Does anyone know what the arc flash / PPE requirements would be for a 120volt terminal box?

Not too many options for determining the PPE. IEEE 1584 is only for three phase. NFPA 70E Table 130.7(C)(15(a) lists Arc Flash PPE Category 1 for "Panelboards or other equipment rated 240 volts and below" provided the maximum short circuit current is less than 25 kA and the duration/device clearing time is 2 cycles or less.

No specific requirement for the simple reason of attempting to get a stable arc at voltages approach or under 200-300 VAC. In the original 208 VAC test work done for IEEE 1584, only a single test point was stable enough to use it for the empirical model.

Right now IEEE 1584 gives guidance that for equipment fed from a transformer rated 125 kVA or less (again, 3 phase), with a voltage of 208 VAC or less, "need not be considered" (under 1.2 cal/cm2). Mind you this is 3 phase. Single phase results will be lower but there isn't the same amount of data and no standard for single phase.

NESC Table 410-1 gives 4 cal/cm2 for either of the following conditions. Note that NESC has essentially a minimum of 4 cal/cm2 (and no face shield) requirement for ALL tasks so there are no tasks where PPE is not required. Note that the table starts at 50 VAC. No requirements below 50 V. NESC is based on actual testing in this table as opposed to theoretical or calculated values used elsewhere.
Case 1: "Panel boards—single phase (all) / three phase (100 A)"
Case 2: "Panel boards—three phase (>100 A)"
Taken together, it's 4 cal/cm2 across the board for all equipment rated 50 to 250 VAC.

Duke commissioned Kinetrics to study DC arc flash for battery purposes. A test at 130 VDC sustained an arc for 80 milliseconds before extinguishing with 20 kA of available fault current with a very narrow (1/4") arc gap. Mind you, this is DC, not AC...it doesn't go through a current zero 120 times per second. This at least in part explains why no 120 VAC results...if an arc at extreme conditions at 130 VDC will only sustain for 80 milliseconds, an AC RMS arc under the same conditions certainly won't sustain.