The lung counting system at Kalpakkam, India, used for the estimation of transuranics deposited in the lungs of occupational workers, consists of an array of three large area germanium detectors fixed in a single assembly. The efficiency calibration for low energy photons was carried out using ²⁴¹Am and ²³²Th lung sets of Lawrence Liver more National Laboratory phantom. The muscle equivalent chest wall thickness (MEQ-CWT) was derived for the three energies 59.5, 75.95 (average energyof ²³²Th) and 238.9 keV for the series of overlay plates made of different adipose mass ratios. Efficiency as a function of MEQCWT was calculated for individual detectors for the three energies. Variation of MEQ-CWT from 16 to 40 mm resulted in an efficiency variation of around 40 % for all the three energies. The array efficiency for different MEQ-CWT ranged from 1.4×10^-3 to 3.2×10^-2, 1.5×10^-3 to 3.3×10^-3 and 1.1×10^-3 to 2.3×10^-3 for 59.5, 75.95 and 238.9 keV, respectively. In the energy response, efficiency was observed to be maximum for 75.95 keV compared with 59.5 and 238.9 keV.