Just circling back to comment that my heat tape is installed, also some snow guards to prevent sliding ice and snow from stripping it off where it has been fastened. I appreciate all the positive comments and help.
I used 230V Raychem/nvent Wintergard wet, one segment up a downspout then across the metal roof. The other segment is in the gutter and goes down the other downspout.
I added a new electrical subpanel very close to the downspout location where both heat tapes emerge. Got very lucky as I already had a pipe through the slab that was perfectly located to add it. Added a dual gang bell box on the outside and a metal NEMA enclosure on the inside with a 230V contactor with 120v coil. Each heat tape is connected to a 20amp GFCI breaker.
I added a simple Dayton on/off temperature control, with an external temp sensor, and I also added an indicator light to let me know when it is active. I set the Dayton to activate about 29F and turn off at 32F. I may adjust that up/down as I get some experience with it.
So far all seems good. I've put an amp probe at each of the two breakers and max amperage draw so far is only 6amps at 20F outside temp.
The flaw is that it runs and uses electricity every time the temp gets low enough, whether ice and snow is present or not. Of course I can flip the breakers to deactivate it-- at least when I am around to do that. I might add more sophisticated snow sensors in the future but this was already a high dollar ($$$$) project.
