At 04:49 PM 11/20/2006, Tom Rauch wrote:
> > Note the last suggestion--install an electronic
> > low-voltage lighting
> > system, with an ELV dimmer. Then comes the big tip: a
> > separate neutral.
> > Lots of times these days electricians will wire a building
> > with a common
> > neutral--combining the white wires and running a lot of
> > circuits back to
> > the panel on a single white wire. This is a bad thing to
> > do--because you
> > get all kinds of nasty noise on the line, and the noise
> > appears on all of
> > the circuits. But it happens (it saves the contractor
> > wire).
>
>I'd be careful with that separate neutral suggestion.
>
>1.) Wiring has to be to code, or it should be.
The code allows almost any connection for neutral, as long as there's
only one of them. You can home run them all or daisy chain. Or, say
you have two hots and a neutral (often the hots will be on opposite
phases, but not necessarily.) You can share the neutral, but it has
to be sized appropriately. If the two hots are on the same phase,
the neutral would have to have twice the ampacity.
What the code doesn't like is multiple neutral paths for the same
circuit. (I think it's mostly because it gets unpredictable where the
current is actually going to flow if there's a fault or an open, and
it makes it hard to debug failures.)
>2.) Even if you break the neutral, you still have the hot
>lead and ground lead. Everything ties together back at the
>box anyway, and a wire will radiate no matter what you do if
>you have an RFI generator connected to it.
I think the idea is that if the supply and return from the dimmer to
the lamp are essentially a pair, then the magnetic fields cancel, and
it's sort of self shielding (a balanced pair), since they are right
next to each other, and the "loop area" is small.
A shared neutral has more potential for having the supply and return
form a loop with some area, and making a better radiator.
The dimmer creates mostly conducted mode differential noise. What
you'd want to avoid is having that turned into common mode or
radiated noise. The whole thing is complicated because the typical
electrican's wiring practices are based upon DC/low frequency
circuits, where the benefit of twisted pairs, home run wiring, etc.
are marginal, if not actually a detriment. They're more focussed on
ease of installation, material cost, and low IR loss than on good
cancellation of differential mode magnetic fields.
Only recently (in the 100+ year history of the code) did the code
start requiring the neutral and ground to be run through the box
holding the light switch. A few decades ago, a common practice was
to run the line to the fixture, and then just run 2 conductors (hot
and switched hot) down to the switch.
Jim, W6RMK
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