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Wire selection and ratings, an introduction

Wiring Harness

If repairing an existing harness or working from prints, the wire size can be easily determined or found on the drawing, and would have been engineered to ensure the wiring harness provides safe and reliable operation. When it comes to wire selection, the current rating, or ampacity as it's also referred to as, is an important part of the process, but not the only one. The wire size you'll see listed (if at all) is dependent on a number of variables, which need to be evaluated to ensure the wiring is correct for your particular application. In this article we go into what these variables are, factors to take into consideration, types of wire, and a couple time tested wire selection methods.

1. Wire Ratings

2. Variables

In general, wires must be sized so that they: have sufficient mechanical strength to allow for service conditions; do not exceed allowable voltage drop levels; are protected by system circuit protection devices; and meet circuit current carrying requirements [2, par. 11-66]. Below is a non-inclusive list of variables to consider.

Fig2-1
Figure 2-1. Single copper wire in free air. [3, Fig. 3]
Fig2-2
Figure 2-2. Altitude derating curve. [3, Fig. 5]
Fig2-3
Figure 2-3. Bundle derating curves. [3, Fig. 4]

3. Types of Wire

There are many different specifications of wire available, from Mil-Spec (MS, now AS), to OEM, to commercial grades. For content length, this article will focus on the Mil-Spec types used for power and signal wires due to its common use in motorsports and high performance applications. This is not to discourage the use of automotive wire, such as TXL, GXL, or SXL, as these are also used very effectively in motorsports. There are two main categories of wires, suitable for either "open wiring" or "protected wiring" applications.

Open Wiring Example
Open Wiring on an aircraft
Protected Wiring Example
Protected Wiring on a ALMS LMP2 race car
Open Wiring Example
Open or Protected Wiring? On a motorcycle harness. This can be debated, but a wire categorized for open wiring is probably more appropriate.

Whether it's open or protected wiring, it will always be stranded rather than solid to minimize fatigue breakage, and always at least tin plated. Bare copper develops a surface oxide coating at a rate dependent on temperature. This oxide film is a poor conductor of electricity and inhibits re-termination of wire. Therefore, all wiring has a coating of either tin, silver, or nickel, that have far slower oxidation rates [2, par. 11-77]. There are several common types of insulation materials: Of these, ETFE is most common in motorsports for open wiring, and XLETFE for protected wiring. This doesn't mean each is the "perfect" insulation. Each insulation has its' own varied properties for; electrical, mechanical, chemical, and thermal performance, not to mention the economics of cost and availability. The selected insulation material is a balance of these properties dependent on the desired performance and application requirements.
Table 3-1. Comparable properties of wire insulation systems [2, pg. 11-37][10, pg. 24].
Most desirable -> Least
Relative Ranking12345
CostETFEXLETFEPTFEPICOMP
SpacePIXLETFEETFECOMPPTFE
WeightPIXLETFEETFECOMPPTFE
TemperaturePTFECOMPPIXLETFEETFE
Abrasion resistancePIXLETFEETFECOMPPTFE
Cut-through resistancePICOMPXLETFEETFEPTFE
FlammabilityPTFECOMPPIETFEXLETFE
Smoke generationPICOMPPTFEETFEXLETFE
FlexibilityPTFEETFECOMPXLETFEPI
Arc propagation resistancePTFEETFECOMPXLETFEPI

Table 3-2. Open Wiring [2, pg. 11-40].
Table 3-2 Open Wiring
Table 3-3. Protected Wiring [2, pg. 11-41].
Table 3-3 Protected Wiring

4. Wire Selection Methods

Wire selection can be a time consuming process, but is necessary to achieve the desired results for reliability, safety, and prevent failure in the particular application. Current guidance relies on the use of published and publicly available standards, not limited to the following:

These standards are a good basis for wire selection, they're time tested, some of which go back decades. Beyond that, more advanced methods can be utilized such as thermal modeling and CAD, which become necessary with complicated wire harness designs.

43.13-1B Example
The full document covering many subject matters related to aircraft inspection and repair can be downloaded as a PDF. Click Here
The Aeronautics Guide is another source covering similar subject matter, but with updated graphics such as the figure below. Click Here
Conductor chart, continuous (top), and intermittent flow (bottom) [3, Fig. 2]
AC43-13 conductor chart
NASA Technical Memorandum 102179 Example
The full document can be downloaded as a PDF. Click Here
Selection of wire size and circuit protection device [6, Fig. 1]
102179 flow chart

Sources:
[1] Society of Automotive Engineers. (Rev. C October 2006, superseded). AS50881 Wiring Aerospace Vehicle. ftp://pklunx.hill.af.mil/data/FA810514R0009_AS50881Rev_C_Wiring_Aerospace_Vehicle.pdf

[2] Federal Aviation Administration. (September 08, 1998). 43.13-1B - Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair. Retrieved September 4, 2019, from http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43.13-1B_w-chg1.pdf

[3] Aeronautics Guide. Wire Size Selection - Aircraft Electrical System. Retrieved September 4, 2019, from https://www.aircraftsystemstech.com/2017/06/wire-size-selection.html

[4] Federal Aviation Administration. (December 8, 2018). Aircraft EWIS Practices Job Aid 2.0. Retrieved September 4, 2019, from https://www.faa.gov/training_testing/training/air_training_program/job_aids/media/EWIS_job-aid_2.0_Printable.pdf

[5] Air Force Space Command. (June 3, 2009). TECHNICAL REQUIREMENTS FOR WIRING HARNESS, SPACE VEHICLE. Retrieved September 4, 2019, from https://apps.dtic.mil/dtic/tr/fulltext/u2/a633334.pdf

[6] Gaston, Darilyn M., National Aeronautics and Space Administration. (June 01, 1991). Selection of Wires and Circuit Protective Devices for STS Orbiter Vehicle Payload Electrical Circuits. Retrieved September 4, 2019, from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910015188.pdf

[7] Rickman, Steven L., Iannello, Christopher J., National Aeronautics and Space Administration. (September 07, 2016). Heat Transfer Analysis in Wire Bundles for Aerospace Vehicles. Retrieved September 4, 2019, from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160011474.pdf

[8] Rickman, Steven L., Iannello, Christopher J., Shariff, Khadijah, National Aeronautics and Space Administration. (June 09, 2017). Improvements to Wire Bundle Thermal Modeling for Ampacity Determination. Retrieved September 4, 2019, from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170006077.pdf

[9] National Aeronautics and Space Administration. (November 01, 2018). Re-Architecting the NASA Wire Derating Approach for Space Flight Applications. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20180007922.pdf

[10] National Aeronautics and Space Administration. (September 01, 1994). New insulation constructions for aerospace wiring applications. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950009641.pdf

[11] National Aeronautics and Space Administration. (September 01, 1994). Kapton wire concerns for aerospace vehicles. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950009630.pdf

Kapton, Tefzel, Teflon are trademarks of DuPont Corporation. Raychem, SPEC 55 are trademarks of the TE Connectivity Ltd. family of companies.

Created: September 4, 2019
Updated: May 15, 2021


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