABLE SIZING CALCULATIONS

( This article is draft stage and is under modification..........)

In electrical system design, cable sizing is one of the major activity.
Simplified Procedure for cable sizing;
• Determine the design current
• Decide the Protective device rating
• Decide the installation method of cable
• Identify the de-rating factors
• Find the Calculated current rating of required cable
• Select the cable
• Check the voltage drop
• Check the short-circuit capacity


STEP-1 : CALCULATE THE DESIGN CURRENT
Use # phase power formula to calculate the design current

Let's assume P=50 kW, v=415 V, and pf=0.8
• Where P is 3 phase load in kW and V is the Line to Line voltage.
• pf is the power factor of the load which may be taken as 0.8 , if otherwise not known.


we would get Ib= 50/ (1.732 * 0.8 * 0.415 ) = 86.95 Ampere

If it is a motor load, we have to divide by efficiency so as to get the input current drawn.


STEP-2 : SELECT THE PROTECTIVE DEVICE RATING

Protective device ( MCB/MCCB circuit breaker) is selected to protect the cable against damage from thermal overload.
Protective device must be selected such that Nominal rating of the device is more than the design current, i.e