Jump to content

Cable Sizing With Given Load

Recommended Posts

hi . m about to install 2 by 10kw motors using star delta starter. flc of each motor is 25A. what should be the size of the cable.

There are a lot of ways to determine the cable size since different type of cable may have different ampere rating. However, for minimum requirement, you may use the following formula:


For DC and single phase motor

S= (2.L.I. pf)/(C.VU) or

S = (2.L.P)/(C.VU.U)


For Three Phase motor:

S=(1.73.L.I.pf)/(C.VU) or

S = (L.P)/(C.VU.U)



S = cable size in mm sq

L = cable length in meter

I = Rated ampere

P = power of motor in watt

CU = Voltage drop allowable( such as 2.5% of usage voltage)..depend on country regulation

U = Voltage

pf = power factor

C = conductivity of cable if Cu = 56, Al = 35, Fe = 8.5


To get the final result, S shall be divided with the temperature surrounding factor:


20 Deg C: 1.4

25 Deg C : 1.3

30 Deg C : 1.22

35 Deg C : 1.13

40 Deg. C : 1.00

45 Deg. C : 0.87


For example, S = 3.5 with surrounding temperature around 45 Deg. C, then the final result shall be


(3.5)/(0.87). From here, you will select the cable size based on the standard manufacturer available like 1.5, 2.5, 4.0 and etc. For safety, I would suggest you choose one step higher then the final result.




Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...