cable sizing calculation

try this

dear nitin

try this

1) calculate ampacity load to be connected

2) select conductor size as par NEC 110.14 (c), 310.16 (wherever applicable)

3) derate cable size (temp derating and group derating)

4) after derating check the rating with your load, if less go for next higher size and repet from step 2 (reverse calculation)

5) check the VD within permissible limit (3%)

6) select the overcurrent protection device NEC 240.3(b)

any problem pls comback to me (email removed)

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Somebody please indiacte method based on IEC.

For IEC sizing you can use the following free online tool:

http://www.myelectrical.com/pages/cables/cableSizing.aspx

Their site also has information on the process for sizing cables to IEC.

Steven

Thanks Steven, the tool is really nice and helpful but it is restricted up to 1000 kv, could you please suggest similar tool for higher level of voltage, i have a 1837 kw motor feed from 6.6 kv bus, breaker size selected 1250 A, length of feeder is 70 m, ambient temp 45 deg C, how would i size feeder for this? please suggest some method.

thanks

To size the current you should refer the relevant bs,is or IEC standards.

Alternatively from cable manufacturer tables(ducab or polycab or oman) one can calculate the current but several factors have to be taken into consideration

1. cables being laid in air,duct or buried ground.

2. cables are touching or spaced - derating factor will be applicable

3. If in trays the no of vertical tiers of tray laid

4.the deration factor due to ambient temperature.

5 . the cable should be able to take the fault current till the time breaker disconnects

(for a 11KV VCB breaker the disconnect time is 80milli second as per abb.)

For this sq of k x sq of S=>sq of I x t has to be calculated where

k = factor depending on cable insulation and conducting material

s = cross sectional area of cable

Sq of I x t = called let through energy of the device(product of fault current and the time of flow through the device)

formula is Power = 1.732 x voltage x current x p.f

I assume that in your case the fault current at the breaker terminal will be 35KA for 1 sec as for 11 KV it is 25 KA max for 1 sec

In this case

I = 1837 x 1000

--------------- = 206 A

6.6 x 1000 x 1.732 x0.8

refer to ducab catalogue or polycab catalogue for quick calculation as it freely available from web and take into account all the derating factors,and the fault current capacity for 1 sec.

As for considering sq of ixt value size the cable according to fault current(35ka) & rated current as any manufacturer breaker will disonnect in 1 sec.

But i dont know why you have gone for 1250A breaker as a 400A or 630A breaker will suffice.

Errata

This 1837 KW of motor is it input or output power.if it is output then it has to be divided by efficiency.actually you will have to look at manufacturer chart( if available) and know the current for 1837 KW.that is the correct method for calculating current.

current is not a problem with ducab or polycab or oman mfr table or tool available at there web but it is voltage 6.6 kv which is not supported by their web based tool.

In ducab catalogue on the web the current capacity of ducab 3.3kv/6.6 kv/11KV cables is available with short ckt withstand current,various derating factors are available in pdf form.If you want i can send the pdf file.

can u please suggest cable size for 2.75 MW, 6.6 KV, 283 A motor for a length of 150 m?

British Standard 7671.