Transmission system works like express highways to evacuate the power generation in remote areas to the load centres. The assessment of the capability of the network to carry power is essential not only from the security point of view but also from commercial use reserving transactions in a competitive electricity market. The main contribution of the paper is i) An evaluation of ATC with heuristic techniques viz. Genetic Algorithm, Particle Swarm Optimization, and Firefly Algorithm, ii) The comparison with Generalized Algebraic Modeling System software (GAMS), iii) Comparison of the results obtained with sensitivity based methods Viz. AC/DC PTDFs based approach. The results have been determined for intact and line contingency cases taking simultaneous as well as single transaction cases. The results have been obtained for IEEE 24 bus RTS.