Electrical circuit puzzle

[David Nebenzahl]

OK, here's today's electric circuit puzzle:

Client wants a new ceiling fan installed. The old one works, they just want a different one up there. No problemo, right?

The new fan has a light, just like the old one (actually several hanging sockets). Unlike the old one, though, the new one has only 3 wires: hots for the fan and light, and a common neutral. Ugh. Not the way I would have designed it.

I hook it up, figuring that I can simply use one of the neutrals in the ceiling box and not the other one. Try it: light doesn't work at all, while the fan does. That's weird.

So I go up there and measure voltages between wires. Here's what I get (apologies to those who don't use monospaced fonts):

(1) blk X---- ----X blk (2) fan light (3) wht X---- ----X wht (4)

Measured voltages:

1 2 3 4 ------------------------------- 1 90* 120 240 ------------------------------- 2 90* 0 90 ------------------------------- 3 120 0 120 ------------------------------- 4 240 90 120 -------------------------------

• The 90 volt readings are from the dimmer switch all the way up.

Hmmm; something ain't right.

I ended up putting the old fan back up, as I couldn't get to any of the wiring above the ceiling and we decided to leave well enough alone.

I figured out what was wrong after I got back home. It's actually pretty simple.

For the solution, look here:

[makolber]

=A0 =A0 =A0 =A0 light

that is crazy to have 240 volts going to one box like that... I would rewire that circuit at the box so that it is all on one phase (polarity is actually the correct term) ...

I'm sure the code experts will chime in shortly..

Mark

[David Nebenzahl]

On 2/8/2009 7:06 PM [e-mail] spake thus:

That does raise an interesting question, though: if you had 2 circuits connected to opposite sides of the incoming service in the same box, then you'd have 240 volts between the hot wires. It would work fine, but is there any code restriction against this? I don't know of any, but I ain't no expert ...

[metspitzer]

here:

>

The drawing doesn't make much sense to me.

The biggest code violation is the dimmer is in the neutral. You can't switch the neutral.

Also the fan will run all the time.

Also I don't know how you get 90V from black to black. You should get

240V on one and 0V on the other.

He is also marking a hot leg black on one end and white on the other.

It is a puzzle alright. It is not an electrical circuit.

[David Nebenzahl]

On 2/8/2009 7:43 PM metspitzer spake thus:

Maybe it's not the best drawing in the world, but it shows what's there accurately.

The 90 volts is on account of the dimmer, which apparently doesn't allow the full 120 to get through.

The black and white wire markings are what exist. I know they're wrong; that's the whole point. (I didn't do this wiring; I only came to install a device in the box. Not my fault.)

[Steve Barker]

here:

>

it's not uncommon at all to have both legs in the same box.

s

[John Grabowski]

*From your diagram it seems that you have 240 volts split between the fan and the light and the neutral is switched using a dimmer. If that is the case when the dimmer is off you will have 240 flowing through the light and fan motor. This is not good. Personally I think that you have an obligation as an electrician to inform the customer of the problem and its hazards and tell them what it will cost to correct it. I would do this in writing and let the customer decide how he/she wants to proceed.

[metspitzer]

You can hook it up right. The only thing that is wrong is that the dimmer needs to go in the hot leg.

You have one circuit for the fan, which stays on all the time and one circuit for the light, which is controlled by the dimmer.

The drawing looked so good, I thought it was designed to be a puzzle. Sorry

I can't tell how it is physically wired, but putting the dimmer in the neutral seemed like it was done just to cause 90V in some of the readings.

[Davej]

Well, at least you avoided a cloud of smoke.

[David Nebenzahl]

On 2/9/2009 8:54 AM John Grabowski spake thus:

No. When the dimmer is off (and that means totally off, infinite resistance when the switch is flipped off), no current flows through the light at all. The only place where there is 240 volts is between the hot leg of the fan wires and the "neutral" (really the hot) of the light wires. The fan is powered from the other leg of 120 and is completely independent, electrically speaking, of the light; it just happens to be wired in the same box.

No, it's not.

I have told them. There really is no hazard with the wiring as it is, electrically speaking. There may be problems later with anyone trying to work on this circuit: I'm going to provide the client with a diagram in case they need work done later.

[JIMMIE]

My guess is that the switch is not in the neutral but it is in the white wire of a switch loop (legal). This could also acount for the 90 volt reading.

I think it is wired right. The OP just doesnt understand what he is seeing. Hes been confused by phantom readings from a high impedance volt meter.

Jimmie

[David Nebenzahl]

On 2/9/2009 12:08 PM JIMMIE spake thus:

Nope.

If it's wired correctly, then how do you account for the reading of 240 volts (definitely not a "phantom" reading, but a very real electrical potential) between a hot wire and a neutral wire? That should never happen in domestic wiring. Think about it.

This whole business of "phantom" readings due to high-impedance DMMs is highly overstated, in my opinion. Yes, it can happen, but not very often. Otherwise, one couldn't trust any readings taken with the damn things, really.