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I am hoping to be able to use a smart plug with my window air conditioner. I currently use the TP Link HS 100 plugs around my home, but I am unsure if they could handle the amount of power an air conditioner uses. Any suggestions?

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If your AC unit is a portable one, yes, it should work. By portable, I mean the unit is shipped with a 13A plug attached rather than designed to be wired in to a fused spur. It also depends on the size of the unit. If the compressor is rated at 1 kW or less, you should be fine. Equally, if it is a slightly larger unit, but with modern electronic 'soft start', there shouldn't be a problem.

The issue is that compressors often provide a significant surge load when they turn on, and this might overload the switch embedded in the plug. The risk is likely to be that the plug fails rather than a fire hazard (but no guarantees).

You must be sure to use a switch type plug, rather than one which is designed as a dimmer if you're connecting it to an electric motor, but it seems you're fine in this respect.

A more reliable control mechanism would be to interface to the control side of the AC unit. Generally this requires the manufacturer to have designed a home-automation interface for their product, and you are probably out of luck there...

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According to this TPLink page the HS-100 units are rated at 13A maximum.

You can switch a load of essentially any size by using the HS-100 to switch a secondary relay (solid state or electromechanical.)
Electromechanical relays able to switch many kilowatts are available at prices that are usually well below the cost of the automation system switches. You may need a small amount of extra circuitry depending on the required coil voltage. If you use a relay with a mains-voltage AC coil you need only the relay plus an enclosure and whatever is required for connections. Note that solid state switches (SSRs) are both far more costly than 'old fashioned' relays, and inferior in energy losses. SSR2 typically drop 2 to 3 VAC across the operating relay. At 50A that would dissipate Power = V x I = 2V x 50A = 100 Watts (or 150W at 3V). This level of thermal energy requires heat sinking. A mechanical relay such as the 90A Panasonic one listed below has a contact resistance of under 2 milliOhm worst case at end of lifetime for a energy loss at 50A of I^2 x R = 5 Watts (or 16 Watts at 90A). At 50A switched ~= 5.5 kW load the 5 Watts (or less) dissipation can be accommodated using standard mounting methods. .

This Digikey product page lists literally dozens of relays rated at 30A, <- 265 VAC for under $US10 in unit quantity.

The Panasonic HE series relays in single quantities cost about
$22 for the 48A, <=277VAC version and
$33 for the 90A, <=277 VAC version

This relay series is not available with 110VAC coil and was used as an example. The HE series has coils rated at 6 9 12 & 24V DC at 2 Watts.
As Harper points out, this relay series is NOT rated for inductive load. However many are - and as a general guideline, Panasonic make about as good quality components as you can buy. In some applications dedicated contactors such as the "Functional Devices" and "Packard" ones that Harper suggested may be good - but I'd want a far better specification sheet than in the links and rather more certainty of the quality regardless of volume sold. Such relays may be excellent - but know why you think any brand is and be sure that both the specifications AND the real world performance do in fact meet the application. Ideally buy through a known reputable supplier who guarantees "provenance". ebay and "even" Alibaba can be a source some very well priced high quality products. But also some extremely shonky ones indeed. If you are able to assess quality and legitimacy of certifications yourself they can be a good idea. Otherwise, maybe not.

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    Maybe worth noting that in the UK, an instalation like this would likely come under the scope of technically requiring a 'safety' certificate. – Sean Houlihane Jan 25 '17 at 10:24
  • Right idea. Wrong parts bin. Electronics != Code electrical. Don't reach into the electronics/hobby parts bin when dealing with mains power, they're not right, not even close. Not quite as tight as UK but we DO have rules. Use Code electrical parts bins, e.g. 30A air conditioning relays made for motor loads that cost about $13, or screw-into-the-knockout type relays. amzn.to/2jyy5Iw functionaldevices.com/pdf/datasheets/RIB2401SB.pdf – Harper - Reinstate Monica Jan 28 '17 at 1:14
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    @Harper No. The relays I gave as examples were, as I said, examples, but are entirely suited for and certified for any use within specifications. They are only "hobby" relays inasmuch as a smart hobbyist may pay the slight extra amount for one of the world's top component brands rather than buying relays with no certain provenance. . Data sheet here - They are rated for 48 A and 90A, resistive load at 277VAC, whereas the the part you suggested is 20A rated, also at 277 VAC resistive. – Russell McMahon Jan 28 '17 at 13:57

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