I want to add a small bar sink in my basement. I have 1 1/2" line transitioning into a 2" before it goes into the floor coming from the kitchen sink above, it is 48" from the location I want put the sink. Do I need to run a vent up or am I close enough not to add a vent
... in my state (VA), the allowable distance between a 1 1/2" drain trap (standard size for a kitchen sink) and its vent is 6 feet. If your trap is farther from the vent than that, then you should consider adding a new vent or an air admittance valve.
If your local codes allow them, the 2" pipe may serve as a wet vent. According to the IPC an
X size stack can handle
Y number of DFU.
1.5" : 1 DFU
2" : 4 DFU
2.5" : 6 DFU
3" : 12 DFU
DFU or Drain Fixture Units, where the UPC allows for bar sinks to count as
1 and kitchen sinks to count as
So long as these are the only two fixtures discharging into the 2" pipe, you're within nominal code not to add a vent but you still need to check your local codes.