Knob and Tube Wiring and GFCI
How to Add a GFCI Outlet to Knob and Tube Wiring or a 2 Wire Outlet: Identify the Polarity of the Circuit Wiring, Wiring the GFCI Outlet, Adding a Ground Wire to the GFCI Outlet
Guide for Adding a GFCI Outlet to Knob and Tube Wiring
Electrical Question: I am working on Mother’s Bathroom electrical wiring in an 1910 vintage home.
- There was knob and tubing wiring still in many areas of the house.
- In particular to the bathroom where I have only the hot and neutral wires that were coming in to an old fluorescent light over the sink cabinet.
- Can a GFCI be installed if there is no ground wire there?
This electrical wiring question came from: Steve, a Handyman from Niles, Ohio.
Thanks for your electrical wiring question Steve.
How to Add a GFCI Outlet to Knob and Tube Wiring
Skill Level: Intermediate to Advanced – Best performed by a Licensed Electrician..
Tools Required: Basic Electricians Pouch Hand Tools and Voltage Tester or Non-Contact Voltage Tester.
Estimated Time: Depends on the personal level experience and ability to work with tools and access to the wiring.
Precaution: Identify the circuit, turn it OFF and then Tag it with a Note before performing any wiring.
Notice: Installing additional electrical wiring should be done with a permit and inspected.
Instructions for Wiring a GFCI Outlet for Knob and Tube and 2-Wire Circuit
- The Basics of How a Non-Grounded GFCI Outlet Works
- When installed correctly, the circuitry of an ungrounded GFCI will still work and detect a ground fault even when there is not a ground wire attached to the GFCI outlet.
- Ground fault protection may be detected with an electrical device or the user of a device that is connected to the GFCI outlet.
- Detected faults to ground will cause the circuit to stop working at the GFCI outlet.
- The GFCI Outlet Test and Reset Functions should be tested routinely.
- Identify the Polarity of the Circuit Wiring
- A GFCI Outlet can be installed where there is no ground wire as long as the polarity is correctly identified.
- Advanced Only: While the circuit is off, remove the outlet and wrap black electrical tape around the screws of the outlet. Take all precautions to keep other occupants in the area away from the outlet.Turn the circuit power on to perform the polarity test.
- Use a non-contact voltage tester to identify the black circuit power wire of the circuit.
- NOTE: The power wire of the circuit should be connected to the side of the outlet that has the smaller plug slot of the two vertical blade slots on the face of the outlet which are used for the cords to plug into.
- Turn the circuit power off.
- Identify the power wire using black electrical tape.
- Identify the neutral wire using white electrical tape.
- Wiring the GFCI Outlet
- Make sure the circuit is turned off before proceeding with the wiring connections.
- The following connections are made to the LINE side of the GFCI outlet.
- Connect the power wire of the circuit to the brass screw.
- Connect the neutral wire of the circuit to the silver screw.
- Electrical Code Requirements for a Ungrounded GFCI Outlet
- The GFCI outlet must be labeled as being an ungrounded outlet, which means that there is no ground connection available.This label will be included with the GFCI outlet and must be attached to the face of the GFCI outlet.
- The GFCI outlet will still function and provide protection in the case of detecting current to ground due to coming into ground sources such as a water facet or pipe.
- Adding a Ground Wire to the GFCI Outlet
- A dedicated ground wire may be installed from the outlet location and connected to the main electrical panel and bonded to the ground terminal of the electrical system.
- The home grounding system will include a bonded ground to earth and other components as required for the specific location.
- The main ground system bonding may include connection to copper water pipe, concrete foundation of the home, metal gas pipe, ground rod(s). Each system will have electrical code requirements as applicable to the specific construction of the home.
- If a dedicated ground wire has been installed and attached to the ground screw of the GFCI outlet, then the non-grounded label is not required.
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