I'm not familiar with the term 'spinning load', but it appears here to be the minimum power required, one that must be sustained come what may, to maintain power to underground operations (at least light and air supply, I imagine).
'Spinning' refers to the generators that are running at any given time. By extension it might even extend to PV, but let us content ourselves with conventional rotary devices. The conventional term 'spinning reserve' (réserve tournante) is the amount of power that can be generated immediately since the generating equipment is already tied in to the system and spinning, albeit slowly.
It is logical that 'spinning load' will be the amount of power actually being generated at any time. This would necessarily be high in times of high demand, and lower at other times. But if the purpose of the 'spinning load' in your case is to 'always have power [underground] if load shedding is required', then it has to be the baseline load, not one that will vary.

So I'm thinking puissance tournante, though this is not, I don't think, any more conventional a term than is 'spinning load'.