Cable Selection as per Theory Calculation

→ British Standard 7671.

Or, in USA it's NEC. Or just search CR4. That's the box to the right of this screen. Type in "cable sizing."

I am not sure about what you mean by 'cable selection as per theory calculation'. Here is a modified form of one of my previous comments for the similar type question:

There are many things to consider for the cable selection but the most important is the standard which you have to follow. Here are some of the major things that are needed to consider for the cable selection:

1. Load current: Cable ampacity must the greater than the load current. If the load is continuous (more than 3 hours continuously), NEC regulates the cable ampacity to be at least 125% of the load current. For non-continuous load, it is 100%.

2. Ambient temperature: The ampacity table values are based on a certain ambient temperature. If the maximum ambient is greater or less than the indicated value, the cable ampacity is multiplied by the de-rating factor.

3. Number of conductor: If the number of current carrying conductor of a cable or conduit is more than 3, NEC applies another de-rating factor for the ampacity.

4. Cable raceway: Ampacity de-rating factor also applies for the type of cable raceway

5. Short Circuit Current: The selected cable must withstand the short circuit current where it will be using

6. Cable length (Voltage drop): The voltage drop within the length of the cable shall be within the permissible range (generally 3%). If it exceeds, the higher size cable is selected

7. Device Coordination: For the device coordination, the cable to be selected so that the upstream breaker protects the cable.

You need to follow the local standard (NEC or CEC or IEC or others) for detail of these items.

Also see the previous thread:

http://cr4.globalspec.com/thread/54875

- MS

Don't forget frequency. Is it power transmission? in AC or DC? 50 or 60 Hz, or 400 Hz aircraft power? Is it for data transmission? RF transmission? Automatic transmission?

The reply from msamad is fairly good, I just want to make it more eleborate with theoretical/ practical touch.

For cable selection you have to satisfy all the following factors.

1- Location of installation. you have to decide the type of insulation and mechanical/ chemical protection as per the location where the cable is going to be installed.

2- current carrying capacity. It is to be 125% the maximum demand.

3- Method of laying: This Factor is to be considered and cable capacity de rated according to the method of laying i.e directly buried in the ground ( soil resistivity to be considered for derating), laid in one way ducts or directly clipped to a surface.

3- Cg Grouping factor. the cable capacity to be derated if more than one cables laid touching. If the distance between cables laid parallel is 2xDiameter of the cable, then ignore this factor.

4- Ambient Temperature. Normally the current carrying capacity tables (Manufacturing catalogue table, NEC or IEEE tables) are showing the current carrying capacity at 30 deg C. the derating factor is to be applied for different ambient temperatures. for example derating multiple for 35 deg C is 0.97, for 40 deg C is 0.94 and for 45Deg C is 0.91 and so on.

5- Frequency is to be ignored if it is 50/60 Hz, but for higher frequencies you have to consider derating.

6- If you are selecting an Aluminium conductor cable and want to terminate on copper bus bar or terminals, consider bimetallic effect.

I hope it shall cover all the possible factors for the selection of cable.