How to Choose Wire Gauge for a Custom Wire Harness

Wire gauge is a system decision, not a catalog shortcut

A wire size cannot be selected from current alone. The same conductor may behave differently in a short, open run than in a long, tightly bundled harness near a heat source. Voltage, continuous and intermittent current, duty cycle, route length, ambient temperature, insulation rating, bundle construction, terminal limits and the applicable equipment standard all affect the decision.

For sourcing, avoid a note such as "use 18 AWG" without the circuit conditions behind it. Record why that size was selected and which drawing or calculation controls it. This helps the harness supplier confirm that the conductor, terminal, seal and connector remain compatible.

Understand AWG and square-millimetre designations

American Wire Gauge is an inverse system: a smaller AWG number identifies a larger conductor. Metric wire is normally specified by nominal cross-sectional area in square millimetres. The two systems describe size differently, and a commonly printed conversion table does not make every AWG construction identical to the nearest metric size.

IEC 60228 defines nominal metric conductor areas and resistance requirements for several conductor classes. IEC Technical Report 62602 provides data for AWG and kcmil sizes. The governing wire standard, conductor material, stranding and finished-wire specification should therefore be stated rather than relying on a rounded conversion alone.

Start with the real circuit current

Document normal current, maximum continuous current, expected transient or inrush current, duty cycle and fault protection. A motor, heater, lamp, solenoid and electronic load can place very different demands on a conductor even when their nominal ratings appear similar.

The design method must come from the product or industry standard that controls the equipment. Current-carrying guidance is conditional on conductor and insulation type, temperature, installation and bundle assumptions. A value copied from a general chart is not automatically valid for a finished harness.

The protective device also belongs in the review. The selected conductor, connector contacts and protection strategy must be coordinated so that an abnormal current is interrupted according to the approved equipment design. A harness manufacturer should not invent the fuse or breaker rating from wire gauge alone.

Check voltage drop over the complete path

Conductor resistance causes a voltage drop that increases with current and path resistance. For a simple DC circuit, an early estimate can use V = I × R, but the resistance must represent the complete current path at the relevant temperature. The return conductor, terminals, splices, contacts and connection interfaces may all contribute.

Long low-voltage circuits are often limited by voltage drop before thermal capacity becomes the deciding factor. The FAA’s AC 43.13-1B illustrates a wire-selection process that checks both current-carrying capacity and voltage drop, and shows that a size passing one check can still fail the other. Its examples are aviation guidance, not universal limits for other products.

State the maximum permitted voltage drop or the minimum voltage required at the load. Without an acceptance limit, the harness supplier cannot determine whether a longer route or a warmer conductor still meets the equipment requirement.

Account for ambient temperature and insulation rating

A conductor generates heat while carrying current and must release that heat to its surroundings. High ambient temperature reduces the available thermal margin. The wire’s insulation temperature rating sets a material limit, but it does not by itself prove that the conductor may carry a particular current in every installation.

Use the temperature correction or derating method required by the governing standard. Include nearby heat sources, enclosure temperature, ventilation and expected operating duration. If the route crosses different temperature zones, identify the most demanding section rather than averaging the environment.

Insulation selection also depends on voltage, abrasion, fluids, flame requirements, flexibility and regulatory rules. Two wires with the same conductor size may have different outside diameters and different environmental performance.

Apply bundle and routing derating

Wires in a bundle cannot dissipate heat as freely as an isolated wire. Bundle size, loading pattern, protective sleeving, conduit, tape wrapping and installation space can change the thermal condition. NASA’s MSFC-STD-3012, for example, determines derated current using both wire size and bundle size for the systems within its scope.

Do not transfer a bundle factor from one standard or application into another without approval. Define which circuits may be energized together, how the harness is covered, the expected ambient condition and which derating method controls the design.

Routing also affects mechanical life. A larger conductor may have a larger bend radius, require more support and occupy more space at branches or connector exits. Confirm that the chosen size can be installed without forcing the harness against sharp edges or placing load on terminals.

Match the conductor to the terminal and connector

Increasing wire size does not help if the selected terminal cannot accept it. Connector families specify approved conductor ranges, insulation-diameter ranges and termination requirements. TE application specifications show these limits for particular contact systems; the values differ by connector family and must not be generalized.

Check the exact terminal part number, conductor size and type, insulation outside diameter, strip length, seal range where applicable, crimp tooling and connector cavity. A wire may satisfy an electrical calculation but still be too large for the terminal barrel, seal or housing, or too small for reliable insulation support.

The contact system may also impose its own current and temperature limits. Review the complete connection rather than assigning the conductor’s theoretical capacity to the connector.

Consider conductor material, stranding and flexibility

Copper, aluminium and copper alloys do not have identical resistance, mass, termination or corrosion behaviour. Stranding changes flexibility and influences how the conductor fits and terminates. A nominal area alone does not completely specify the wire.

For moving assemblies, repeated flexing may control the wire construction, routing and strain relief. For fixed harnesses, installation bends and vibration may still matter. State whether the cable is static, occasionally moved or continuously flexed, then use a wire specification approved for that service.

Any change in conductor material or strand construction should trigger a review of resistance, terminals, crimp settings, joining method and validation. It should not be accepted as a purchasing substitution based only on nominal gauge.

Do not oversize without checking the consequences

A larger conductor can reduce resistance and temperature rise under the same conditions, but oversizing is not a free safety margin. It can increase bundle diameter, weight, stiffness, connector size and routing space. It may also require different terminals, seals, tooling and minimum bend radius.

Choose the smallest approved size that satisfies all electrical, thermal, mechanical and regulatory requirements with the required design margin. If extra margin is desired, document it and verify the complete connection system again.

What buyers should include in the RFQ

Provide system voltage, normal and maximum current, transient profile, duty cycle, circuit length and the allowed voltage drop or load-voltage requirement. Identify the circuit-protection strategy and the standard or customer specification that controls wire sizing and derating.

List the required conductor material, AWG or metric size, stranding or flexibility class, insulation specification, color, outside-diameter limits and environmental exposure. Include the connector and terminal part numbers, seals, splices, branch points, coverings and routing constraints.

Mark any value that is provisional and request an engineering review before release. If the supplier is expected to recommend a wire size, provide the operating data and require the approved calculation or design record to be returned for review.

A short wire-size checklist

1. Are normal, continuous, transient and fault conditions defined?
2. Has voltage drop been checked over the complete supply and return path?
3. Are ambient temperature, bundle loading and coverings included in the approved derating method?
4. Does the wire insulation meet the electrical, thermal, mechanical and environmental requirements?
5. Do the exact terminal, seal and connector accept the conductor and insulation diameter?
6. Are routing, bend, weight, flexibility and circuit-protection constraints documented?

Frequently asked questions

Can wire gauge be selected from amperage alone?

No. Current is essential, but length, voltage drop, ambient temperature, bundling, insulation, duty cycle, terminals, routing and the governing standard can change the required size.

Is a larger wire always better?

No. A larger conductor may improve electrical margin, but it can create connector, sealing, tooling, weight, stiffness and packaging problems. The entire connection system must be checked.

Are AWG and square millimetres directly interchangeable?

They can be compared by conductor area and resistance, but they are different designation systems. Use the approved wire standard and finished-wire specification instead of substituting the nearest rounded value automatically.

What information is most often missing from a wire-size request?

Common gaps are the allowed voltage drop, actual route length, simultaneous bundle loading, ambient temperature, transient current, terminal part number and insulation outside diameter.

References

These primary sources show that conductor sizing depends on voltage drop, thermal conditions, bundling, conductor standards and the approved terminal range. Their application-specific values are not presented as universal limits, and this article makes no unsupported HPC capability claim.

FAA AC 43.13-1B: Aircraft Inspection and Repair

NASA MSFC-STD-3012: Electrical, Electronic and Electromechanical Parts Derating

IEC 60228:2023 Conductors of insulated cables

IEC TR 62602:2009 Data for AWG and KCMIL sizes

TE Connectivity QP 6.5 Connectors Application Specification

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