Ampacity Audits and Uprating

For additional information on ampacity audits or uprating, please contact Jay Williams.

Our engineers routinely perform cable ampacity analyses, for all cable system types, and for land and submarine cables. We perform these studies as part of conceptual analyses, detailed cable system design, or evaluations of existing cable systems.
Evaluating existing cable systems, "ampacity audits," is becoming more common as utilities find increasing need to improve the power transfer on existing cable systems. Since the earth portion of the cable thermal circuit is so important - and so variable - we team with Geotherm, Inc, the world's foremost firm for soil thermal analysis, to accurately characterize the native soil and the trench backfill.
PDC engineers and Geotherm teamed for the first detailed ampacity audit, performed for the United Illuminating Company. In that case, we found a situation where two cables crossed at a twelve foot depth, in an old landfill area - and the circuit ampacity decreased 25%. However, we worked with UI to design a fluid-filling and rapid circulation system that increased the ampacity by 34%, for a net ampacity increase of 9%.
Since that time, PDC engineers have performed more than a hundred ampacity evaluations, and we have performed more than a half-dozen full-scale ampacity audits of existing cable systems.

Ampacity Programs

PDC engineers have several ampacity programs that are used for our analyses. PowerAmp is an easy-to-use Windows-based ampacity program developed by PDC that calculates steady-state and emergency ampacities for all cable types. In addition, PDC engineers developed the ACE program used in the EPRI UT workstation. We maintain that program for EPRI, and may use it for EPRI member utilities. PDC also has several programs developed in-house that evaluate specific ampacity conditions such as deeply-buried cables in plastic or steel casings, extruded cables in steel pipe, cable crossings, rapid circulation and forced-cooling, etc.

Ampacity Audits

An ampacity audit is intended to carefully determine the actual ampacity of an existing cable system. An audit begins with evaluating the utility's load shape, the circuit plan and profile drawings, manufacturer's cable data, any soil thermal analyses, etc. to permit accurate calculation based upon the utility's records.
However, a "paper study" is seldom sufficient. We conduct a thorough field analysis, walking the line route looking for evidence of steam mains, distribution ductbanks, etc. that may affect ampacity. In addition, we look for areas where fill has been added or other changes have been made that may affect ampacity.
Utilities seldom have had a thorough soil thermal analysis performed. We typically retain Geotherm, Inc to perform that analysis, and the field analysis mentioned above is usually conducted during the thermal survey. PDC and Geotherm will select the best locations for thermal property analyzer tests, typically 1000 - 2000 feet apart. We will also select locations for test pits, where we can test the controlled backfill and take samples for laboratory analysis, evaluate backfill dimensions, etc.
We provide calculations and results for steady state ampacities, and emergency ampacities as a function of cable preload. Importantly, our thorough review of the circuit permits our recommending potential mitigative measures that can improve circuit ampacity.