You can also attenuate on/off controllers if your tape is too high output for the application, or you already have racks with tubs sitting on bare tape by simply putting a 600w dimmer DOWNSTREAM of your on/off controller.
Simply adjust it to 100%, and watch your thermostats click on and off.
I use a kill-a-watt meter set to watts/VA, start at 100% at say 120 watts for a section of tape, then put a mark on the faceplate of the dimmer switch every 10 watts, and label them.
You just calibrated your dimmer for wattage.
Now unhook your kill-a-watt, then turn it down 10 watts, and time how often it clicks. Plug a low wattage trouble light into your strip and check your watch. Turn it down 10 watts at a time until you get your thermostats cycling once every 10 minutes to a half hour, depending on the temp.

Also, I check the 'draw' of my tape every 6 months. If I have any tape that draws more than the rest, I swap it out for my bottom tape, then keep an eye on it.

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