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[dhouston]

While incandescent bulbs are almost 100% resistive, when turning them on, the inrush current might be a problem when loading a switch or module to the maximum wattage. So I recommend against loading to maximum wattage and would probably reduce it even more for CFL or LED loads with unknown power supplies. I would be cautious with all non-incandescent loads when loading any circuit beyond 50% when the total current is unknown.

Given that you have a Kill-A-Watt, use it to measure things like PF when current draw is unknown. Low power factors mean there are inductive or capacitive currents that, while out of phase, are still real and might require derating beyond what might seem to be the case.
 
As for warnings on new fangled light cartons, I have 2 dimmable CFLs (retired or never used). One says it works with 'all' dimmers while the other says 'most' dimmers. All of my dimmable LEDs are in use and I did not save the boxes.

It's been years since I've needed a new module but I believe they usually had warnings like "Not for inductive loads" or had lower ratings for inductive loads. Both the LM465 & LM14A say for 'indoor incandescent' only.

[JeffVolp]

Ok, so it comes down to calculating the current from the other available information and making incorrect assumptions.  If the device lists current, then that will be an acceptable peak value to use when sizing things.

Correct

If the maximum number of incandescent bulbs are installed on a switch with no margin and then later changed to CFL or some other type of bulb, does that mean you need to check to make sure it won't overload the circuit?

Since CFL and LED bulbs consume so much less energy than the incandescent bulbs they replace, even with a relatively low power factor the current drain should still be less.  You could always verify it with the Kill-A-Watt.

Jeff

[bkenobi]

A kill-a-watt is great for plug in loads, but not so good for switched circuits.  That said, I have a test fixture (diy lamp basically) that I've used for various purposes that allows bulbs to be plugged into the kill-a-watt individually.  There's no reason one bulb couldn't be used to determine a baseline and then multiply by number of bulbs.  This doesn't help with larger non-plugin devices like furnaces or water heaters.  I think the problem for larger things is the controls though, so it would probably be better to isolate the controls and add an inline style filter anyway.  I can't imagine needing more than 20A for the controller.  But, those are much more complex and I assume wouldn't be fully resistive.

[dhouston]

A kill-a-watt is great for plug in loads, but not so good for switched circuits.  That said, I have a test fixture (diy lamp basically) that I've used for various purposes that allows bulbs to be plugged into the kill-a-watt individually.  There's no reason one bulb couldn't be used to determine a baseline and then multiply by number of bulbs.  

I use this.
http://www.amazon.com/GE-54173-Adapter-Converts-Polarized/dp/B002DN2OEM?ie=UTF8&psc=1&redirect=true&ref_=od_aui_detailpages00
As long as you use the Kill-A-Watt ammeter you should be OK. For example, I measured a 15W incandescent (0.12A) and a 9W/40W equivalent CFL (0.14A). Going by claimed wattage, you would expect the 9W CFL to draw less current (~0.075A) but its power supply is non-linear and the actual current is nearly twice what's expected.

But, it still is much less than a 40W incandescent (~0.33A) so that supports Jeff's point .

This doesn't help with larger non-plugin devices like furnaces or water heaters.  I think the problem for larger things is the controls though, so it would probably be better to isolate the controls and add an inline style filter anyway.  I can't imagine needing more than 20A for the controller.  But, those are much more complex and I assume wouldn't be fully resistive.

I'll leave HVAC and water heater controls to others. It has been 20+ years since my spinal cord allowed 'hands-on' with things like those.

[bkenobi]

I have a couple of those but never thought about using it with the kill-a-watt.  I also have the other direction that I've used with the XTBM and a line tracer I've used in the past (for mapping my home circuits).  Nice cheap tools if you remember they are in your toolbox.   

[dhouston]

Nice cheap tools if you remember they are in your toolbox.

Speaking of toolboxes, I think a $20 Kill-A-Watt belongs in most X10 and HA enthusiasts toolboxes.

This thread caused me to search comp.home.automation for my original review. I did not find it but did find a post about my review being  published in Home Energy Magazine (published by Lawrence Berkeley National
Laboratory) in mid-2002 (which I had forgotten). After 15+ years it's nice to recall that and to see how successful the Kill-A-Watt has since become. It was also nice to see (some of) the names of those who frequented comp.home.automation in those days.

The earliest post I could find where I mentioned the Kill-A-Watt was in late 2001 and, from it, surmise I purchased it in mid-2001. I cannot recall the name but someone whose opinion I respected mentioned coming across it but being reluctant to risk $65 on it. I took the risk and was impressed enough to become a  technology evangelist for it.