Does the ac condensate line have to be rigid pipe where it attaches to the tail pipe of a vanity. Need code reference
[FONT=Times-Bold][size=2]M1411.3.2 Drain pipe materials and sizes. [/size][/FONT][FONT=Times-Roman][size=2]Components of [/size][/FONT][size=2][FONT=Times-Roman]the condensate disposal system shall be cast iron, galvanized steel, copper, polybutylene, polyethylene, ABS, CPVC or PVC pipe or tubing. All components shall be selected for the pressure and temperature rating of the installation. Condensate waste and drain line size shall be not less than [/size][/FONT][FONT=Times-Roman][size=1]3[/size][/FONT][FONT=Times-Roman][size=2]/[/size][/FONT][FONT=Times-Roman][size=1]4[/size][/FONT][FONT=Times-Roman][size=2]-inch (91mm) [/size][/FONT][size=2][FONT=Times-Roman]internal diameter and shall not decrease in size from the drain pan connection to the place of condensate disposal.Where the drain pipes from more than one unit are manifolded together for condensate drainage, the pipe or tubing shall be sized in accordance with an approved method. All horizontal sections of drain piping shall be installed in uniform alignment at a uniform slope.[/size][/FONT]
I would think you need to look at the bigger picture as to where the condensate drain is connected.
Code requires that the condensate be connected to the building drain though an air gap connection and NOT physical to the drain. If the drain system backs up the there is potential that it can back up into the evap coil
Not true. Model codes like the IRC only require the condensate to be conveyed to an “approved place of disposal”
(e.g. IRC M1411.3 Condensate Disposal … http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_14_sec011_par002.htm)
Acceptable places for disposal include roof gutters, discharge at grade (preferably not draining across a walking surface), and fixture waste connectors before the trap (e.g. sink tailpiece before the trap). For the last option the AC unit just needs to be above the sink (attic installations) or connected through a condensate pump (basement installations) to physically connect the condensate lines to the waste drain.
Family mechanical business has been doing it that way for quite some time, including new construction with some very strict/picky building inspectors.
JMO & 2-Nickels … 
Use of rubber type hose to make the connection from the PVC condensation drain to the sanitary side of the sink tailpiece is common practice, at least in this part of TX. I prefer solid PVC connections but only call out hose type connections if they are not secure or the hose is deformed due to too sharp a bend.
I wouldn’t worry too much about the sink backing up into the evaporator coil up in the attic 
Use of rubber type hose to make the connection from the PVC drain to the sanitary side of the sink tailpiece is common practice, at least in this part of TX. I prefer rigid PVC connections (which makes servicing the drain more difficult) but only call out hose type connections if they are not securely clamped at both ends or the hose is deformed due to too sharp a bend.
I wouldn’t worry too much about the sink backing up into the evaporator coil up in the attic
Flexible piping used to connect the condensate drain to a sink tailpiece should be polyethylene or polyvinylchloride tubing (usually milky white or clear) … see Jeff’s post.
Using flexible poly tubing for connecting to a sink tailpiece is the preferred method for many installers, because it’s only a matter of time before gunk, hair, etc. clog that tailpiece wye fitting and it needs to be taken apart to properly clean. It looks similar to a dishwasher drain hose connection.
JMO & 2-Nickels … 
But can the rubber hose be reduced in size ? I think no.
And the answer behind door # 3 is no:D