First Class Changing A Two Prong Outlet To A Three Prong
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Ensure first that you have armored cable with a metal strip inside the armored cable this was done to improve its grounding capability.
Changing a two prong outlet to a three prong. Since two pronged outlets are not grounded it is essential to replace these receptacles with three pronged versions. Yet it is possible to retrofit a new three prong or gfci receptacle into the same outlet box without any rewiring as long as the box itself is grounded. If you re going from a 2 prong to a 3 prong outlet and your home has in fact been grounded but there is no ground wire at the outlet you will also need to run a wire from the green ground screw to the back of the metal outlet box for grounding the outlet.