Both colors are caused by oxidation as the copper reacts to elements in the environment. The green oxidation is commonly seen when pool chlorine is stored in the same room with electrical equipment and minimal ventilation, producing copper sulfide or hydrated copper sulfate. The example above is a close-up of a service lug at an electric panel in a pool equipment room with stored pool chemicals. 

    Normal oxidation darkens copper gradually over years, and this is observable in copper water pipes as they age. But overheated copper wires will form a dark-gray-to-black oxidation patina on the exposed wire surfaces, similar to what happens to an overheated copper pan on a stove. The overheating can be caused by excessive current in the wires or a lightning strike, and it will usually be accompanied by melted or discolored insulation near wire connections.

    High moisture level in the air, combined with water intrusion in the panel box can also cause an uneven green patina on exposed copper wiring over time. Older service panels for mobile homes, the kind that are mounted on a post next to the home and have accumulated rust-through holes and open knockouts, are where we see it most often. 

    Recent incidents here in the The Villages, Florida, where recharging a bank of golf cart batteries in a small garage releases hydrogen and hydrogen sulfide into the air that sets off a carbon monoxide alarm (they are also sensitive to high levels of hydrogen) and sends the fire department racing to the home, is another example of an environmental contaminant that will blacken nearby copper wires. 

   The defective “Chinese” drywall that was imported during the height of the building boom around 2006 is one more source of blackened copper. It gives off a distinct sulfurous smell, which is a secondary indicator that defective drywall is causing the problem.

     Then there is also the blackened copper at wire connections to breakers that is caused by someone obsessively adding a thin coating of black anti-oxidant paste to all the breaker connections, even though only required for aluminum wire. Look closely for this one. 

    And sometimes, unfortunately, it will just be a mystery. The hydrogen sulfide in a sewer gas leak in the area of the wires is an example of a possible cause that would not be discernible if it was a one-time event long ago. Luckily, copper oxide is still a good conductor, unlike aluminum oxide, which is not.

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Here’s links to a collection of our other blog posts about ELECTRICAL WIRING:

• Which house appliances need a dedicated electrical circuit?

• Can a short circuit cause a high electric bill?

• When is an electrical panel overcrowded with too much wiring?

• Is it alright to just put wire nuts on the end of unused or abandoned NM-cable or wiring?

• What causes copper wires to turn green or black in an electric panel?  

• What are typical aluminum service entrance wire/cable sizes for the electrical service to a house?

• Why is it unsafe to bond neutral and ground wiring at subpanels?

• Should I get a lightning rod system to protect my house?

• Why is a strain relief clamp necessary for the cord connection to some electric appliances?  

• Does a wire nut connection need to be wrapped with electrical tape?

• What is the minimum clearance of overhead electric service drop wires above a house roof?

• What are the requirements for NM-cables entering an electric panel box?

• What is the color code for NM cable (Romex®) sheathing?

• What is the life expectancy of electrical wiring in a house? 

• What is an "open junction box"? 

• How dangerous is old electrical wiring? 

• What is a ground wire? 

• I heard that aluminum wiring is bad. How do you check for aluminum wiring?  

    Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.