[FAQ] Two-prong outlets
I would like a CEC translation of the NEC codes cited below. PM me if you can help.
So, you have two-prong outlet receptacles and want three-prong outlets. I will try to address common questions regarding two-prong outlets, and what can be done about it. If you have specific questions not covered(or needs clarifying) regarding the subject of two-prong outlets, please read this entire post before posting a new thread. This thread will be monitored indefinitely. Please understand that this post is very limited and the unique situations homes have are limitless.As an electrician, I will always advise you get a professional look at your wiring and discuss your options. You may accidentally make your situation worse. PROCEED WITH CAUTION
##Code followed: NFPA 70 NEC 2011
Updated 5/28/14
##Part 1: Common Questions Are my existing two-prong outlets dangerous?
Assuming the wiring and outlet itself are okay, most likely not. There are a couple of issues that surround two prong outlets:
1.The age of the outlets are a factor. Frequently used outlets may get worn out and will not provide a solid connection to your equipment/appliance plug. This may lead to arcing.
2.If hazards were to occur in an ungrounded outlet, there may not be a way for the breaker to trip. Metal parts may stay energized and may potentially be a shock hazard.
The biggest hazards of two-prong outlets are the work-a-rounds that people do.
I currently use two to three prong adapters. Aren't those good enough?
The adapters should not be considered a way to properly ground equipment or an appliance. They simply make a three prong fit into a two prong. The same hazards may also apply to adapters as the following question.
Why can't I just replace my two-prong outlets to three-prong outlets?
NEC 406.3(D) Forbids a simple replacement of two-prong plugs with a three-prong if there is no equipment ground available. In my opinion, it is the worst case scenario. If hazards were to occur, and the metal parts become energized, that power can "leak" into the metal parts of what you have plugged in. This is a serious shock hazard! Just changing the outlet itself, does not provide a ground.
Can I just replace an old two-prong outlet with a new one?
Yes, it is perfectly legal and safe to replace an old two-prong with a new two-prong device. Like stated above, outlets may get worn out and need replacing. This does not mean that you have to change to a three-prong and possibly create a hazard. You can still purchase new two-prong outlets.
NEC 406.3(D)(3)(a): "A non–grounding-type receptacle(s) shall be permitted to be replaced with another non–grounding-type receptacle(s)."
What is the ground in an outlet, and what does it do?
The ground is an additional path for electricity to return to the earth in the event of a fault.
Definition from the NEC:
Grounding Conductor, Equipment(EGC). - The conductive path(s) installed to connect normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.
What that means is, if anything that could be energized does become energized, it will cause a ground fault and trip the breaker.
##Part 2: Identification of branch circuit cable
Identifying the circuit's wiring type is essential to figure out your next step. In some cases, there is a ground present in the box. You will need to open the device box and an have electrical testing meter to identify a present ground. The most common cable types that feed two-prong outlets are: knob and tube, and amored cable(without a bonding strip).
Knob and Tube
Knob and tube wire is usually identified by a black cloth outer finish, and each conductor may enter the box through different holes. There is no ground run with this cable type.
Armored Cable (BX/AC)
Identifying a bonding strip Image 1 | Image 2
The bonding strip may be hard to see but is usually wrapped around the armored jacket at the it's ends. If the cable does contain a bonding strip, the jacket may be used for a ground. Without a bonding strip, the cable may test correctly as having a ground, but it is not longer approved to be used as one.
Metal Clad Cable (MC) This type of cable looks like AC/BX but has a green insulated ground(Exception: self grounding MC) which can be identified.
Nonmetallic-Sheathed Cable (NM/Romex)
Without a ground - For a short while, they produced a NM cable without a ground. It can be identified by having a cloth sheath and may have rubber insulation covering the conductors.
With a ground - Usually a bare copper conductor located within the jacket of the cable is used for the ground.
##Part 3: Testing to ensure there is a ground present
There are many types of testers that can be used to test the grounding. In my opinion, using a digital multimeter is the best way to make sure that there is a present ground in the box. There should be +-120volts between the live wire and the ground wire / metal box, and continuity between ground and neutral. (Note: Even though there is continuity between neutral and ground, they should not be in contact with each other inside the box.
##Part 4: Using the Existing Ground
If the cable is approved to supply a ground in your branch circuit, then you can safely install three-prong outlets. A bonding jumper should be installed from the box, and or grounding conductors, to the device. Alternatively, self-grounding outlets can be used. I will not cover every single method used to do this, and I suggest you post pictures and ask in this thread.
##Part 5: GFCI Remediation
Definition from the NEC Ground-Fault Circuit Interrupter (GFCI). - A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds the values established for a Class A device. Image
NEC 2011 406.4(D)(2)
This article allows three-prong outlets to installed on an ungrounded system as long as the following conditions are met:
- It is a GFCI type receptacle, or is "protected" from a GFCI receptacle.
- Is marked "No Equipment Ground" (These stickers usually come with the GFCI).
Here is a quick wiring diagram that shows how to protect a standard three-prong outlet with a GFCI. Image In this case, you would not have the ground that is shown.
##Part 6: Rewire Options
If you do not have a equipment ground, and the GFCI remediation option was not what you were looking for, a rewire is your only other option. One way would be to just run a cable with a ground to the location you want the 3 prong outlet. This would be cheaper than completely removing the old wiring, and installing new wire. Any rewiring is beyond the scope of the FAQ.
Credit for suggestions, additions, and help: u/lookatthatsquirrel u/fcisler
Posted May 28, 2014, 1:15 AM UTC · edited