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Wire Ampacity Changes in DM Electrical RT 2.0

Posted on August 17th, 2021. Last updated on January 11th, 2024.

We added two new fields to the Wire Ampacities command in DM Electrical RT 2.0. These values were added to handle calculations for derating wire due to ambient temperature and more than 4 conductors in a conduit.

When projects created using previous versions are updated to the new version, default values are provided for these two fields. The default values are conservative. If you plan to use the new calculations and want more precise calculations, you will need to update these values in your project.

Wire Ampacity @ 30°C

Previously, the dialog box only listed Breaker Ampacity. It was assumed the wire ampacity was as large as or larger than the breaker ampacity. For derating calculations, however, the software needs the actual ampacity of the wire itself.

When a project is converted, the Wire Ampacity @ 30°C values are set equal to the Breaker Ampacity divided by the Parallel Runs. That value is conservative; there are cases where the wire ampacity will be higher.

For example, a 50A Breaker Ampacity is set to use #6 wire. When the project is converted, the Wire Ampacity @ 30°C will default to 50A. However, per NEC Table 310.16, the correct value is 55A.

Temperature Rating of Conductor

This value is used for derating calculations due to ambient temperature based upon NEC Table 310.15(B)(1).

When a project is converted, the Temperature Rating of Conductor values are set to 60°C for #1 wire and smaller, and 75°C for 1/0 wire and larger.

If you have adjusted your wire sizes to assume different conductor temperature ratings, review these values to make sure they match.

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