ICRP 60 has defined the personal dose equivalent Hp(10) as an estimator of the effective dose E. Personal dosimeters, worn on the trunk, allow the measurement of the quantity Hp(10). However, the characteristics of the instrumentation and the definition of Hp(10) itself can generate differences between the two quantities, depending on the energies and on the directional distribution of the incident radiation. The objective of this study is to evaluate the possibility of the measurement of the effective dose E using an instrumented anthropomorphic phantom at workplaces. In the first step of this study, calculations of the effective dose for standard configurations are made using the Monte Carlo code MCNPX. This paper presents the model of the numerical anthropomorphic phantom and the results for whole body irradiations by broad unidirectional or plane-parallel photon beams. The results agree with those calculated by Zankl et al., so confirming the good suitability of the code and the phantom used. Then, the dose distributions inside some organs are presented and the locations of future detectors for the instrumented phantom are discussed.