Q333 : Sizing Conductors for Ampacity

Question
What are the criteria that should be considered when sizing conductors?

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Answer
The dominant sizing factor is the ampacity of the conductor. IEEE Guide 80 - 2000 (Section 11.3) gives all the relevant formulas. These are the ones implemented in our Ampacity software.

The two main variables to consider are:
1. The RMS current flowing in the conductors
2. The maximum temperature allowed for the conductors

The value to be used for the current should normally be at least half as large as the total available fault current, including local contributions. This is because the current will normally split into at least two branches (no matter where the fault occurs), each branch carrying approximately half the current. On the other hand, to be absolutely safe, it is recommended to use the total fault current for this value (not half of it). Note that this current is the RMS current; for a fault, a derating factor to account for the asymmetry of the current could be applied. This is usually a rather small effect which tends to reduce the effective current circulating in the conductors.

Note that MALZ isn't needed to compute this limit current: all that is required is a knowledge of the total fault current.

The maximum allowed temperature should be selected according to the tables in the standard. Note that the standard mentions that the maximum temperature should not exceed the annealing limit for conductors requiring mechanical strength (such as overhead conductors). This limit is 250 C for copper. For buried conductors, the limit is about 1000 C.

As mentioned above, the Ampacity program that ships with CDEGS can perform the sizing calculation when you know the fault current and the maximum allowed temperature. To start the program, select Tools | Ampacity from the CDEGS main program. The data entry requirements of the program are quite simple and self-explanatory. You need to specify the fault current under Symmetrical RMS Current Magnitude, the Maximum Allowable Temperature (you can let the program use the fusing temperature, which is appropriate for buried conductors), and select the type of conductor. Then, click Compute to get the minimum conductor size, or click on Report to get a more detailed computation report.

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