ISO/TR 25087:2025

Space systems - Study of electrical wire derating

This document provides information on how electrical designers determine the allowable limit of electric wire in space condition. This document provides the basis of the allowable wire current and its derated value in published technical standards. This document also provides the results of comparing the derated wire current values depending on the number of bundled wires, wire type and temperature environments, which can help the system designer to handle the difference when determining the limit of wire ampacity.

Systèmes spatiaux — Étude du déclassement des fils électriques

General Information

• Status: Published
• Publication Date: 01-May-2025
• ICS: 49.060 - Aerospace electric equipment and systems
• Technical Committee: ISO/TC 20/SC 14 - Space systems and operations
• Drafting Committee: ISO/TC 20/SC 14/WG 1 - Design engineering and production
• Current Stage: 6060 - International Standard published
• Start Date: 02-May-2025
• Due Date: 24-Jan-2026
• Completion Date: 02-May-2025

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).

Standards Content (Sample)

Technical
Report
ISO/TR 25087
First edition
Space systems — Study of electrical
2025-05
wire derating
Systèmes spatiaux — Étude du déclassement des fils électriques
Reference number
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
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Phone: [phone]
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Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General description . 2
4.1 Electric wire design approach for space vehicle .2
4.1.1 General .2
4.1.2 Understanding the environmental requirements .2
4.1.3 Determination of allowable current .2
4.1.4 Wire selection .2
4.1.5 Wire placement .2
4.1.6 Testing and verification .2
4.2 Wire current and derating in standards .3
5 Wire current derating in standards . . 3
5.1 Single wire allowable current .3
5.2 Bundled wire allowable current .3
5.3 Study of the difference in derating factor for bundled wire .4
Annex A (informative) Single wire allowable current in standards of governmental institute . 5
Annex B (informative) Delated current and derating factor in governmental standards . 7
Bibliography .11

iii
Foreword
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The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 14, Space systems and operations.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
During the design of electrical wires or wire bundles, an understanding of their allowable current is
necessary. Nowadays, space system designers refer to aerospace standards to determine the allowable
current to avoid exceeding wire temperature limits due to resistive heat dissipation.
Generally, in these wire standards for space systems, some margin is added to the allowable current value
when the wire is used in the Earth's atmosphere. There are several standards for wire derating for space
systems depending on government agencies and commercial organizations; and the allowable currents differ.
For engineers designing space vehicle systems for the first time, the differences in allowable wire current
values in these standards can cause confusion. Although the differences stem from differences in the
assumed thermal environment, several evaluations of the correlation between the current allowances of
each standard and the thermal environment have been conducted; and understanding this background can
assist the system design engineer in wire selection.
This document contributes to risk mitigation in space vehicle system design by providing general technical
information on the allowable wire current, its limit in space application and information on mounting
conditions and associated thermal environment considerations.

v
Technical Report ISO/TR 25087:2025(en)
Space systems — Study of electrical wire derating
1 Scope
This document provides information on how electrical designers determine the allowable limit of electric
wire in space condition. This document provides the basis of the allowable wire current and its derated
value in published technical standards. This document also provides the results of comparing the derated
wire current values depending on the number of bundled wires, wire type and temperature environments,
which can help the system designer to handle the difference when determining the limit of wire ampacity.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
derating
reduction of electrical and thermal stresses applied to a part during normal operation to decrease the
degradation rate and prolong its expected life
3.2
derating factor
multiplier applied to the upper limits of voltage, current, temperature, etc. for derating (3.1) on electronic
components, the values of which vary depending on the operating environmental conditions and materials
3.3
American wire gauge
AWG
standard way of describing wire sizes in North America
3.4
ethylene tetrafluoroethylene
fluorine-based plastic that has high corrosion resistance and strength over a wide temperature range
3.5
single-layer insulation
sheet material that provides passive heat shielding through radiative insulation
3.6
multiple-layered insulator
insulation system consisting of multiple radiation shields separated by low thermal conductive spacers
which works to limit the amount of radiative heat transfer

4 General description
4.1 Electric wire design approach for space vehicle
4.1.1 General
Current rating is only one factor to consider when choosing a cable. The choice of wire for a spacecraft is
generally made with the following points taken into consideration:
— understanding the environmental requirements;
— determination of allowable current;
— wire selection;
— wire placement;
— testing and verification.
Once environmental requirements are determined, space vehicle system engineers further evaluate the
thermal, radiation and vibration resistance.
For example, polytetrafluoroethylene (PTFE), a commonly used insulating material, has excellent resistance
to high-temperature environments, but ethylene tetrafluoroethylene (ETFE) is superior to PTFE in radiation
degradation.
4.1.2 Understanding the environmental requirements
A correct understanding of the environmental requirements for the spacecraft can enhance the design
capabilities of spacecraft systems engineers. In general, space vehicle system engineers evaluate factors
such as temperature, radiation, vibration, air pressure, radioactive particles and static electricity.
4.1.3 Determination of allowable current
A correct understanding of environmental conditions can support space vehicle system engineers to
determine the allowable current of the wire. Factors such as wire sizing, wire material, cross-sectional area,
ambient temperature and ambient radiation levels are considered in this document.
4.1.4 Wire selection
Correct understanding of relationship between the allowable current and the environmental requirements
can support space vehicle system engineer to select wire. This process includes comparing materials,
checking approved industry standards, and evaluating manufacturers.
4.1.5 Wire placement
Placing wires in the appropriate locations can be one approach to achieving optimal spacecraft design. This
includes selecting wiring routes, adding protection and installing cooling systems if necessary.
4.1.6 Testing and verification
To check that the selected wire meets the environmental requirements, it is effective to conduct tests such
as current capacity, thermal characteristics, vibration characteristics and radiation resistance. In addition,
thermal analysis is an effective method for verifying the suitability of cables, especially their jackets, to
thermal environments.
4.2 Wire current and derating in standards
Today, it is common practice for electrical engineers in the aerospace industry to apply the derating rules
[1] [2]
of SAE AS 50881 and ECSS-30-11C when designing wiring bundles for aircraft and space systems.
The derating rules of MIL-W-5088, the origin of SAE AS 50881 was built on considerations from the 1950s
onward; and this derating rule is based on natural convection cooling of free wires in air and was updated in
the 1970s with the addition of a bundled wire loading condition.
Derating rules for space environments where natural convection cooling is not effective have been
[3] [4]
considered based on several tests and are reflected in MIL-STD-975M and EEE-INST-002 .
There are no cross-agency standards in the U.S. NASA that specify current and derating of wires and wire
bundles; and GSFC, MSFC, JSC, and JPL have issued individual standards addressing wire ratings. This fact
can cause spacecraft system engineers to worry about which documents to refer to.
Most NASA standards use the derating curves in MIL-W-50881 or SAE AS 50881 as guidance. MIL-W-50881
or SAE AS 50881 derating curves only provide wire bundle derating up to 30 480 m altitude, but additional
derating curves for vacuum environments have been considered and published, for example, as JPL D-8208
[5]
(Rev. I) .
[6]
Another wire current standard is JERG-2-212N1 issued by JAXA (Japan Aerospace Exploration Agency).
While the single wire current standard follows MIL-STD-975M, it is characterized by the fact that the
derating curve is shown to change when the wires are bundled and under the influence of the thermal
[7]
environment. There is also GJB/Z 35-93 published by Commission for Science, Technology and Industry
for National Defence, China, which is compatible with MIL-STD-975M.
5 Wire current derating in standards
5.1 Single wire allowable current
Single wire allowable current of Cu wire under vacuum conditio
...