Choosing the right conductor for a terminal connection is one of the most consequential decisions a panel or design engineer makes, yet it is often reduced to a quick glance at a chart. Undersizing a conductor invites overheating, accelerated insulation ageing and nuisance trips; oversizing wastes copper, strains terminals and complicates routing. This guide walks through current-carrying capacity, the relationship between metric cross-section and AWG, the derating factors that matter in real installations, and how to match wire correctly to a terminal block’s rated range.
What Current-Carrying Capacity (Ampacity) Means
Current-carrying capacity, or ampacity, is the maximum continuous current a conductor can carry without its temperature exceeding the rating of its insulation. Current flowing through the resistance of copper generates heat (I²R losses). That heat must dissipate into the surrounding air, enclosure or adjacent conductors. When generation exceeds dissipation, the conductor temperature climbs.
Why correct sizing matters comes down to four interlinked concerns:
- Heat and insulation life. Most installation cabling uses insulation rated for 70 °C (PVC) or 90 °C (XLPE/EPR). Sustained operation above the rated temperature degrades insulation, embrittles it and eventually causes failure.
- Voltage drop. A conductor that is electrically adequate for current may still drop too much voltage over a long run, starving the load.
- Safety. Overheated conductors and terminations are a fire and shock hazard, and a primary reason protective devices are coordinated to conductor ampacity.
- Terminal rating. The terminal block, lug or connector in the path has its own current and temperature rating. The connection is only as strong as its weakest element.
Conductor Cross-Section: mm² and AWG
In IEC/IS practice, conductors are specified by cross-sectional area in square millimetres (mm²). North American practice uses American Wire Gauge (AWG), an inverse logarithmic scale where a larger number means a thinner wire. Because the two systems do not map onto identical values, AWG equivalents are always approximate.
The table below lists common metric sizes with their nearest AWG and an indicative ampacity. These current values are indicative only. Actual permissible current depends on insulation temperature rating, installation method (free air, conduit, bundled), ambient temperature and the governing standard (for example IEC 60364-5-52, IS 732 or the equipment standard). Always size to the applicable standard for your installation rather than to a generic chart.
| Cross-section (mm²) | Approx. AWG | Indicative current (A) |
|---|---|---|
| 0.5 | 20 | 6–9 |
| 0.75 | 18 | 9–12 |
| 1.0 | 17–18 | 12–15 |
| 1.5 | 16 | 16–20 |
| 2.5 | 14 | 22–28 |
| 4 | 12 | 30–37 |
| 6 | 10 | 40–48 |
| 10 | 8 | 55–66 |
| 16 | 6 | 75–88 |
| 25 | 4 | 100–115 |
Treat the ranges as a starting point. A 2.5 mm² conductor bundled with several others in a hot enclosure may be limited to well below 22 A, while the same conductor clipped to a surface in free air could safely carry more.
Derating Factors
Published ampacities assume a reference condition — typically a single conductor at a stated ambient temperature. Real installations rarely match those assumptions, so correction (derating) factors are applied:
- Insulation temperature rating. A 90 °C-rated conductor can carry more current than a 70 °C conductor of the same size, but only if every device in the path — including the terminal — is rated for that temperature. The lowest-rated component governs.
- Ambient temperature. Higher ambient air reduces the temperature headroom available for I²R heating. Above the reference ambient (often 30 °C), a correction factor below 1.0 applies; below it, a factor above 1.0 may apply.
- Bundling / grouping. Conductors grouped together heat one another and dissipate less effectively. Grouping factors reduce ampacity as the number of loaded conductors in a bundle or conduit rises.
- Enclosure heat. Inside a sealed panel, internal air can run significantly hotter than the room. Power dissipation from drives, power supplies and other conductors raises the effective ambient seen by every conductor and terminal.
Apply these factors multiplicatively to the base ampacity. The corrected value, not the table value, is what must exceed the load current.
Solid vs Stranded Conductors and IEC 60228
IEC 60228 defines conductor construction in classes:
- Class 1 — solid conductors.
- Class 2 — stranded conductors for general fixed wiring.
- Class 5 — flexible stranded conductors.
- Class 6 — extra-flexible (very fine strands).
Solid conductors are easy to insert and hold well under clamping pressure, but they are stiff and fatigue under repeated movement. Stranded and flexible conductors tolerate vibration and routing much better, which is why they dominate panel wiring — but their loose strands can splay, escape a clamp or be damaged by a screw, producing a high-resistance joint that overheats.
Ferrules for Stranded Wire
For this reason, flexible stranded conductors (Class 5/6) terminated in screw or many spring terminals should be fitted with bootlace ferrules. A crimped ferrule (per IEC/EN 60947 practice for terminations) gathers the strands into a solid pin, ensuring uniform clamping pressure, preventing strand spread and maintaining a low, stable contact resistance over the life of the joint. Use the correctly sized ferrule for the conductor cross-section, and a calibrated crimp tool.
Voltage Drop
Even a conductor with ample ampacity can fail to deliver usable voltage over distance. Voltage drop is the voltage lost to conductor resistance along the run. For a DC or single-phase resistive circuit, the basic relationship is:
Vdrop = 2 × L × I × R
where L is the one-way length (m), I is the current (A), and R is the conductor resistance per metre (Ω/m); the factor of 2 accounts for the go-and-return path. Annealed copper at around 20 °C has a resistivity giving roughly 0.0179 Ω·mm²/m, so R per metre ≈ 0.0179 / (cross-section in mm²).
Worked example. A 24 V DC load drawing 10 A over a 30 m one-way run on 2.5 mm² copper:
- R per metre ≈ 0.0179 / 2.5 ≈ 0.00716 Ω/m
- Vdrop = 2 × 30 × 10 × 0.00716 ≈ 4.3 V
That is roughly 18 % of 24 V — far too much. Stepping up to 6 mm² (R ≈ 0.00298 Ω/m) gives Vdrop ≈ 2 × 30 × 10 × 0.00298 ≈ 1.8 V, about 7.5 %. For sensitive 24 V control and instrumentation, low-voltage DC runs are frequently limited by voltage drop rather than ampacity, so check both.
Matching Wire to the Terminal Block
A terminal block specifies a rated wire range (minimum and maximum cross-section it can clamp reliably) and a rated current. Both must be respected:
- The conductor must fall within the terminal’s accepted cross-section range — too thin and the clamp cannot grip; too thick and it will not seat properly.
- The terminal’s rated current, established under its own standard test conditions, must not be exceeded.
- The terminal’s temperature rating must be compatible with the conductor insulation rating you are relying on.
The governing rule is simple: the continuous current must not exceed the lowest-rated element in the path — conductor, ferrule, terminal block, lug or busbar. A 6 mm² cable rated for 40 A delivers nothing extra if it lands on a terminal rated for 24 A.
Sizing Checklist
Before committing a conductor size, work through the following:
- Determine the maximum continuous load current, with margin for inrush and future growth.
- Select a base cross-section from the applicable standard, not a generic chart.
- Apply derating for ambient temperature, grouping/bundling and enclosure heat; confirm corrected ampacity exceeds load current.
- Verify the insulation temperature rating is matched by every device in the path.
- Calculate voltage drop for long or low-voltage runs and upsize if it exceeds the permitted limit.
- Confirm the conductor cross-section sits within the terminal block’s rated wire range.
- Fit correctly sized bootlace ferrules on flexible stranded conductors.
- Coordinate protective devices with the corrected conductor ampacity.
- Re-check that no single element in the path is rated below the actual continuous current.
Get these steps right and a connection will run cool, hold its contact resistance and last the life of the panel. Unison Connectors’ terminal block range spans a wide spread of wire-ranges and current ratings to suit conductors from fine control wiring up to heavier power feeds, making it easier to match the terminal to the conductor rather than the other way around. Always validate final sizing against the standards and installation conditions that apply to your project.