This paper presents a set of specifications and design guidelines to ensure data center electrical system stability and to reduce voltage unbalance. Facebook experienced electrical stability and resonance at multiple data centers and identified that the impedance interaction between the building electrical system and the power supplies are the root cause for the instability. Higher building impedance drives the power supplies to instability; therefore, it is critical to keep the impedance the electrical system under a certain limit. This paper focuses on defining inductance limits for the passive electrical components including switchboards, reactors, transformers and busways. The goal is for equipment vendors to comply with the limits to make sure system stability and to avoid high system voltage unbalance by design. Impedance-based power stability analysis and generalized Nyquist stability criterion are used to define the limits given on the specification. Field impedance measurements are presented to demonstrate the stability limit on selected data centers.