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The ground fault circuit interrupter made its debut about a year after Neil Armstrong walked on the moon. Only one of these actually involves rocket science.
So what are the rules about applying GFCIs? If you look in the index of the NEC, you’ll find a long listing of applicable sections. Don’t let this intimidate you. The basic requirements are in 210.8. Section 215.9 refers back to 210.8, and the other references are for specific applications such as fountains or garages.
If GFCI isn’t required for a particular circuit, there’s nothing stopping you from providing GFCI protection for that circuit. Well, almost nothing. It is expressly prohibited in some applications, such as fire pumps [695.6(G)]. From an operations point of view, you want to apply the same principle to critical equipment even where the NEC doesn’t prohibit GFCI protection.
Generally, GFCI protection is required on circuits where water is nearby. That’s why, for example, it’s required if a receptacle is near a sink. And why Article 680 (pools) contains GFCI requirements in multiple sections.
Because the word “ground” is in the name of the device, many people believe it doesn’t work on the old two-wire ungrounded circuits found in many older buildings. This is not true. The device does not interrupt ground; it interrupts on a ground fault. It needs a neutral and a hot, and it monitors the neutral current to determine when to open.
Source: Mark Lamendola | Mindconnection