Why we do not publish current ratings
Every sizing table online gives a current next to a cross-section, and almost every one hides the conditions behind it. The same copper conductor can carry very different currents depending on whether it is clipped to a wall, laid in a perforated tray, pulled into conduit with other circuits or buried in dry soil. Current rating is therefore set per installation method to IEC 60364-5-52, BS 7671 or the NEC as applicable, and confirmed by the project engineer. Strandquay supplies the construction your engineer specifies and checks that the conductor delivered is the size marked. That second job is where buyers lose money, and it is covered below.
The five inputs that decide a size
- Installation method. Conduit, trunking, tray, ladder, direct burial, ducts or free air. Each wiring rule has reference methods with their own rating tables and correction factors.
- Grouping. Circuits run together heat each other, so a group of cables is derated against a single circuit.
- Ambient and ground conditions. Air temperature, soil temperature and soil thermal resistivity change how fast heat leaves the cable. Insulation matters here: XLPE runs at 90 C continuously, PVC at 70 C.
- Voltage drop. Long runs, low voltage circuits and DC solar strings are often sized by voltage drop rather than heating. Drop depends on conductor resistance, length and load.
- Short circuit. The conductor and any screen or armour must survive the fault current until the protection clears it. XLPE tolerates 250 C during a short circuit; the engineer checks the cross-section against the fault level and clearing time.
The largest result of these checks sets the size. The voltage drop explainer and cable size selection basics go further into each input.
How a size is proved: IEC 60228 resistance
IEC 60228 does not define a conductor by its diameter or number of wires. It defines it by the maximum DC resistance per kilometre at 20 C for each nominal size and class. A conductor that meets the resistance limit has enough metal for its marked size; one that fails is undersized, whatever its diameter looks like. This is why the desk asks for conductor resistance on the routine test record for every drum.
The class matters as much as the size. Class 1 is solid, Class 2 is stranded, Class 5 is flexible and Class 6 is more flexible still. Flexible classes use more, finer wires and are allowed slightly higher resistance for the same nominal size, so a Class 5 cord and a Class 2 power cable marked 2.5 mm2 are checked against different limits. State the class on the spec sheet so that the test record can be read against the right column.
| Nominal size, mm2 | Copper Class 1/2 | Copper Class 5, plain | Aluminium Class 2 |
|---|---|---|---|
| 1.5 | 12.1 | 13.3 | - |
| 2.5 | 7.41 | 7.98 | - |
| 4 | 4.61 | 4.95 | - |
| 6 | 3.08 | 3.30 | - |
| 10 | 1.83 | - | 3.08 |
| 16 | 1.15 | - | 1.91 |
| 25 | 0.727 | - | 1.20 |
| 35 | 0.524 | - | 0.868 |
| 50 | 0.387 | - | 0.641 |
| 70 | 0.268 | - | 0.443 |
| 95 | 0.193 | - | 0.320 |
| 120 | 0.153 | - | 0.253 |
Tinned Class 5 conductors, used in solar cable, have slightly higher limits: 5.09 ohm/km for 4 mm2 and 3.39 for 6 mm2. Note how aluminium 95 mm2 has a limit of 0.320 ohm/km, close to copper 50 mm2 at 0.387: an aluminium feeder is usually one or two sizes larger than the copper cable it replaces, which your engineer confirms. See aluminium cables and IEC 60228 conductor classes.
AWG and mm2 are different series
US and Canadian projects size wire in AWG and kcmil; IEC markets use mm2. The two series do not line up, and rounding to the nearest metric size can leave a conductor short. The table gives the nominal area calculated from the AWG diameter formula next to the nearest common metric size.
| AWG | Calculated area, mm2 | Nearest common metric size |
|---|---|---|
| 23 | 0.258 | Cat6 conductor, about 0.57 mm diameter |
| 18 | 0.823 | 0.75 or 1.0 mm2 |
| 14 | 2.08 | 2.5 mm2 (1.5 mm2 is smaller) |
| 12 | 3.31 | 4 mm2 (2.5 mm2 is smaller) |
| 10 | 5.26 | 6 mm2 |
| 8 | 8.37 | 10 mm2 |
| 6 | 13.3 | 16 mm2 |
| 4 | 21.2 | 25 mm2 |
| 2 | 33.6 | 35 mm2 |
| 1/0 | 53.5 | 50 mm2 (smaller) or 70 mm2 |
| 4/0 | 107 | 95 mm2 (smaller) or 120 mm2 |
A metric cable is not a substitute for an AWG size under US rules, and vice versa: US building wire must be the listed type, such as THHN/THWN-2 to UL 83, and NEC 310.3 sets a minimum of 14 AWG for copper and 12 AWG for aluminium or copper-clad aluminium. More conversions are in AWG vs mm2.
What to send the desk
Send the sizes your engineer has fixed, the conductor material and class, the standard and the metres per size. If you only have a load and a route, the desk can list the inputs your engineer still needs but will not choose the size. With sizes fixed, use the quote form; resistance records per drum are part of every order checked under quality control.