Adding the Sense electricity monitor to my breaker box
Attach it to the dryer's breaker.  This is fine because Sense is entirely contained inside a steel panel, so even if it entered the worst case scenario of the device drawing more current than its wires can handle, yet less than 30A, the panel enclosure would contain the damage.   

Look at your dryer's breaker and see whether its labeling or instructions authorize use of two wires of dissimilar sizes. If not, you will need to get some #10 wire and pigtail it.  When using wirenuts on #10 wire, tighten it with great fervor especially if it is solid wire.   This would also be a great place to use mini-lugs, i.e. alumiconns.