Tap Rules

[Philip Barr]

240.24(a) requires OCP to be readily accessable, unless considered supplimentry OCP, ie a fused disconnect next to an above ceiling AHU. More reasons not to put panelboards in attics.

 

[Philip Barr]

If you go with the tap idea, although I've never done this and most electricians may look at you crosseyed for doing it, code compliant though it may be.
From what I know for branch circuits with taps you can't exceed 50 amps for your circuit wires. But since you load for 15amp devices should be 12 amps or less your circuit shouldn't exceed 48 amps.
Table 210.24, and article 210.19(4) seem to point to a 50 amp circuit with a #6awg run with #12 taps not exceeding 18" long.
May run that by your AHJ first but sounds like a sound installation to me.
On putting panelboards in attics, unless it's a dwelling unit you will be hard pressed to come up with 6-1/2' of headroom 110.26(e). And if you had that you can add a light and switch to your installation 110.26(d). And building codes would also probably require a work platform as well. Rule of thumb IMO...don't put panelboards in attics.

After further thought, I recalled that if your installing receptacles for these AC units, you will need 20 amp ocp max for the receptacle devices. I suppose a pigtail type fuseholder could work in each receptacle box, you'll probably need a deep 4-11/16 box though.
This is what I call "creative" wiring, alot of variables to think of to make sure it's right. Good luck with it, to bad there's no accessable spot for a subpanel must be a tiny building, it seems you'd find 30" of clear space somewhere.