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Cable Resistance ( 20 Degree Vs 90 Degree Celsius)

Nick Parker

While reading the cable parameters from the OEM catalogue, I see that  cable resistances are produced for both AC (at 90 degree celcius) and DC (at 20 degree Celsius).

Some other manufacturer catalogue also shows both AC resistance (@ 90 degree Celsius) and DC resistance (@ 20 degree Celsius).

Now the questions,

1. Why not the DC resistance @ 90 degree Celsius?

2. Which resistance to be used for cable sizing? for both AC and DC applications?

  • 0

    Well, are you sizing for voltage drop on a long run - cool cables, or maximum power transfer on a short run - hot cables?

    AC and Dc resistance will be the same except for very large CSA where EM effects push the current out of the core of the wire to the outer.

    AC reactance (inductance) depends on the layout of the flow and return currents, and is a measure of the energy spent and recovered creating, destroying and then  reversing the magnetic fields around the wires, and is not usually an issue where L and N are close and low current.

    Mike.

  • 0

    The AC and DC resistance are  actually the same at both temperatures, it is the reactance part of AC that is different. This is a way to minimise the entries in the table of data, and may well come from the cable standards document. Resistance change with temperature is a material based function only (eg copper), whereas the reactance part varies with cable size, construction, and other things as Mike says above. We normally use the AC figure at maximum temperature (90C) although we may adjust the maximum conductor temperature to suit other installation details often 70C. The DC value is what you will measure with a normal meter, often used as a quick way to estimate length.

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