It applies to what the title says [424.90], but if you don't know what a radiant heating panel is that doesn't help much. And the definitions provided in 424.91 are vague. So let's explain that. The heater is radiant (heats by emitting heat energy from an element to warm people and objects), as opposed to convection heat (heats the air). And it's a panel, as opposed to being a floor system. It could be a wall panel or a ceiling panel, for example. And it's ready for connection to a branch circuit [424.91].
This is not a Code question, but understanding this can help with the installation. These kinds of heaters are typically used to supplement other forms of heat. For example, a house may be heated by a central air system but the master bathroom might have a wall panel heater. Note also that each unit must be identified as suitable for the installation [424.92(B)].
They must be at least two inches away [424.93(A)(3)].
- Yes, but only if you follow the requirements of 424.98. For example, you can't install them across expansion joints unless you make provision for expansions and contractions [424.98(B)].
- It must have GFCI protection 424.99(B)(5). This "could" be acheived with a GFCI receptacle ahead of that portion of the circuit supplying the heater(s). But this is a poor design for multiple reasons. A better way to do it is to use a GFCI breaker for that circuit. Though not NEC-required, using a combo AFCI/GFCI breaker would be the best way to go. This way, you protect individuals from electric shock and also protect the circuit from arc fault.