Overview
ISO/TR 25087:2025 - Space systems - Study of electrical wire derating - is a technical report that explains how electrical designers determine allowable wire current (ampacity) for spacecraft. The report summarizes the basis for single‑wire and bundled‑wire current limits in published aerospace standards, compares derated current values across different standards and environments (vacuum vs. atmospheric), and provides guidance to manage differences arising from wire type, bundle size and temperature conditions.
Key topics
- Derating and derating factor: definition and role of multipliers applied to allowable current to limit electrical and thermal stress.
- Single‑wire ampacity: basis for allowable current under space (vacuum) conditions; comparative data summarized in Annex A.
- Bundled‑wire derating: how allowable current reduces with number of wires in a bundle; comparative tables and plots provided in Annex B (including Figure B.1).
- Environmental influences: impact of ambient pressure (vacuum vs. atmosphere), ambient and wiring temperature, and thermal environment on allowable current.
- Material and insulation considerations: differences between insulation types (e.g., PTFE, ETFE, single‑layer insulation (SLI), multiple‑layer insulation (MLI)) and their effects on thermal performance and derating.
- Design workflow: recommended electrical wire selection steps - define environmental requirements, determine allowable current, select wire, plan placement, and perform testing/verification (thermal, current capacity, vibration, radiation).
- Standards comparison: analysis of common aerospace documents (MIL‑STD‑975M, EEE‑INST‑002, JPL D‑8208, ECSS‑Q‑ST‑30‑11C, JERG‑2‑212N1, SAE AS 50881, etc.), noting where values and margin assumptions differ.
Practical applications
- Provides engineers with a consolidated reference to select correct wire ampacity and apply appropriate wire derating for spacecraft harnesses and power/signal routing.
- Supports thermal analysis and verification planning for spacecraft electrical systems, helping to avoid overheating due to resistive heating in vacuum or low‑pressure environments.
- Useful for harness designers, avionics and systems engineers, test laboratories, and procurement/specification authors who must reconcile differing derating rules across agencies and suppliers.
- Aids risk mitigation by clarifying assumptions behind published derating curves and by offering comparative data to inform conservative or optimized design choices.
Who should use this standard
- Spacecraft electrical/system engineers, harness designers, thermal analysts, quality managers, and anyone involved in aerospace wiring specifications or compliance.
Related standards
- SAE AS 50881 / MIL‑W‑5088 origins, MIL‑STD‑975M, EEE‑INST‑002, JPL D‑8208, ECSS‑Q‑ST‑30‑11C, JERG‑2‑212N1, GJB/Z series (as referenced in the report).