Glossary · Safety and certification

What Is a GFCI? Ground-Fault Outlets and Plug-In Solar

GFCIGround-fault circuit interrupter

A fast safety device in an outlet, breaker or cord that cuts power when current leaks to ground, for example through a person.

  1. Current outOn the hot wire, to the appliance.out
  2. ApplianceCurrent should all return on the neutral wire.back
  3. GFCI comparesAbout 5 mA missing? It trips in as little as 1/40 s.
A GFCI compares the current going out with the current coming back. If they differ by a few thousandths of an amp, some is leaking, so it cuts the power.

At a glance

Trips at
A difference of about 5 mA between current out and back
Speed
As little as 1/40 of a second
Where
Set by the NEC: outdoors since 1973, bathrooms 1975, kitchens 1987
Plug-in solar issue
GFCI standards assume current flows one way

A ground fault is an unintentional electrical path between a power source and a grounded surface, the CPSC explains, and a person can be that path. A GFCI watches for it by comparing the current flowing out on a circuit with the current coming back. If they differ by as little as 0.006 amperes, it cuts the power, before the electricity can affect your heartbeat. OSHA’s guidance puts it at about 5 milliamperes, in as little as 1/40 of a second.

Where are GFCIs required?

The National Electrical Code decides, and the list has grown: according to the CPSC, outdoors since 1973, bathrooms since 1975 and kitchens since 1987, among other places. The code mostly applies to new construction and major renovations, so an older home may not have them everywhere.

Why do GFCIs matter for plug-in solar?

Outdoor outlets, where a balcony or patio kit would plug in, are usually GFCI-protected. UL’s white paper on plug-in PV warns that GFCI standards assume current flows one way, while a plug-in system feeds current back, potentially blinding the GFCI. It’s one of the problems UL 3700 is meant to solve. Never bypass or remove a GFCI to make a kit work: our UL 3700 guide explains what a certified system has to handle.

Published by the Plugged Sun editorial team

Sources

  1. What Is a GFCI? (CPSC fact sheet 099) (Official document, US Consumer Product Safety Commission, 2026)
  2. Ground-Fault Circuit Interrupters (GFCI) (Official document, US Occupational Safety and Health Administration, 2026)
  3. Plug-In Photovoltaic (PIPV) Systems: Safety Considerations for Residential Use (Source, UL Solutions, 2025)

Where GFCI comes up