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I'm replacing a few 2-prong receptacles with GFCI ones in an old house. My understanding is that ideally we should identify head of circuit and only replace the head receptacle with GFCI one, while replacing downstream 2-prong receptacles with regular GFCI ones.
However since it is an old house and I might not be able to identify all head circuit correctly. I'm wondering if is it ok to replace all 2-prong receptacles with GFCI ones, is there any downside besides higher cost on GFCI receptacles?
You can do it either way. You can...
In this case you spend $16 per outlet. Every outlet is responsible for its own GFCI protection. Therefore you do not use the "Load" terminals at all. Don't even remove the "For Wizards Only" warning tape.
Since the outlets are not grounded, you will need to use the "No Equipment Ground" stickers on each one.
The upside of this method is you never have to go hunting for the reset button, you know exactly where it is.
Or...
The "Load" terminals have exactly one use: to extend GFCI protection to a circuit downline. They should never be used for anything else.
So having found the correct outlet, guess which wires come from the panel, and attach them to "Line". Cap off the others temporarily. Power up for testing and if the GFCI works, you guessed right.
Now attach the remaining wires to "Load".
Affix a "No Equipment Ground" sticker here.
All other plain receps that trip with this GFCI get "GFCI Protected" stickers and "NO Equipment Ground" stickers.
The upside of this is you only need 1 GFCI per circuit, so fewer GFCIs to test every month.
If you want, you can certainly put GFCI outlets everywhere. Just be sure to not use the LOAD terminals on any of them. Instead, use pigtails to connect both incoming wires and outgoing wires to the LINE terminals. If you have any outlets where multiple hot and/or neutral wires connected (any the tabs are not broken), do not use the LOAD terminals to reproduce that.
Having said that, it's not that hard to eliminate the need for many GFCI outlets. Start with an outlet in a room. With the power off, remove it from the wall and disconnect all of the wires (make sure you note how they were connected if the colors are not obvious) and cover them with wire nuts. Turn the power back on and see which outlets are now dead: these are all downstream and can be replaced with normal 3-prong outlets, labeled "GFCI protected/no ground". After idntifying the incoming wires (with a voltmeter or non-contact tester), turn the power back off and replace the outlet you pulled with a GFCI outlet. Make sure to connect the incoming wires to LINE and outgoing to LOAD.
Now try another outlet, if it turns out that a new GFCI outlet is downstream, either make that an isolated GFCI or go back and replace the downstream GFCI with a normal 3-prong (labeled).
Don't forget to label all GFCI outlets "no ground".
One warning: if you try to use an outlet tester with a GFCI trip button to test your wiring, such as
be aware that this will not work without a ground. You can get around this by using a long wire with one end connected to a ground (a water faucet my work) and the other end connected to the tab on a 2-3 prong outlet converter
and plug the tester into that.
The other answers cover the basic requirements, so I'll get to a requirement not mentioned. To comply with the National Electrical Code (NEC), replacing a receptacle requires Arc-Fault Circuit-Interrupter (AFCI) protection. This has been a requirement since 2014. There are variations of adoption by state and local ordinance, but the NEC requires AFCI protection in most locations in a house, except bathrooms. An example of a state difference is Idaho, where AFCI protections is only required in bedrooms. California adopted all NEC requirements. Also, tamper-resistant outlets are a requirement in changing receptacles.
To recap requirements:
Non-grounding: You are allowed to replace a non-grounding (2-prong) receptacle with another non-grounding receptacle, but I've found none available for sale. So this option is likely only available if you have extras on hand, or get them another way, perhaps replacing a broken receptacle with one salvaged when you upgrade another branch to grounding receptacles. Tamper-resistance is not required for non-grounding receptacles. However, the NEC does not exclude the AFCI requirement if the replacement is in an area that requires AFCI protection, which complicates the simple replacement of a damaged 2-prong receptacle with the same.
GFCI: But in most cases, people want to install a 3-prong grounding receptacle. Without upgrading the wiring (running hot neutral ground), GFCI protection is required. That means, for a given branch circuit, either a GFCI breaker at the panel, a GFCI receptacle that can feed regular 3-prong receptacles as loads downstream on the branch, or more GFCI receptacles to attain complete coverage—at the extreme that means a GFCI receptacle at each outlet.
Tamper-resistance: All receptacles need to be tamper-resistant, whether plain or GFCI.
AFCI: In addition, the NEC requires AFCI protection as prescribed. Because of code difference, you should find the requirements for your location (state, and possibly county, city). Per the NEC, bathrooms are excluded, and some large appliances. Basically, the protection is most important where people might plug in many items (bedrooms, living rooms, kitchens, garages), and a spark could cause a fire. Large appliances with motors can trip AFCI protection unnecessarily, hence the exclusions. Bathrooms are primarily a water hazard, and are excluded.
For a branch that requires AFCI protection, the options are similar to GFCI. You can get an AFCI breaker—including a combination AFCI/GFCI breaker. You can use AFCI outlets—including combination AFCI/GFCI breakers.
You're required to tag ungrounded outlets properly—"No Equipment Ground" for GFCI ungrounded outlets, and both "GFCI Protected" and "No Equipment Ground" on protected standard outlets (stickers come with GFCI receptacles).
Note: These requirements greatly complicate updating old outlets. I'm giving an overview of the important details—check code or hire an electrician as needed. I'm not saying the police will be knocking at your door to check your outlets. But if you do a lot of work and your place burns down, or someone gets injured, maybe your insurance provider or other party may consider negligence, and that may even happen after you've sold your house to someone else. Either way, it's good to know what requirements are before taking on such an upgrade task. Maybe you intend to be 100% compliant with code, do all the work, then find out a few months later no one mentioned AFCI, etc., and you could have taken care of it the first time.
Code access:
You can get free access to the NEC—here's the 2023 code: National Electrical Code 2023
You can find state and major local codes here, choose your state and click Electrical Code: Building Codes
