Hello. My first visit to this very informative site. I have a question about wiring in a pole building. I'm using 12-2 Romex simpull on a 20 amp outlet circuit along the walls. My question is, will heat in the summer months on the metal siding be hot enough to damage the wire if it is stapled on the top edge of 2x4 purlins inside the finished wall space? It won't touch the metal siding but will be within a half inch or so from it. I don't want to drill holes through the poles and this looks like a good place to run wiring. If this is OK to do for an ag building, can I run several wires in this space to various circuits? This part of the building will be partitioned off for a machine shop. Hope this makes sense. Thanks.

Probably won't be a problem with the romex degrading from temperature, but the higher temp will heat the wire and drop the current carrying capacity by raising the resistance. Make sure it's fused at no more than 20 amps; if you operate a 2 KVA or higher device for extended periods in the summer, expect the breaker to trip now and then.

At temps up to 140 deg F there is no need to derate the cable.
As Rob says, keep to the standard 20A breaker and you'll be fine.

I've shot the hood of my truck and a new blacktop driveway with a digital thermometer in the middle of a 90+ deg day in summer and it was under 140 deg.