New Capability: Cable Ampacity Calculations


New Capability: Cable Ampacity Calculations

Teppics Engineering now has new capability provided by a new module from SKM PTW. We can now provide Cable Ampacity calculations given a temperature rise from the surrounding conditions.

The design challenge is to account for in your calculations soil type, duct bank encasing type, type of conduit (metal vs. plastic), conductor size, current ratings, ambient air temperatures, depth of duct bank, conduit quantities, conduit configuration (2x4, 3x3, etc). These calculations are required to adequately derate your cables and ensure you have the proper rating / size / quantities of cables going to the equipment. This is especially important when you have large ampacity requirements from equipment to equipment - the standard NEC tables / quick references are not adequate.

Cable Ampacity Calculations Example

As a cable carries more current, the amount of heat rejected increases exponentially with the product of the resistance (P=I^2 * R). Most cables have a protective, insulative coating typically made of PVC. These coatings have a temperature rating - 60C, 75C, and/or 90C. These ratings are temperature rise over ambient. So, if it's 40C (104F) and your cable is rated to 60C rise over ambient - you're left with ~100C which boils water.

However, if you increase the current going through the wire still, you will exceed the temperature rating of the insulation. This will cause this protective coating to melt or worse, catch fire. In the calculation, you have cables routing through a raceway (conduit) and it's trying to reject heat to the surrounding area. Depending on if you're directly burying these conduits in soil vs. encasing in concrete vs. free air - these surrounding conditions can insulate or conduct the heat. Another concerning condition is when conduits are close enough together, the heat from conduit 1 can heat conduit 2. This mutual heating causes unnecessary temperature rise - thus, if you don't want a fault condition, the current you can route through the conductor is less (essentially de-rating the conductors).

A common mitigation technique for arid / dry environments like the Southwest region is to encase the conduit banks in concrete. By comparison to sand and/or rocky soil, concrete is a very good conductor of heat. This allows you the ability to locate the duct bank closer to the surface of earth, allowing convection to wick heat in to the atmosphere, while still providing appropriate protection encasing these conduits in concrete. Additionally, you can design the duct bank with space between to provide more thermal mass between heating elements (cables) to sink more thermal energy.

The Result

Lastly, after Teppics Engineers have performed these calculations, the visualization of the 3D product can be realized. Utilizing our new CAD platform, AutoDesk Revit, you can see from the attached pictures that spacing conduits out between each other is relatively easy. However, in the example shown, having a 40" x 40" entrance and exit pathway in electrical equipment is just not feasible. A transformer may be 40" x 40" total - let alone just the customer side of the equipment. Thus, routing conduits in 3D allows for visualization of at least 1 possible solution for routing conduits.

Teppics would then provide this solution to the contractor and express language in notes to allow field modifications to occur if there is a more optimal solution. Key elements such as minimum conduit distances, concrete bank material, and closeness to surface material are defined while the rest of the measurements such as bend radius, sweep start/stop, and angle of sweep compared to rotating on the longitudinal length, are left to the contractors in the field. The point of 3D dimensions is not to handcuff the field installation, rather it's to show that there is a possible solution and allowance for field personnel to modify if a better solution exists.

The pictures below show one example of how Teppics Engineering can provide the level of technical understanding in the design for a project. Let us know if we can help you with your next design!

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