Q76 : Types of quantities that can be computed with the Ampacity utility.
Question What can I use to compute the maximum conductor size?
Answer The Ampacity utility can compute the following quantities:
1- Minimum conductor size;
2- Ampacity (i.e., maximum fault current rating);
3- Temperature rise during a fault (computed in terms of the final temperature after the fault).
One of the 3 values is computed each time the calculation is launched. Note that when one value is to be calculated, the other two values become required input data. The input form reflects this by greying out the quantity being computed and allowing the other two to be specified.
Assumptions made in this computation are as follows:
No heat loss into the surrounding soil (i.e., the heating process is adiabatic);
The thermal capacity per unit volume (i.e., the product of specific heat and specific weight) of each conductor remains constant during the heating process: this is usually the case for fault durations on the order of a few seconds or less.
The basic equation used in this module is the detailed equation provided in ANSI/IEEE Standard 80 for the calculation of ampacity for symmetrical currents. To account for asymmetrical current characteristics (i.e., dc offset), the symmetrical current is increased in accordance with the decrement factor presented in the same standard.