I am replacing a bathroom light fixture in an old condo built in the 70s. There is armored cable coming into the fixture from the wall.

From my understanding is that I need to do these steps,

  1. Install an old work/junction box w/ armored cable connector and attach cable.
  2. Attach the three wires (black to black), (white to white), (ground from light fixture to ground in old work box. Is that right and safe?

Do I have to voltage test to make sure my old wires can handle the voltage of the new light fixture wires without starting a fire?

Pardon my ignorance on any of this. This is my first time doing electrical work.

Am I safe doing this without starting a fire?

Let me know if I am missing any important steps. Thanks!!!

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Work Complete Update - I had to extend the armored cable with an EMT connection and new cable as I couldn't pull cable out of the wall to reach to the newly located floating work box. No mc flex cable connection available at the stores to extend the cable so a EMT connection was used.

As instructed, tested for ground and also installed anti-short bushings. Connected wires and connected ground to the ground screws. Used CFLs to create less heat for the older wires in the walls.

1.5 Year Update - No Problems with light fixture / light

  • A photo of the new fixture would be helpful. Not all fixtures require a box. While no test is required you are likely entering the abyss where 90ºC wire is required for the new fixture and you have 60ºC existing wire. Commented May 23, 2020 at 15:08
  • Built in the 70’s the metal clad is probably the ground.
    – Ed Beal
    Commented May 23, 2020 at 15:39

2 Answers 2


Not all fixtures need junction boxes. Some fixtures have a built-in junction box like florescent light fixtures or bathroom fan/light combination fixtures, so it will depend on what you're putting up there. If one is needed, then you're correct with your plan. Turn off the breaker that controls the circuit. You'd want a metal old work box and would connect your armored cable to with with the connector from your old fixture. Test you wires for voltage, 120V, black to white and black to the armored shield, which would mean the armored cable is grounded. Make your fixture connections and mount it to the junction box. Turn on the breaker and flip your light switch.

  • 2
    They should also add an anti-short bushing into the mc cable. Commented May 23, 2020 at 14:56
  • I'm using a multimeter to test voltage. Black probe to white/neutral wire and do you mean the red probe to the armor cable/junction box to test the ground? Will it give me a 120v reading if I do this and thus meaning proper grounding?
    – Jumper89
    Commented May 26, 2020 at 0:49
  • @Jumper89 One probe on the white and the other probe on the black, you should get 120V. that is what will feed your light. Then one probe on the metal part of the armored cable or metal box if cable is connected to it and the other probe on black. You should get 120V if cable is grounded.
    – JACK
    Commented May 26, 2020 at 1:16

That lamp has a huge metal surface. If a user takes a long hot shower - typically teenage female persons - there might be the danger that these metal parts are having some dangerous voltage from the hot wires or other hot parts inside the lamp which will be also wet. Even the wall could thus have dangerous voltages, its resistance being low due to frequent cleaning with detergents which leave residuals on the surface with conductive chemical components, if wet.

This danger was even a little bit lower in times of incandescent bulbs since they heated the hot wire ends and the total lamp a little bit, lowering the probability of condensation.

The mixture of electricity and water is why in many countries a GFCI is now mandatory in bathrooms, because it would detect any dangerous current that could flow from the hot parts inside the lamp to the grounded metal casing - or worse from any hot parts through the human body to ground. If the threshold is reached, a GFCI cuts the power in a few miliseconds.

GFCIs are so reliable that TV stations in Central Europe would no more accept any script where the murder is done by throwing a hairdryer, shaver or a kitchen mixer (like in a 70ies Columbo episode ) into the bath tub.

If not yet done, an option would be to replace the breaker(s) for the bathroom with breakers that have an additional GFCI. A private building from the 70ies most likely didn't have GFCIs with low thresholds of maximal 30mA when it was built.

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