replacing 2 slot receptacle with 3 slot


Old 10-21-14, 06:45 PM
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replacing 2 slot receptacle with 3 slot

Hello everyone

I just bought a 1950's house which has a new panel and 3 slot receptacles in the (finished) basement and some rooms. 3 of the upstairs rooms have old 2 slot receptacles.

I have tested the the 2 slot receptacles with a multimeter, and they all read approx 110v when i test to the ground screw on the outlet. I presume this must mean that these older sockets are fitted with grounded armored cable, and that I can replace them with a new 3 slot outlet and pigtail the ground to the grounding screw. Does it matter that I do not know what the boxes are actually grounded to?

When I was having the house inspected the inspector seemed to suggest that I would be best to have GFCI receptacles fitted, but i would much rather have regular plugs if possible.

I am confident that I can change the outlets over (and have a book!), but wanted to be certain that this doesn't represent some crazy risk that I don't know about.

many thanks in advance for any help you can offer
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Old 10-21-14, 07:22 PM
Tolyn Ironhand's Avatar
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Welcome to the forums!

While your meter may read 120 volts to ground, it may not be a reliable ground. You need to determine if your cables are Armored cable with, or without a bonding strip, or Flexible Metal conduit. The only one that is approved for grounding is AC cable with the bonding strip. If you can determine that you do have an approved ground, you are OK you ground to the box.

Is it allowed to change ungrounded receptacles with GFCI's as you mentioned. This will not give you a ground, but will give you the safety to use a grounded plug.
Old 10-21-14, 07:27 PM
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What is the voltage hot to neutral? 110 is within limits but lower than expected. That may be because old style BX does not have a grounding strip and are not considered a reliable ground.

Not an approved test but I would connect a 100 watt incandescent bulb between ground and hot and measure the voltage.
Old 10-22-14, 04:31 AM
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If you do decide to use the GFCI protection method, you only need one at the beginning of each run. The remainder of the receptacles can be of regular type if wired from the LOAD side of the GFCI. HOWEVER, you must use the stickers in the GFCI package to denote "GFCI Protected", and "No Equipment Ground" on each cover plate.
Old 10-22-14, 07:53 AM
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Hi guys

thanks for the welcome and the prompt advice.

All of the offending receptacles are on 1 circuit. I just figured out which one it is at the panel (the only unlabelled one!). It is supplied by modern grounded NM cable, so I figure I should be able to just change the boxes out after all.

I did notice that several circuits in the panel are supplied by rubberized, grounded cable, which my book suggests has a life expectancy of approx 25 years. The house had a new panel installed in 2009 when they finished the basement, and obviously they didn't change these wires (I guess would need a whole rewire of the house). Is this something that I should be overly concerned about? There are no overt issues with the electrics in the house, just didn't know whether it should be on my radar.

Thanks again

Old 10-22-14, 08:08 AM
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It would depend on the condition of the cable. High loads will decrease the lifespan as the heat will degrade the insulation.

If it is easy to replace I might put it on the list to change out.

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