As voltage and power factor, considered with in limit, as W=VICos(phi), current carrying capacity of line would decides the, capacity of power transmission, for local LT distribution network, supplying load to local load and for industrial load at normal supply voltage. For HT power line depending on length of line, the short, medium or long model(to find A,B,C,D constant) should be used to find Vs,Vr,Is,Ir,Ps,Pr and Pmax, with efficiency, could be obtained, using respective equations ......

You should first know about the size or name of the acsr conductor which you are using. After that, you can see the manufacturer data and gets its ampacity. Finally, just multiply with the transmission line voltage to get the power rating. 

V(sending)*V(recieving)*sine angle between sending voltage and recieving end voltage/(Reactance of line) 

in general line complete capability is based on its thermal limit

plz Refer ATC (avaliable transfer capability )calculations

If you are talking about the capacity of the line, then its thermal limit can be calculated based on its current carrying capacity.   But whenever the transmission line is in a system its limit is controlled by system operating conditions and not on its thermal limits.   Normally for a given line you can fix up its capacity and distance over which that power can be transferred using normal steady state stability limits.

Samundra Gurung got it correct, above.

up to my knowledge concern,acsr conductor type and manufacturer data sheet specifications has to be considered ,but along with line length is also impourtant...

voltage drop depends on line length ,also recieving end voltage depends on Vrecieving=Vsending end-Vdrop

Vdrop varies with conductor type specs.and length...

Pmax=Vs*Vr/X

It depends on the definitions of transmission system limit.If you consider for a particular line. Limit is always begin with thermal, voltage stability ,and steady state stability limits respectively. For the concept of ATC (Available Transfer Capability), either point to point or zone to zone, thermal limit or voltage stability limit is the limitation of transferring power. Finally, for a load ability limit of a power system, all generation plants and loads are assumed to be increased until reaching the nose point of a system. This simulation is a continuation power flow thereby voltage stability limit is a limitation of a transmission system.

Narasimha Prasad,  Vdrop is also a function of ampacity per IEC and BS7671

Vdrop=function (Z for the conductor size and type x length x amp load)

In general, it depends on the thermal and steady state stability limits.

If you ask about the capacity of the line, it depends on the thermal limit.

If you ask about the loadability margin of the network, you can utilize continuation power flow to determine the MVA margin for each load increase

Komson Daroj: we can increase(extend in power axis) the nose point by ,reactive power compensation  techniques,there by improving power transfer capability whil maintaining voltage limit right?

Dear Narasimha, what you say is right and this can be simply done in simulation. However, this is not a suitable measure to increased loadability of a system.

We can increase the loading capacity by transmitting power at high voltage, or high phase order transmission line.