Ampacity Table and Calculations

Cable sizing is one of the most common question in this forum. You can search it within this forum and find more detail information.

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.

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.

- MS