How to differ the cable conductor qualified or not?

If the square of the wire is not enough, it will not be qualified? In fact, not always! For many people, this is already a “common sense” existence, but is this really the case? With the improvement of conductor material production process and scientific and technological progress, the advanced production process of oxygen free copper has been widely used. The resistivity of copper conductor material is enough to ensure that the copper wire less than the nominal diameter can meet the requirements of DC resistance of corresponding specifications, while the conductor of wire and cable has strict requirements on the resistivity of conductor, for example, The DC resistance of 25 mm2 copper core conductor specified in the standard at 20 ° is not greater than 0.727 Ω / km (the class 2 starnded conductor), so the conductor is qualified as long as it is not greater than this value. Even some square of the wires are eough or above, if they use recyle material or bad material which is not pure, the DC resistance at 20℃ can’t meet the standard, the conductors are not qualified, too.