NASA's interoperable power bus is a contract between impedances

What the standard actually standardizes

Revision A establishes 120 VDC as the interoperability power standard for vehicle-to-vehicle, element-to-element, and internal equipment interfaces. A 28 VDC interface is allowed by approved exception. The permissible steady-state ranges are deliberately broad: 98 to 136 VDC for the 120 V class and 23 to 36 VDC for the 28 V class.¹

Those numbers are not promises that a generator sits at exactly 120 or 28 volts. They define what equipment must tolerate at the specified boundary after generation swing and distribution loss. The standard stays design-neutral behind that boundary.

Stability is split across source and load

A tightly regulated electronic load can behave as negative incremental impedance: when bus voltage falls, it draws more current to preserve power, deepening the disturbance. The standard therefore requires characterization of source impedance and equipment input impedance, including measurements from 30 Hz to 100 kHz. Compatibility depends on their relationship, not on either component alone.

The same logic governs inrush. For an example 10 A interface, the standard derives an illustrative capacitance limit of 1,000 microfarads at 120 V but 4,286 microfarads at 28 V from the allowed charge envelope. It also defines ripple spectra, normal load-step transients, abnormal overvoltage and undervoltage behavior, fault coordination, return isolation, and single-point grounding.

Where a skeptic should push

A common electrical contract reduces integration uncertainty; it does not guarantee plug-and-play operation. Each actual program still defines connectors, harnesses, tiered interfaces, protection settings, and available fault energy. A load that passes the generic interface can still destabilize a specific feeder if margins are assigned carelessly.

Revision A was approved by NASA program leadership pending multilateral coordination and partner signatures. That status matters. The document is a released engineering baseline, not proof that every Artemis or Gateway element has implemented every clause unchanged.

The bottom line

High confidence: the standard creates a precise common verification language for 120 VDC and 28 VDC exploration systems. Moderate confidence: it will reduce bespoke interface redesign if programs preserve the generic boundary through procurement. The benefit disappears if exceptions multiply until every pairing again requires a custom stability and fault study.

For a lunar grid, interoperability is stored engineering work

The non-obvious implication is institutional: a verified impedance envelope lets a future load reuse analysis performed before that load existed. The opportunity is modular growth across agencies and contractors. The threat is nominal-voltage thinking, where matching labels conceal incompatible inrush, ripple, grounding, or protection behavior. The grid is interoperable only when its transient contract survives contact with real hardware.