New install: What's causing a tripping arc fault breaker?
This behavior is because arc-fault breakers have an interesting quirk: they also provide a small degree of ground fault protection as well! (They are not GFCI's because their ground-fault trip threshold is set to the 30mA used for equipment protection (GFPE) devices as opposed to the 5mA ground-fault (differential) trip of a Class A personnel protection GFCI.)

You will need to contact your electrician and tell them to get their neutrals straight.  Tell him he violated NEC sections 200.4:


  Multiple Circuits. Where more than one neutral conductor associated with 
  different circuits is in an enclosure, grounded circuit conductors of each 
  circuit shall be identified or grouped to correspond with the ungrounded 
  circuit conductor(s) by wire markers, cable ties, or similar means in at least 
  one location within the enclosure.


300.3(B):


  Conductors of the Same Circuit. All conductors of the same circuit and, 
  where used, the grounded conductor and all equipment grounding conductors and 
  bonding conductors shall be contained within the same raceway, auxiliary 
  gutter, cable tray, cablebus assembly, trench, cable, or cord, unless 
  otherwise permitted in accordance with 300.3(B)(1) through (B)(4).


and 310.10(H)(1):


  General. Aluminum, copper-clad aluminum, or copper conductors, for each 
  phase, polarity, neutral, or grounded circuit shall be permitted to be 
  connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG 
  and larger where installed in accordance with 310.10(H)(2) through (H)(6).