Dear Mr. Electrician: What do I need to replace ungrounded outlet that has only two prong holes?
Answer: You need to determine what your options are when replacing the two-prong ungrounded electrical receptacle outlets. The BIG question is whether or not you have an approved grounding conductor in the outlet box which would allow you to replace the old outlet with a new 3-prong grounding type tamper resistant electrical receptacle. NOTE: Some text links below go to applicable products on Amazon.com
You can easily check this with a voltage tester. Put one lead of the voltage tester on the screw for the wall plate. Make sure there is no paint on the screw. Put the other lead in each hole of the receptacle. If it indicates that you have power, you probably have a ground connection to the box. You can also use a pigtail light bulb socket to do this simple test.
If the test indicates that you do not have a ground at the electrical receptacle outlet’s location, see about using GFCI’s as a replacement further down below.
STEPS TO REPLACE A TWO PRONG UNGROUNDED OUTLET
To replace the two prong electrical receptacle outlet, turn off the power to this circuit at the circuit breaker panel. Use a voltage tester to confirm that the power is off and double check at each step throughout the process.
You must take apart the receptacle by removing the wall plate first. Unscrew the center screw (Or upper and lower screws) counter-clockwise. Pry away the wall plate from the receptacle. The wall plate may be held onto the wall and/or the receptacle by many coats of paint.
Use a razor knife to score along the outer edges of the wall plate. Also score along the outer face of the receptacle. If the wall plate does not loosen from the knife, try getting a small flat putty knife behind it. In some cases you may just have to break off the wall plate, but the face of the receptacle may come off also. Do this with the power off.
Some newer style wall plates have the screws hidden. You must pry away the front of the wall plate using a small flat screwdriver. Pry gently and evenly from all corners.
Remove the two screws securing the receptacle to the wall box and gently pull the receptacle away from the wall. If the screws break off, you might be able to grab them with a pair of Vise-Grip Pliers and slowly turn them out. If that doesn’t work you may have to drill out the old screw and tap the hole and install a new screw.
I had to use my razor knife to clean out the slots on the 6/32 screws that held the above outlet in the metal outlet box for many years.
The standard receptacle screw thread is 6/32. It is a number 6 machine screw with 32 threads per inch. The screw head is usually a flat head or a low profile round head. If necessary you can re-tap the hole to 8/32 and use an 8/32 flat head machine screw. Use my drill, tap, and screw chart here.
Pull the receptacle away from the electrical box. The wires are suppose to be long enough that you can easily work on the receptacle. Quite often though, the wires are too short or the insulation is old and brittle, and sometimes you have both together. Not to panic, but care must be taken so as to minimize damage to the insulation. If the wiring is pre-World War 2 it may be soldered and taped.
If you need to make changes or splices you should use a sharp knife to cut back any old electrical tape. Do not cut the wires. They are all that you have to work with. Identify all of the wires before taking them apart. Use white electrical tape for the white neutral wires. Use red for switch legs. Use black tape for the hot wires.
Check the wiring for polarity with a two wire lead voltage tester or volt meter. Turn the power on and be very careful. With one tester lead held against the metal box, put the other on each of the wires to see which one shows voltage. That one is the hot wire and should be labeled black.
Now check the black hot wire with one lead and the touch the other lead on the other wires until you get a reading on your tester. That wire is the neutral and should be labeled white. Shut the power off before getting back to work replacing the two prong outlet.
After all of the taped-on insulation is removed you will see the soldered wires twisted together. Using a pair of pliers, slowly untwist the wires. The old solder has a very high lead content which makes it very soft. Straighten them out a little and tape them with several layers of electrical tape using the appropriate colors.
If the wires attached to the existing two prong receptacle are in good condition and the color coding is easily seen, then you can remove them from the receptacle. Do not cut them. Loosen the screw or insert a paper clip or small screwdriver into the rear quick-stab clips to release the wire from the receptacle.
If the wires are too short, you can use wire connectors to splice short pieces of wire called pigtails to connect them to the receptacle. Pig-tailing is a good wiring method to use when wiring electrical outlets because the receptacle will never carry the full load of the circuit this way.
In the photo above you can see that the existing wiring is BX cable and is clamped tightly into the box with an old style brass bushing. This provides an adequate ground for the new tamper resistant receptacle. I tested each wire with my Wiggy Voltage Tester with one lead on the box and the other on the wire to determine hot and neutral. I was able to tape the wires with white and black electrical tape and reuse them on a new receptacle.
Look inside of the electrical box to see the type of wiring. There are several choices depending on geography and age that were used as part of general construction techniques. A Pre-World War 2 house could have knob and tube wiring, or old style BX metal armored cable or even rigid metal electrical conduit with wires pulled inside.
The above metal outlet box had two BX cables entering it, one from each side. There wasn’t any tapped holes in the back of the box for a ground screw so I had two options. I could have drilled a new hole in the back of the box and tapped it for a ground screw or I could use a grounding clip.
Homes built after World War 2 could have BX or non-metallic cable wiring. Look closely inside the metal electrical outlet box (Does not apply to plastic electrical boxes) to see if you have BX cable, conduit, individual conductors, or something else feeding power to the box. If so you can replace your two prong outlet with a tamper resistant self-grounding receptacle. By doing this you will have a grounded three prong electrical receptacle outlet and be code compliant.
When working with old steel electrical boxes, I use my 6-in-1 tap tool to clean out the threaded screw holes. This helps keep the new screws from getting stripped.
The alternative method of grounding is to install a machine screw into the threaded hole in the back of the box and loop a green or bare wire around it to be used as a grounding conductor, also known as a grounding pigtail. The grounding pigtail then gets connected to the green screw on the receptacle. The threaded holes on old black electrical boxes were 10/24 while new boxes have 10/32 threaded holes.
WHEN THERE IS NO GROUND AT THE ELECTRICAL RECEPTACLE
If there isn’t any effective and approved ground, then you must replace the existing non-grounded electrical receptacles with a new non-grounded receptacle replacement. This is not always acceptable in a modern household. Consequently the National Electrical Code has made an allowance for using grounded receptacles on an ungrounded circuit. You must use GFCI tamper-resistant receptacles and label them as not having a ground. Read article 406.4(D)(2) in the National Electrical Code.
Just because you added a GFCI receptacle onto a non-grounded circuit that does not mean that you have a grounded third prong. For sensitive appliances with microprocessors and computer equipment, you should always use a grounded circuit. If the house is so old that it does not have grounded wiring, then it is overdue for new updated wiring. Some of my other posts may come in handy for that.
It would be best if the GFCI receptacle is installed at the beginning of the circuit, which would be the first receptacle on that circuit. Connect the wires going to the other ungrounded receptacles to the LOAD side of the GFCI. After that all electrical receptacles downstream will be GFCI protected. You can then install 3 prong receptacles however they must all be labeled that they are GFCI protected and that no ground exists. Labels for this come with each new GFCI receptacle.
It is also approved in the National Electrical Code to install a single conductor wire to a grounded water pipe or bonded ground rod to get the necessary ground path. However for the amount of work and effort required to install a single conductor ground, you may as well install brand new circuit wiring with a built-in grounding conductor. Read article 250.130(C) in the National Electrical Code for the allowances to install a grounding conductor to an ungrounded electrical receptacle.
INSTALL REPLACEMENT FOR UNGROUNDED OUTLET
Most electrical receptacles in a home are now required to be tamper resistant to prevent children from putting things in the slots. Install the new tamper resistant receptacle (or tamper resistant GFCI receptacle) by attaching the white neutral wires to the white screws which are usually on the same side as the green ground screw. Attach the black hot wires to the brass colored screws and then attach the bare or green grounding conductor to the green screw.
If the metal electrical box is properly grounded you can use self-grounding, tamper resistant receptacles or GFCI’s. For GFCI receptacles, take note of the LINE and LOAD terminals. The hot and neutral wires connect on the LINE side. The wires that go to the downstream outlets connect on the LOAD side of the GFCI.
Fold the wires back into the box as you push the receptacle back in. Screw the 6/32 screws into the box. Sometimes they can be short and you will need to buy longer 6/32 screws.
Install a new wall plate and the job is done. Turn the power back on and test. Note that the GFCI receptacle will need to be reset when it is first energized. Push the reset button which can sometimes be difficult depending on the brand of GFCI. Sometimes I have to use a screwdriver and push the reset button with some force.
ANOTHER UNGROUNDED OUTLET REPLACEMENT
An ungrounded two prong outlet extender with a burn mark was cause for concern. Not only was the extender burned, but also the outlet that it was plugged into. Notice the burn mark below.
With BX cable, the grounding conductor is the metal armor of the cable. It is important to have all BX clamps and connectors tight for this very reason.
The extra cable clamp on the right side should have been removed during the original installation of this outlet. According to the National Electrical Code (NFPA 70) each clamp inside the electrical box counts as one wire towards the allotted wires for that box. Read article 314.16(B)(2).
I used the screw from the clamp that I removed to attach the grounding pigtail to the grounded metal outlet box.