The main thrust of the NEC is 'safety'. It must be IMPOSSIBLE for the LV wiring
to make contact with the HV wiring regard less of fault condition (ie. overload and wires
melting). This is usually handled by a having grounded barrier
between the two halves of an electrical box, encompassing the SSR in this case.
In other words, you bring the HV in and and out on one side of the box, and the LV wiring
on the other, and have a fireproof physical separator that makes it impossible for
the two wire classes to come in contact with each other.

In the case of a standard wall box, you will likely want to mount the SSR to the
metal wall with screws (to get heat sinking), so the LV control will have to enter the
box immediately next to the terminals so that there is no possible contact with the
HV wires also in the box.  The SSR must be INSIDE the box so that any possible flame
it generates due to failure is contained by the box. It may be easier to turn the
switch into a double gang box and put the SSR into the empty compartment with with the
HV leads coming through a hole drilled in the separator plate.

IANA electrician but I've had to deal with this on equipment I've built to pass
CSA regulations & inspection.

Robert

Richard J. Pytelewski wrote:

> Hello All!:
> 
> Need a little help here.  I am wanting to control a SSR (+12 vdc) placed in
> a standard ac wall outlet box to parallel a standard 120v ac SPST ac light
> switch. The application is in the US so I am inquiring about what is
> accepted practice in the US (i.e., the National Electric Code (NEC)).  The
> NEC is really hard to deal with and I find very little in the way of clarity
> in its' dealing with mixed (low and high voltage) wiring.
> 
> Any help here would be greatly appreciated!  Thanks!
> 
> Rich
> 
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