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Uniform Temperature - Ampacity Calculation Software

 

Cable Thermal Analysis Software Uniform Temperature Ampacity Calculation

This cable ampacity calculation approach is based on the equal temperature criterion for ampacity calculation. The cable ampacity assessment tool determines the maximum allowable load currents when all the cables in the system have their temperature within a small range of the temperature limit.

Since all the conductors in a cable branch are assumed to equally share the load current, in the case where these conductors are not located in the same conduit/location, they may not have the same temperature. When this situation occurs, the temperature of the hottest conductor in this cable branch will be used to represent this cable branch.

The uniform-temperature ampacity calculation involves an iterative process, which adjusts cable loading current in each iteration so that the cable temperature approaches the temperature limit. In this ETAP cable ampacity module, the load adjustment in each step is determined based on the gradient of cable temperature change and therefore, offers fast convergence to the solution. This method is part of the overall cable thermal analysis program incorporated into ETAP.

This cable ampacity calculation approach is based on the equal temperature criterion for ampacity calculation. It determines the maximum allowable load currents when all the cables in the system have their temperature within a small range of the temperature limit. Since all the conductors in a cable branch are assumed to equally share the load current, in the case where these conductors are not located in the same conduit/location, they may not have the same temperature. When this situation occurs, the temperature of the hottest conductor in this cable branch will be used to represent this cable branch.

The cable ampacity assessment involves an iterative process, which adjusts cable loading current in each iteration so that the cable temperature approaches the temperature limit. The load adjustment in each step is determined based on the gradient of cable temperature change and therefore offers fast convergence to the solution. The following steps are involved in the calculation:

  1. Determine an initial loading level based on the base ampacity from the Cable Library and using cable derating factors for the given configuration.
  2. Calculate cable temperature as in the steady-state temperature calculation described above.
  3. Check cable temperature values against the cable temperature limit. If the temperature values of all the cables are within close range of temperature limit, the solution has been reached. If not, determine the load change required for the cable temperature to approach the temperature limit based on the gradient of cable temperature change.
  4. Update the cable loading and go back to step 2 to recalculate cable temperature.


If the Update Currents from the Cable Ampacity Calculation option is checked in the study case, the cable allowable current will be updated by the calculated ampacity.

If for any of the cables the Fixed Current option from the Loading page of the Cable Editor is checked then Uniform Temperature calculations cannot be conducted. In this situation ETAP stops the calculations and provide an error message informing the user that UGS contains a cable with fixed ampacity.