For large facilities having several floors or containers, floor response spectra, FRS, other than ground response spectra need to be developed. However, FRS can have error especially when components are not small in their masses. In this paper, error is estimated in order to specify applicability of the FRS by deriving and comparing with analytic results for two degrees of freedom system. An identity regarding modal vectors and participation factors in the modal method is used to measure the FRS error. It is found that FRS is sufficiently accurate if the mass of a component is one hundredth or less than that of the floor. On the contrary, it is shown that fixed frequency of a component does not affect the FRS accuracy considerably. A compact power plant system consists of two main assemblies with spring mounts is applied as an example for the derivation of the FRS and several aspects associated with its modeling and calculation are discussed.