Why is the copper wire in my cable black?

This may be seen on an existing installation, where the exposed copper conductor has a black powdery substance formed its surface. Sometimes, the copper wire in a new cable may have a mottled black surface when the insulation is stripped back.

The black substance is most likely to be copper oxide, which is formed when copper comes into contact with Oxygen in the air. Performance wise, this can be a problem as oxidised copper is not electrically conductive. This means that its presence can lead to an increase in the conductor's electrical resistance causing it to heat up, which is inefficient in terms of conducting electricity. Heat from conductors is never good and too much heat can cause the insulation to melt or burn.

Does the black colour effect the performance of copper wire?

Copper wire is used in electrical cable because of its excellent conductivity. It is also durable and corrosion resistant. However, when exposed to the oxygen in air the surface of the copper will oxidise to form a thin protective oxide layer which acts as a barrier between the metal and the environment. Exposure to moisture can accelerate the oxidation process.

When stripping the insulation from new electric cable, the copper wire should be shiny and pure in appearance. If it is black, then it’s a sure sign that a cable has not been manufactured properly. The advice from Prysmian is not to use the cable, even if the oxide is removed from the exposed conductor, because you have no way of knowing how far the oxidation extends beneath the insulation.

This effect can be a sign that the copper has been overheated during the production process. Copper is “drawn” from thick rods into thin strands for conductors and then “annealed” in a process that makes the copper more compliant and malleable. Overheating at either of these stages can affect the performance of the copper conductor.

Oxidation of the copper can also result from poorly sealed cable ends. If the cable has not been properly sealed and carefully stored, then air or moisture could permeate the exposed conductor and cause the copper to oxidise.

Over time, even with good quality cable, copper exposed at terminations or joints can eventually turn black. Where these connections need to be re-made, a quick buff with wire wool should remove any superficial copper oxide to restore the copper's lustre and conductivity, ensuring a reliable connection.

Of course, if you keep having to wire-wool a connection to remove the outer layer of oxide you will over time end up removing so many layers that the cable will start to degrade.

Does anything else turn copper wire black? 

The black colour on copper can also be caused by a reaction with sulphur over time. Unless you live on a volcano, this patina is usually the result of copper coming into contact with the sulphur present in rainwater. (If you do live on a volcano, it is quite likely that sulphur gas in the air will react with any exposed copper in the same way.) 

Just as with copper oxide, copper sulphide is not electrically conductive, and the corrosion it causes will increase the conductor's electrical resistance and cause it to lose conductivity.

Whatever the cause, if the copper wire in an electrical cable has a black surface colour it is an indication that the conductor will not be as efficient as it should be, and any new cable revealing this patina on the conductor should not be installed. 

Cable manufactured in first-rate conditions and to proper quality standards should not be discoloured. Purchasing quality-marked cable from a reputable manufacturer is the best way to ensure that it will perform safely as it should.

With its three UK factories and rigorously employed quality control, Prysmian cable offers electricians and their clients the reassurance of a high-quality product that will stand the test of time. If the copper conductor in new cable has a black coating, we’re willing to bet that it’s not been made by Prysmian.