How to Replace a Two-Prong Receptacle on an Ungrounded Circuit
Why this matters
Every house wired before grounding conductors became standard in cable has two-prong receptacles, and every owner of one eventually wants a three-prong for a computer, an air fryer or a phone charger with a ground pin. The tempting answer, a three-prong receptacle with nothing on its green screw or a jumper from the neutral to the ground screw, is the one that puts a tech in front of an investigator after a shock. The code gives you exactly three legal ways to replace a nongrounding receptacle where no grounding means exists, plus a fourth route that brings a real ground to the box, and each one tells the customer something different about what the third pin does.
The full list of what a receptacle replacement can trigger (GFCI, AFCI, tamper-resistant, weather-resistant) is the SOP Receptacle Replacement and Required Upgrade Assessment. This card owns the ungrounded case. Citations are NEC 2023 (2020 where the section moved), as adopted, local amendments checked.
First, prove the box really has no ground
Half the "ungrounded" boxes a crew meets are grounded through their wiring method, and the answer changes completely if they are. Before quoting, with the circuit locked out and proved dead (see step 1 of the worked example for the sequence), look at what enters the box:
- Metal conduit or EMT run continuously to the panel is an equipment grounding conductor under 250.118. A three-prong receptacle bonded to the box is grounded.
- Armored cable with a bonding strip (Type AC) also qualifies. Older armored cable without the strip is a judgment the AHJ makes, not you.
- Two-wire NM, cloth-covered cable, or knob and tube carries no grounding conductor. This is the case for the rest of this card.
A live reading of 120 V from hot to a metal box proves only that the box connects to something grounded, not that the path is continuous and low-impedance to the panel. Confirm the raceway's continuity back to the panel, or treat the box as ungrounded.
The options, as 406.4(D)(2) and 250.130(C) give them
Where the replacement location has no grounding means, 406.4(D)(2) permits, in substance:
| Option | What goes in | Marking | What the third pin does |
|---|---|---|---|
| (a) | Another nongrounding (two-prong) receptacle | none | No third pin |
| (b) | A GFCI receptacle | "No Equipment Ground" | Nothing; the GFCI protects people on that device and downstream |
| (c) | Grounding-type receptacles supplied through a GFCI | "GFCI Protected" and "No Equipment Ground" on each, visible after install | Nothing; protection comes from the upstream GFCI |
| Fourth route | A grounding receptacle with a real equipment grounding conductor run per 250.130(C) | none needed for the ground | Clears a fault to the breaker |
Options (b) and (c) carry one more rule: no equipment grounding conductor may be connected from the GFCI-type receptacle to outlets it supplies, nor between the grounding-type receptacles. A daisy-chained bare wire stapled between ungrounded boxes would look like a ground and be connected to nothing.
The fourth route uses 250.130(C), which permits the grounding conductor for a replacement grounding-type receptacle to be run separately and connected to, in substance, the grounding electrode system, the grounding electrode conductor, the equipment grounding bar in the enclosure where the circuit originates, an equipment grounding conductor of another circuit from that enclosure, or the grounded service conductor inside the service equipment. A nearby water pipe is not on that list unless it is within the first 5 ft of entry under 250.68(C). Size it to Table 250.122, which Equipment Grounding Conductor Sizing (NEC 250.122) owns.
What is never on the list. A jumper from the neutral terminal to the ground screw, the "bootleg ground". A plug-in tester reads it as correct, which is what makes it dangerous: the receptacle's ground pin is now the neutral, and if the neutral opens upstream, every grounded appliance frame on that receptacle sits at 120 V.
What the customer is told
The marking is the code's way of telling the next person the truth; the conversation is yours. GFCI protection under (b) or (c) trips at the 5 mA personnel level when current leaks to a person, so it is real shock protection. It is not a ground. A surge strip on an ungrounded receptacle cannot shunt surges to ground, some equipment's instructions require a grounded receptacle, and a three-prong appliance with a frame fault keeps running on (c) until a person completes the path and the GFCI trips. Write that on the ticket in a sentence the customer signs.
Worked example: a 1955 bedroom with a desk
The house is wired in two-wire NM without grounding conductors, in a basement-panel ranch with an unfinished basement. One 15 A bedroom circuit feeds four receptacles, R1 to R4 around the room. The owner wants three-prong receptacles throughout, and a desktop computer and printer at R3.
1. Lock out and prove dead. Breaker off, lock and tag on it, and absence of voltage verified at each box with a CAT III or better meter proved on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with work practices at 29 CFR 1910.333(b)(2). Every box: two cables of two-wire NM, metal boxes, no grounding conductor, no continuous raceway. Confirmed ungrounded.
2. Find the first receptacle in the run. With the breaker still locked off, disconnect both cables from the receptacle you suspect is first (here R1, nearest the panel's direction), cap each conductor alone and fold the two cables apart. Remove your lock, stand to the side of the panel, breaker on; this is energized testing, so hands stay off the capped conductors and you wear the PPE your shop's NFPA 70E assessment calls for. In LoZ mode, one cable reads 120 V hot to neutral and the other 0 V, and R2 to R4 are dead. R1 is first, and its live cable is LINE. If R2 to R4 had stayed live, R1 would not be first, and you would repeat the test at the next box. Breaker off, lock back on, prove dead again before touching anything.
3. Settle the triggers. A bedroom is an area where 210.12 requires AFCI protection, and 406.4(D)(4), as adopted, requires a replaced receptacle there to get it. Every replacement in a location where 406.12 calls for tamper-resistant receptacles, which includes this bedroom, must also be tamper-resistant (406.4(D)(5), see Tamper-Resistant and Weather-Resistant Receptacle Rules). One device at R1 settles AFCI and GFCI for the run: a dual-function AFCI/GFCI receptacle.
4. Install R1. LINE cable on LINE terminals, the cable to R2 on LOAD, torqued per 110.14(D). Nothing on the green screw. "No Equipment Ground" sticker on the faceplate.
5. Install R2 and R4 as tamper-resistant grounding-type receptacles with nothing on the green screw, each marked "GFCI Protected" and "No Equipment Ground". That is option (c).
6. Give R3 a real ground (the fourth route). The computer is the reason. From the panel's equipment grounding bar, run a 14 AWG copper grounding conductor (Table 250.122, 15 A) across the basement joists and up through R3's bottom plate into its box. 250.120(C) wants a conductor this size protected from physical damage unless it runs inside a hollow wall or where it is not exposed to damage; along the joists it is stapled in the bay beside the existing cable, and where it rises past the sill it is sleeved. Land it on R3's green screw and the box. R3 connects to nothing else: no jumper to R2 or R4, which would break the rule in (c) and turn their marking into a lie. R3 is still downstream of the GFCI, so it keeps the "GFCI Protected" sticker and loses the other.
R1, FIRST IN THE RUN R2 AND R4, DOWNSTREAM
+----------------------+ +----------------------+
| +----------------+ | | | | | | |
| | TEST RESET | | | | | () | | |
| | AFCI / GFCI | | | |
| +----------------+ | | | | | | |
| | | | | () | | |
| [ NO EQUIPMENT ] | | [ GFCI PROTECTED ] |
| [ GROUND ] | | [ NO EQUIPMENT ] |
+----------------------+ | [ GROUND ] |
+----------------------+
R3: [ GFCI PROTECTED ] only; its own ground, run
from the panel bar, lands on its green screw
7. Restore. Plates on, lock off, stand to the side of the panel, breaker on.
The finish test
Polarity at every receptacle. Without a ground at R1, R2 and R4, a plug-in tester cannot tell you much. Use the meter: one lead in each slot, then the other lead to a known equipment ground brought on a long test lead from R3 or another grounded receptacle. Narrow slot to ground 120 V, wide slot to ground near 0. Reversed polarity on a GFCI downstream run is invisible until somebody changes a bulb.
The plug-in tester's readings, and what they should be. R1, R2 and R4: "open ground". That is the correct reading for a marked option (c) receptacle and goes on the ticket as expected. R3: correct wiring. A "correct" reading at R2 or R4 means someone jumpered a ground or bootlegged the neutral, and it fails.
GFCI function. Press R1's own test button: R1 to R4 all go dead. Reset. The plug-in tester's GFCI button works only at R3, the one receptacle with a grounding conductor, because the tester creates its imbalance by leaking current to ground; pressed there, it trips R1. Pressed at R2 or R4, it does nothing, which is not a failure. The AFCI function is tested with R1's test button per the device instructions.
What changes the answer
- The owner will not have new grounding conductors run anywhere. Options (b) and (c) alone are complete, and the computer conversation above is the job.
- A kitchen, bath or other location where 210.8 now requires GFCI. The replacement needs GFCI protection whether or not you add three-prong receptacles (406.4(D)(3)).
- A local amendment that deletes the AFCI replacement rule or adds its own; check the adopted code before quoting the dual-function device.
References
- NFPA 70, National Electrical Code, 2023 edition (2020 where noted), as adopted: 406.4(D)(2) through (D)(5), 250.130(C), 250.118, 250.120(C), Table 250.122, 250.68(C), 210.8, 210.12, 110.14(D)
- NFPA 70E, current edition, Article 120; 29 CFR 1910.333(b)(2)
- See related: Receptacle Replacement and Required Upgrade Assessment
- See related: How to Check Receptacle Wiring with a Meter
- See related: Equipment Grounding Conductor Sizing (NEC 250.122)