Electrical Cable/Wire Sizing

This time i want shared about how to sizing the electrical cable.This topic is important because it’s involved with safety issue,cost and time line  for the electrical installation project.

Cable or wire sizing matter already discussed for a long time ago and it’s a hot topic for Electricians and Engineers.So i hope from this post,it can give a clear view and  understanding how to select current rating for wire,cable rating,voltage drop, and wire sizing for wiring project application

.From cable/wire sizing calculation we can estimate the proper size for cable to our wiring installation project.One thing that we need to prepared before cable or wire sizing  is several data or information about wiring project includes :

How to sizing the electrical cable/wire?

1) Type of load : For feeder,control panel,distribution board (DB) or motor

2)  Power Source : AC /DC power for Three phase or single phase

3) Full load current (Ampere)

4) Circuit Breaker size

5) Types of cable ( PVC,XLPE,Fire proved)

6) Installation Condition ( Underground, trefoil,trunking,tray or conduit)

7) Distance / length of cable run from source to load

Calculation for sizing electrical cable/wire

Ib ≤ In ≤ Iz

Ib = Amperage  load

In = Circuit Breaker size

Iz = Current rating for cable/wire

Example :

We plan to install 1 unit induction motor 75 hp for running a water pump.Power factor value is 0.8 and motor efficiency is 0.85.Power supply for this motor is 415 volt 3 phase 50Hz.Distance from panel to motor is 75 meter.This panel and pump is located at outside of factory.Please sizing the suitable circuit breaker,electrical cable and also type of cable for this application.

Calculation

Ib ≤ In ≤ Iz

Ib = ( hp x 746) / (√3 x Volt x PF x Efficiency)

Ib = (75 x 746) / (√3 x 415 x 0.8 x 0.85)

Ib = 114.5 Ampere

In = Ib ≤ breaker sizing

In = 114 ≤ 150 ampere

In = 150 ampere (we decide to used MCCB 3 pole with 150 ampere rating)

Iz = In ≤ Iz

Iz = 150 ≤ Iz

Iz = 187 ampere ( carried capacity for 50mm² cable size)

So we decide to used cable size = 50 mm² with 4 core cable and clipped direct to cable tray.To confirm this cable size,we need to calculate the voltage drop.

* For full table of cable size,please download cable size chart / table below.Clink link to download PDF file for Cable Current Rating Table

DOWNLOAD CABLE SIZE CHART

Voltage Drop Calculation

Below is a voltage drop calculation that i made for cable power supply from panel to induction motor.

From Panel to Induction motor = 75 meter

mV = 0.87 ( refer table below )

Formula voltage drop :  Vd = Ib x length(meter) x mV / 1000 

Vd = (114.5 x 75 x 0.87) / 1000

Vd = 7.47 ( Acceptable refer to IEE 17th edition )

* Voltage drop acceptable value is 4% x 415 volt = 16.6 Volt

Conclusion

From this calculation, we fully confident to laying cable size 50mm² with 150 ampere of MCCB,using multicore PVC Armored cable (due to outdoor wiring application) and also cable laying with cable tray.

That why all Electrician and Electrical Engineer need to know about cable sizing calculation to make sure wiring installation project successful and meet the requirement of Electrical Safety and Regulation without any disaster.