How to test for a neutral line in a switch box?

Oh, I forgot. The voltage drop between line and load is only the conductor. Think of where you measurement points are. There is not a "load", but there is resistance. Most meters will not show anything. The drop noticed is only the difference that one might observe between voltage at the main versus that which would be observed say between the circuit breaker and the neutral in the panel. Or, if one found a way to intercept the line before the load while in the outlet box. A probe into a wire nut, perchance.

And you forgot something else too. If there is no load connected, there is NO voltage drop. E=IxR. The voltage drop is E. The resistance of the conductor is R. If the current is 0, it doesn't matter WHAT the resistance is, there can be no voltage drop, so whether the voltage is flowing directly to the switch, or through the load to the switch, the voltmeter will read exactly the same.

If the house is wired with "romex" you cannot "add" a neutral - it needs to be totally rewired to get a neutral.

Basically there are two ways of wiring a switched load, like a light. You can switch the power TO the load - running the cable (line and neutral) from the panel to the switch, and from the switch to the load, with the neutral wire-nutted together, or you can switch the power "from" the load - running the cable from the panel to the load, and the load to the switch, wire-nutting the neutral together at the light - and running a cable down to the switch where both black and white wire can be "live".

My assertion is with a load connected. Maybe I did not explain it correctly. The allowable drop for branch circuits is 2% (3% in some locations).

A common practice for ferreting out issues with motor starters is to measure the voltage across the input and output of each pole. Though, this is primarily with poly-phase systems. Reading a voltage indicates a termination/contact problem.

Same thing a mechanic does when looking for starting/charging/other electrical problems on a car. Check voltage drop from - battery post to engine block with starter cranking to check integrety of the ground circuit - then across every connection if there is a problem until the high resistance is found - and the same on the positive (load) side. Same thing in the charging system, lights, heater, etc.

But what if your sitting there, in the tub or elsewhere, and you don't move for a while. Then you're sitting in the dark!!

More specifically, there should be *ONE CABLE* that has a black, white, and bare (there will be more than one). In that one cable, one that's not connected to a switch will likely be the neutral. Those that are connected to mechanical switches should be marked clearly as something other than neutrals (colored tape or marker) but electricians are often sloppy.

This only works if you haven't separated the wires yet. If you have, there may indeed be voltage on the neutral. WATCH OUT!

BTW, what are you doing poking around in a box with the power on? ;-)

Not necessarily. There could be all sorts of things going on, like no load connected.

Not necessarily. In the above case, you'll often see a phantom voltage between load and neutral (or ground). It's better to measure between hot and ground, then hot an neutral. None of this is a very good way to identify the neutral, though.

A mechanical timer is just a switch. It doesn't need a neutral. It doesn't solve all of the problems that a programmable switch does but some.

The problem is you can't just say the drop between line and load. What does that even mean? You have to define where the meter is actually connected. And in the context presented here, ie investigating wires at a switch box, the way I would think it means is between the line and the wire going to the load, with the wires apart.

Otherwise, what is between line and load? Across the switch, with the switch closed? In that case, you should see near zero. Open the switch and you'll see near 120V.

Hi, If it is simple switch box with wires only for that it may be simple to sort out which is which but crowded box with other wires pass through or doing some thing else it may take a while to figure that all out. Our one bathroom has 3 ganged boxes full of wires doing several things, Jacuzzi pump timer, IR heat lamp/vent fan, light fixtures..

Sure, you can often visually determine which wire is which but

So you wave your hand and the light comes back on??? Generally "occupancy switches" can be adjusted for different turn-off times up to half an hour.

In which case there WILL be a neutral in the box which could be pressed into service, although if that neutral is associated with a circuit on a different breaker or fuse it is technically illegal to use.

And if you have GFCIs or AFCIs will cause them to trip, if the current through the devices is greater than 5 mA (GFCI) or 30 mA (AFCI)

nate

I do believe I accounted for this. I was referencing someone else's point on the subject. I suppose I could have gone into extreme detail, but that really was not the point of the discussion.

In a pure trouble shooting scenario, all points of possible readings would need be taken into account. One might be able to find a faulty termination in a j-box in the attic. Tracing circuits is not fun, but must be done at times.