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Thermal Rating Properties for Cables in PLS-CADD and PLS-CADD/Lite

This document details the units entered into the Thermal Rating Properties section of the Electrical tab in the Cable Data window (accessible via Sections | Cable and Concentrated Loads Files | Create New Cable File ...). Formulas for derived properties are also provided as a guide to creating custom cables.

Thermal bimetallic properties for a cable are enabled by checking the Bimetallic check box in the Thermal Rating Properties input group. The properties for the outer strands are always visible and represent all of the cable's strands when bimetallic properties are disabled. The properties for the core strands are only visible when the bimetallic properties are enabled.

Most thermal properties will have two inputs when modeling thermal bimetallic conductors – one value for the outer strands and another value for the core strands. Be sure to properly differentiate between outer and core strand material/geometric properties to ensure that the final composite cable properties are correct.

Note that the use of  °C and °K temperature units are interchangeable for thermal properties because the rate of change for Celsius and Kelvin are the same. PLS-CADD typically uses °C while some properties may traditionally be provided using °K.

Cable files also allow the bimetallic modeling of stress/strain via a similar, but independent, check box under the Physical tab of the Cable Data window. The two independent bimetallic settings allow a cable to be modeled as monometallic for the stress/strain relationship and as bimetallic for thermal calculations if so desired.

Input Properties

Property SI Units Imperial Units Comments
Emissivity Coefficient dimensionless dimensionless See applicable standard.
Solar Absorption Coefficient dimensionless dimensionless See applicable standard.
Measurement Temperature °C °F
Radial Thermal Conductivity W/(m·°K) W/(m·°K) The units for this property are typically provided in SI so Imperial units are the same.
Heat Capacity (W·s)/(m·°C) (W·s)/(ft·°F) Calculate using (c·γ·A) from the physical properties table below.
DC Resistivity MΩ·km MΩ·mile
DC Resistance Coefficient 1/°C 1/°F
Unit Weight N/m lb/ft Calculate using (g·γ·A) from the physical properties table below.
Cross Section Area mm2 in2 Same as A in physical properties table below but with different units.
Heat Capacity Coefficient 1/°C 1/°F
Layers dimensionless dimensionless
Core Diameter mm in

Physical Properties

The above formulas for heat capacity and unit weight require different units for some properties compared to what is used in the PLS-CADD cable file editor. Refer to the following table for the correct units to use with those formulas.

Symbol Property SI Units Imperial Units Comments
A cross section area m2 ft2 Same as input area but different units for formulas above.
c specific heat capacity (W·s)/(kg·°C) (W·s)/(lb·°F)
γ density kg/m3 lb/ft3 SI requires mass density and Imperial requires weight density.
g gravitational acceleration or conversion factor 9.807 m/s2 1 Imperial value is dimensionless.

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