Table 220.12 of the National Electrical Code (NEC) provides a standardized method for calculating the minimum ampacity and size of service and feeder conductors based on the calculated load of the branch circuits. In this lesson, we will discuss how to use Table 220.12 to determine the minimum ampacity and size of service and feeder conductors for a given installation.

Determine the Load The first step in using Table 220.12 is to determine the load of the branch circuits in amperes. This can be calculated by adding the loads of all the branch circuits that are supplied by the service or feeder.

Determine the Demand Factor Once the total load has been calculated, the next step is to determine the demand factor for the type of load. The demand factor is a percentage that represents the expected simultaneous use of the loads in a given area.

Find the Appropriate Column and Row Using the calculated load and demand factor, locate the appropriate column and row in Table 220.12. The columns correspond to the rated voltage of the system, while the rows correspond to the type of load, such as lighting or heating.

Determine the Minimum Ampacity and Size Once the appropriate column and row have been located, the minimum ampacity and size of the service or feeder conductors can be determined. This information is listed in the table and corresponds to the load and demand factor.

It is important to note that the minimum ampacity and size calculated using Table 220.12 are the minimum requirements for the installation and may need to be adjusted based on other factors such as ambient temperature, conductor material, and conductor insulation.

In addition, Table 220.12 is only one method for calculating minimum ampacity and size and may not be appropriate for all types of installations or situations. It is important to consult the specific requirements of Article 220 of the NEC and to use professional judgment when calculating conductor size and ampacity.

Overall, Table 220.12 provides a standardized method for calculating the minimum ampacity and size of service and feeder conductors based on the calculated load of the branch circuits. By following the steps outlined above, it is possible to use Table 220.12 to determine the minimum conductor size and ampacity for a given installation.

It is important to note that Table 220.12 is updated every three years with each new edition of the NEC. Therefore, it is important to use the most current version of the table to ensure accurate calculations.

In summary, using Table 220.12 of the NEC involves determining the load of the branch circuits, determining the demand factor, locating the appropriate column and row in the table, and determining the minimum ampacity and size of the service or feeder conductors. It is important to use professional judgment and consult the specific requirements of Article 220 of the NEC when calculating conductor size and ampacity.