Cable sizes
Cable sizes
(OP)
How to calculate the correct cable size by taking into consideration distance,impedance,motor size,voltage drop,starting current.
Here is an example:Motor size:160 kw motor (4 pole)
:distance:90 meters
Here is an example:Motor size:160 kw motor (4 pole)
:distance:90 meters






RE: Cable sizes
There are so many formulas that we had studied in education life but i could not exactly remember them! but the best way of finding the cable size is using standard cable tables!
RE: Cable sizes
You have to use also any catalogue of any cable manufacturer to get the cable size which can carry the load and meet the required voltage drop.
I hope that to be useful
Tawakol
RE: Cable sizes
In the US, a motor feeder must be sized at 125% of the full-load current, based on tables in the National Electrical Code. Voltage drop concerns can cause the size to be increased, but it can never be smaller.
RE: Cable sizes
RE: Cable sizes
The calcs are not that hard. It all pretty much comes down to V=IZ. Z can be determined from tables in the NEC or from manufacturer's data.
Suggestion: if you don't know how to size cables, well, get an engineer, that's what we're here for.
RE: Cable sizes
How to calculate the correct cable size by taking into consideration distance,impedance,motor size,voltage drop,starting current.
Here is an example:Motor size:160 kw motor (4 pole)
:distance:90 meters
///Additional information is needed:
1. Copper conductor or aluminum conductor
2. Rated or specified minimum motor transient starting voltage, e.g. 85% of motor nameplate rated voltage.
3. Cable sizes and their ampacity ratings at certain ambient temperature
4. Cable insulation temperature rating\\\
RE: Cable sizes
RE: Cable sizes
Also check if the cable are not laid underground with heated pipes or other high current cables. This might require a software for ampacity calculation .
RE: Cable sizes
RE: Cable sizes
in case cable is being sized for a MCC feeding , say 10 motors, would the starting current of 10 motors will be summed up to size the incommer of MCC.
RE: Cable sizes
1. Ampacity of the cable to meet the load current (Remember to derate the cable for the installation conditions and grouping factors)
2. If it is a medium voltage cable you would also have to check the minimum size to be used for the short circuit point of view.
The simple formula is S = I x sqrt(t)/k.where S is the minimum cross section of the conductor, I is the short circuit current , 't'is the operating time of the disconnecting device(you can consider 100ms)and k is the constant depending on the type of cable (conductor, insulation with lead sheath or not etc)which can be found from any cable catalouge.
3. Now you have to look for the voltage drops during starting and normal running with the formula VD=sqrt(3)xI(R x cos(phi)+X x sin(phi))xL where R and X are the resistance and reactance of the cable and L is the total length of the cable in km.
The size satisfyng all the three conditions to be selected.
If you want I have an excell sheet to find the size for cables satisfying the above conditions.
Cheers
vrj
RE: Cable sizes
thanks in advance.
RE: Cable sizes
RE: Cable sizes
"These ampacities were calculated as detailed in the basic ampacity paper, AIEE Paper 57-660, The Calculation of the Temperature Rise and Load Capability of Cable Systems, by J. H. Neher and M. H. McGrath. For additional information concerning the application of these ampacities, see IEEE/ICEA Standard S-135/P-46-426, Power Cable Ampacities, and IEEE Standard 835-1994, Standard Power Cable Ampacity Tables."
John
RE: Cable sizes