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Oxidation of copper wire

I am re-wiring our house and I found some of the positive insulated 5-10amp cables have a layer of black oxide on all the strands for the entire length of the cable.

Does this cause more resistance and if so, is there a danger that the cable might get hotter?

Is there also a chance that these oxidised cables will lead to higher energy consumption and higher electricity bills?

Update (04/14/2016)

Hi Dan, thanks also for your very expert advice and links.

As I mentioned to Jayeff, I am in Northern Brazil where things are far more lax and sadly often very badly done, which is the main reason that I am doing a step by step re-wire, as I can afford it.

You say that for 15amp and above, you need solid conductor wiring. As the cables will only be feeding a domestic refrigerator, 4 lights and two wall sockets for plugging in tablets, cellphone chargers etc., I have already bought 6mm cable which can handle up to just over 30amp (correct me if I'm wrong).

They will be running from a single phase (positive) trip fuse rated at 20amp.

I chose the multi-strand cable as I have to pull them through some internal and external conduits (using a cable guide) that go through quite a few turns and the solid wire would be almost impossible.

So it would seem that I might have over-rated the amperage, but surely that would be no bad thing?

Regarding the oxidation of the original wiring, I think it might well have been due to very bad workmanship - mixing up of cable thicknesses, very humid conditions (buried in the earth) and also heating up due to overloading the cable.

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Hi, It could just be reaction between the insulation and the copper. Black is usually copper oxide. As long as the connections are 'bright and shiny' there should be no problems regarding resistance. If the insulation doesn't look 'faded' (due to heat) then again there should be no problems


Hi Jayeff, actually the whole cable is oxidised varying from green to black, including the connections. What I forgot to mention was that it's mostly the cables outside buried in the ground, feeding lights and wall sockets that seem to be be the most oxidised. I am in Northern Brazil, where it is very humid and tropical. Worse, whoever did the original wiring used inner wall flexible conduit, not high grade outside /weatherproof conduit, and there were a lot of holes and moisture in the conduit itself.

They had also mixed up various thicknesses of cable, sometimes using 5 amp for positive, 10 and 15 amp for neutral and vice versa - what idiocy!

So anyway, to be safe, I am replacing any cable I find oxidised with new and also any cables with mixes of thickness. I will send the old cable to a recycling depot (if I can find one).

Many thanks for your input!


@ David Yowell - you will also want to put a GFI breaker on these lines. And Yes! You don't want to mix wire gage's (here in the state's all the wires in the cable are the same gage) some older cable had a smaller gaged ground wire which caused load issues so now it's the same gage as the conductors.

Thank god your house didn't burn down or someone get electrocuted!


Many thanks for the advice Dan and Jayeff!

And yes thank God, the lucky stars and all the spirits of the good, that my house didn't burn down. My next step will be to try and find (if possible) an experienced and qualified electrician to check all the circuits over 100%, no matter what the cost!


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Soluzione Prescelta

Here's a good paper on corrosion: Comparative Corrosion and Current Burst Testing of Copper and Aluminum Electrical Power Connectors. While it's quite techie it does point out some of the risks of oxide corrosion on copper wiring.

The one thing here that worries me is the fact you imply the conductor is multi strand. In the US you must use as a minimum 12 gage solid conductor wiring for a 20 Amp service and 14 gage solid conductor wiring for 15 Amp service for 120 Volt AC. Multi strand wiring (copper or aluminum) is not recommended for home in wall wiring and in some states nolonger approved.

There was some bad wire back in the 70's where the wires insulation was corrosive causing the wire to degrade. You might want to check the manufacturing code on the cable to see if you have this wire. If you do there was a fund to help people replace it (class action suit).

The other cause of oxide build up is when the cable was over loaded. This can happen when the wire is not the correct gage for the fuse/breaker, i.e. the gage of the wire is for a 15 Amp service but the breaker is 20 Amp so it will not trip when the wire is over loaded.

You may want to spit the lights/outlets load across two circuits when you rewire. You also may want to increase the wire gage to a thicker gage. And make sure you are using new breakers as the one on this circuit could be bad.

Here's a bit more on overloaded wiring: The Basics of Electrical Overheating

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David Yowell sarà eternamente grato.
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