How did I mess up this wiring?

Question

I had to take down a ceiling light to remove an old wire while renovating.

I eventually managed to wire everything up correctly, but I'm still puzzled by how I miswired it the first few times. I know I got the pairs of wires mixed up, but I still can't picture any circuit that would do what is described below.

3 pairs of wire come into the box - one set of black and white is the power coming in, one pair goes to the switch for the light, the other continues to the outlet on the wall. The light had a CFL bulb in it, and the outlet had an LED night light plugged in.

Once I restored power:

• With the switch up (can't say for certain if this was on or off), the ceiling light was on, but the night light didn't.
• With the switch down, the ceiling light was off, and the night light was on dimly
• When flicking the switch, the nightlight would get brighter or dimmer for a second
• With the bulb in the ceiling removed, the nightlight wouldn't work at all.

At no point did the circuit breaker trip. Also, the entire circuit has no ground connection.

What kind of messed up wiring would make it do that? I've since figured out the correct wiring, but it still bugs me.

Edit: The closest I can come up with is that maybe I wired the light and outlet in series (incoming black to outlet black, outlet white to light black, light black to incoming white) with the switch between the black and white of the outlet. I think that might do it...

Answer 1

Don't worry about why. If you try to only fix the problem, you may miss something. Worry about getting it correct!

Black == HOT
White == Neutral
bare == ground

(in North America -- Europe is different!)

Open it up again and make sure it is connected as follows:

• white from panel to white from outlet to white from light
• black from panel to black from outlet to black from switch
• white from switch to black from light
• all grounds together (and to box)

The wire going to the switch has no neutral in this case. The black wire is "always hot" and the white wire is "switched hot". I like to wrap black electrical tape around the end of the white insulation (without covering the copper wire) to indicate that, despite being white, it is not actually "neutral".

Speculation on how it was mis-wired: (copied and corrected from my own comment later on) If removing the ceiling bulb made the outlet light not work at all, then you were getting some sort of current flow through the bulb. So yes, you must have had it in series. CFL has circuitry so could perhaps pass more current while starting up (hence the increase in brightness of the outlet light for a second) after turning on the switch. As for where the power was going when the switch was off... It could be that the CFL was "absorbing" and "returning" some small amount of current (a capacitive load), enough to lightly switch an LED light; they don't need much.

Given the capacitors and inductors involved in even a simple circuit for a CFL, it's not unimaginable that this is possible. I don't have an LED night-light to test this but you could do this yourself.

If properly wired, the smaller prong of a plug is the hot wire. Bend the larger prong of the night-light or otherwise set it up so that only the "hot" prong is connected. Then touch the exposed prong to the side and/or tip of a CFL and see if the night-light lights.

As a more personal example... Think of standing on a rubber mat (or wearing rubber-soled shoes) and touching the hot wire with your finger. Don't do it -- just think it! You'd feel a "buzz" even though you're not grounded or touching a "neutral" wire. Most of us can relate to this from some experience in our past. Your body has a capacitance and will absorb and return electrons as the voltage in the hot wire oscillates at 60Hz.

Answer 2

I can think of a few things that would cause this. The first is just a bad connection with either a hot or neutral - maybe a wire wasn't all the way in a wire-nut or had lots of corrosion on it. Also possible (if you trimmed any wires the second time) that a wire had a big nick in it.

The second possibility is an open neutral where one of the neutrals was not at all connected. This can cause some circuits to have a higher voltage and others a lower voltage - hence the dim light and brighter second light.

As someone pointed out in a comment it is also possible that you wired it in series instead of parallel though I don't think this would cause a light to get brighter than normal.

Without seeing the issue occurring its really hard to tell for sure though.