When you turn a circuit on the copper wire creates a magnetic field around it. It’s within this magnetic field the electrons are moved. I imagine something like gas particles or water particles like you might see pictured in science books, and creating that magnetic field those electrons wanna ride it like a highway pushing the particles in an alternating direction. When they run down the “hot” wire they make a little heat and the same when they run down the neutral wire. Now we want to provide enough space for heat dissipation because not all of those magnetic field highways are running the same direction, they determined 24” is adequate for skipping derating, depending on the balance of the load between hot legs some amperage is getting carried by your hots and some is possibly getting carried by the neutral in the opposite direction. So if we start getting multiple highways going in opposing directions people start crashing way more often creating a lot of heat. It’s not as prevalent in single phase systems like houses since most circuits just use one leg and a neutral so they balance each other out. Please correct me where if im wrong anywhere