What is the range of TCAS?
Short answer

If a single range should be stated for ACAS II, it would be the reliable range of 14 NM, even if targets can displayed by current TCAS II up to 30/40 NM.

Reliable means here: a track can be established with a probability of at least 90 per cent for aircraft within the surveillance range. The reliable range of 14 NM is achievable with a ground plane antenna, a power of 250 W and a Mode S sensitivity of –74 dBm.

The mean number of aircraft in a spherical volume is proportional to the cubed radius. Each aircraft tracked by a ACAS system must be interrogated, and its position/altitude data stored. This creates stress both on the ACAS equipment and on the radio spectrum used for ATC (SSR and ADS-B in addition of ACAS). The current balance between stress and benefit is a range of 14NM which can be reduced to 4.5 NM if the number of aircraft or the number of ground interrogators prevent managing the nominal range. The spectrum stress should be relieved with the forthcoming ACAS-X.

In addition, while the nominal monitoring range is 14 NM and the guaranteed range is 4.5 NM, RA/TA are issued based on estimated time before separation failure (more accurately before the closest point of approach):


From Eurocontrol ACAS guide

(In the answer, ACAS is related the principle, TCAS is the technical implementation of the principle.)



Diversity of ranges

There are different range values that can be considered:


Overall pulse detection range is 30 NM.
Reliable surveillance range is 14 NM.
Recommended RA range is 12 NM.
Guaranteed range is 4.5 NM.


The range must be limited because it impacts the number of transponder addresses to be stored for a given transponder density as well as the number of interrogations within a volume, hence radio spectrum use and reliability.

ACAS II recommendations by ICAO

ACAS II ICAO recommendations are found in Doc 9863, Airborne Collision Avoidance System (ACAS) Manual:


Interrogation pulse detection range is 14 NM for Mode A/C and 30 NM for Mode S.
The equipment should have the capacity for surveillance of any mix of Mode A/C or Mode S targets  up  to  at  least  30  aircraft.
Surveillance must be reliable within 14 NM, with an aircraft density of 0.3 aircraft per square NM within 5 NM (and up to 142 aircraft within 30 NM).
The equipment should assess as possible collision threats only those targets within a maximum range of 12 NM. No target outside this range should be eligible to generate an RA.
As density increases, reliability decreases.
If the overall target count ever exceeds the surveillance capacity at any range up to 14 NM, the long-range targets may be dropped.
The system reliability is guaranteed within the minimum range of 4.5 NM. This radius is deemed adequate for providing protection for up to 500 kt encounter. 


TCAS version 7.1

TCAS II is the sole implementation of ACAS II which fully comply with ACAS II. The reference (paying) documents are: DO-185B (RTCA) and ED-143 (EUROCAE).

From the EASA document ACAS Guide


  TCAS II can simultaneously track up to 30 aircraft, within a nominal
  range of 14 NM for Mode A/C targets and 30 NM for Mode S targets. In
  implementations that allow for the use of the Mode S extended
  squitter, the normal surveillance range may be increased beyond the
  nominal 14 NM. However, this information is not used for collision
  avoidance purposes.


This is confirmed in FAA document Introduction to TCAS II V7.1


  TCAS has a requirement to provide reliable surveillance out to a range
  of 14 nmi and in traffic densities of up to 0.3 aircraft per square
  nautical mile. [...] TCAS can simultaneously track up to 30
  transponder-equipped aircraft within a nominal range of 30 nmi.