I'm concerned about the safety aspects of the comments so far on this thread.
Even though it's only 120 volts I guess it still bites and means your PIC is
live.
The real way to do this is with a current transformer which provides safety
isolation and is easily made from a miniature audio transformer salvaged from
and old fashioned transistor radio (Germanium transistors usually). These are
nicer to work with as they are usually only wax impregnated and come apart
easily.

Pass a piece of insulated wire through the core and put this in series with the
load. Experiment by connecting the existing windings directly to the + and -
inputs of an OP-amp. bias the + input with equal resistors say 10K ohms across
the supply to create a mid rail and decouple to 0v with a 10 microfarad cap. Fit
a low value resistor between the op and - input say 100 ohms to start with.
The output should give a voltage proportional to instantaneous current in the
load. Your PIC can then find the peak or work out the RMS values as required.


Current transformers are easy if you stick to the rules.

The secondary needs to work into a virtual earth (hence the op amp)

The core must not saturate - unlikely with VE as there is little power transfer

Primary current * no of primary turns = Secondary current* no of secondary turns

NB the voltage from the op amp is simply Secondary current * feedback resistor.

Safe Experimenting

Bob Minchin
Romsey UK

bob.minchin.roke.co.uk