Why does the voltage drop when my office circuit (115V) is under load?
You have a lost neutral. Stop using the circuit until fixed.

This is a classic "lost neutral" we get all the time either on main panels or multi-wire branch circuits (MWBC, which is what this is).  The whole thing is one large circuit since they share neutral. MWBCs are complicated and have special rules that need to be honored if you don't want things like this happening.  Given the casual nature of speech and this question blowing over from ee.se, I suspect you may be out of your depth, no offense.

Particularly if "melting wires together" is a thing that is happening to you, it's really time to put the tools down and call a pro, or else "get religion" about doing good electrical work to Code and then read a book on electrical cover to cover and not skip through it as people with "smartphone attention spans" are prone to.  Google cannot substitute for a book because it only answers questions,and your field knowledge is not sufficient to know which questions to ask.

This is a 3-phase MWBC, implying New York City residence. My advice on MWBCs here applies.  Particularly the part about pigtailing neutrals. You are 3-phase so where it says 2, read 3. 

The "symptom" for this problem is that if you plug a bulky load into one leg of the MWBC, say a resistive heater, and measure voltage at other legs of the circuit, it will be other than the expected 120V.  Another giveaway is a high difference in voltage between neutral and ground, on a 100' run, 1-2 volts is to be expected but no more than that. 

It sounds like you did not fix (or find) the problem with the neutral.  If you insist on continuing to work on it, and you follow my advice, you'll be revisiting a lot of it, but also look in the service panel at the neutral termination there. The overcurrent may have fried it.