Coaxial Power Cable Introduction
A coaxial power cable for DC power transmission has at least the following advantages over other forms of conductors:
- Information content can be combined with DC power from a power supply on a single coax cable saving cost and encouraging simplicity.
- The controlled geometry of a coax cable confines high frequency power supply switching noise components to the space between the center conductor and shield, preventing radiation from the cable and resulting pickup by unintended receivers. The coax cable eliminates undesired electromagnetic radiation.
Since coax cables are not usually thought of for DC power transmission, this article will discuss some important cable specifications pertinent to DC power handling capability.
Coaxial Power Cable Specifications
Dielectric Type – There are two main types of coaxial cable dielectrics: polyethylene (PE) and polytetrafluoroethylene (PTFE) also known by the DuPont trade name of Teflon. The pertinent properties of each are summarized in the following table:
PE | PTFE | |
Melting Temperature |
120C | 327C |
Dielectric Strength |
20kV/mm | 60kV/mm |
Dielectric Constant |
2.1 | 2.25 |
Generally speaking, PTFE is superior in each of the listed properties.
Center Conductor Ampacity – The ampacity of the center conductor is not specified in coax cable data tables for center conductors. However, some tables give the diameter of the center conductor or the center conductor gauge size, from which the ampacity can be derived for a given ambient temperature and coax cable dielectric type. An authoritative table for extensive coax cable data is found in each of the following publications:
- Reference Data for Radio Engineers
- The ARRL Handbook for Radio Amateurs
A useful relationship to remember for coaxial power cable applications is that as conductor diameter doubles, the conductor cross-sectional area quadruples and therefore the DC ampacity quadruples. Derated ampacity can be found from the Wire Parameter Calculator which is based on MIL-STD-975 and using 200C insulation and with the ambient temperature equal to 70C. You must make an adjustment to find the ampacity for use with either PE or PTFE dielectrics or at other ambient temperatures. In determining the relationship for the ampacity adjustment factors, an understanding of the following relationship is needed: conductor temperature rise is proportional to power dissipated in the conductor, and power dissipated in the conductor is proportional to the square of the current in the conductor. Therefore,
the ampacity adjustment factor accounting for the melting temperature of the chosen dielectric is:
and the ampacity adjustment factor accounting for the actual ambient temperature is
Multiplying the above factors together and simplifying gives a consolidated ampacity adjustment factor of:
Therefore, the derated ampacity for a chosen coaxial dielectric and operating ambient temperature is determined by multiplying the ampacity found from the Wire Parameter Calculator for a given conductor size by the consolidated ampacity adjustment factor.
For example, coaxial cable RG-58, Belden part number 8240 has a AWG20 center conductor and a PE dielectric with a melting temperature of 120C per the table above. The Wire Parameter Calculator reports a single wire ampacity for a 200C rated wire at 70C ambient temperature of 6.15A. If the maximum operating ambient temperature is 40C, then the consolidated ampacity adjustment factor calculates to 0.78. The ampacity of RG-58 at 40C operating ambient temperature is 0.78 * 6.15A = 4.82A.
Center Conductor Resistance – Another factor to consider in coaxial power cable applications, especially when the cable run is very long is the resistance of the conductor. Knowing the center conductor gauge size, any wire table or calculator can be entered to find the resistance per length of the conductor gage size. Multiply the resistance per length by the cable length and multiply be two again to account for the outbound and return paths to get the total cable resistance. The current squared times resistance losses must be small compared to the transmitted power for the system to be practical.
Dielectric Strength – If the coaxial power cable application involves high voltage transmission, we should estimate our margin to dielectric breakdown. It is important to realize that generally speaking, dielectric strength figures are given for perfect conditions. Dielectric materials are typically stretched, compressed, dinged, nicked, abrased, etc., leading to degraded dielectric strength. Knowing the dielectric thickness and dielectric strength, the safety margin between operating voltage stress and dielectric breakdown stress can be determined. Safety factors of 10X are not unreasonable.
For example, RG-58, which has a PE dielectric, has a dielectric outer diameter of 0.119 inches and a center conductor outer diameter of 0.0285 inches. Therefore, the dielectric thickness is found to be (0.119 – 0.0285) / 2 = 0.04525 inches = 1.78 mm. With a published dielectric strength of 20kV/mm, the breakdown voltage of the insulator is about 35.6kV. However, prudent design requires a very large safety factor. Using a 10x safety factor results in an maximum operating stress level of 3.56kV. This DC level put us in the same ballpark as the published RMS voltage rating of 1900V for AC applications.
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