An online expert recently expounded on detailed design of a balun, this is an excerpt about wire sizing.
The wire gauge used limits the current handling capacity of the wire, run too thin a wire and it will heat up. Run much too thin of a wire for the power in use and it will fuse open. Current carrying capacity of wire is typically rated for either power transmission applications or chassis wiring applications. The latter, and higher, current capacity for a wire is relevant to designing a balun. How much current your 50 watt signal generates depends on the impedance its looking into. If you’re talking about a 50 ohm system, with a perfect match you’ll deliver one amp through your balun wires when driving 50 watts into it. Allowing for say a 4:1 SWR the worst case current(@12.5 ohms) is 2 amps. If you’re using this as a tuner balun, perhaps to drive a multi-band doublet then the impedance can vary widely so over sizing the wires is easy insurance. Here’s a table of wire current carrying capability: https://www.powerstream.com/Wire_Size.htm
For convenience, the relevant part of the table linked above is quoted for discussion.
So, the poster recommends wire with chassis wiring rating of 2A for 50W with reserve capacity for worst case VSWR=4. Continue reading Baluns – wire size insanity