The cable schedule says 50 mm². The voltage-drop calculation says the cable drops 3.8%. The CT datasheet says 14 mm. The three numbers describe different properties of the same conductor, and a monitoring project needs all of them.
This article covers conversion between sizes, resistance, voltage drop and the physical fit of a current sensor. Current-carrying capacity, protection and fault duty come from the wiring rules that apply to the site (IEC 60364-5-52, BS 7671 or the NEC), and this article does not cover them.
Five separate quantities
“Cable size” can refer to any of these:
| Quantity | What it is | Where to get it |
|---|---|---|
| AWG or nominal mm² | A standard designation for the conductor | Cable schedule, sheath marking |
| Bare solid-round diameter | The diameter of a solid circle with the nominal area, d = √(4A/π) | Calculation, or the AWG table in NIST Handbook 100 |
| Resistance | Ω per km at a stated temperature, normally 20 °C | Cable datasheet; IEC 60228 gives the maximum for each size |
| Voltage drop | The volts lost between the origin and the load at a stated current and power factor | Calculation from datasheet R and X, route length and load |
| Finished outside diameter | The size across the sheath, which is what a sensor must go around | Cable datasheet, then a caliper measurement on site |
Convert AWG to mm²
American Wire Gauge is a geometric series. Every 6 gauges halves the diameter, and every 3 gauges roughly halves the area. The NIST Copper Wire Tables give the diameters, areas and resistances. The AWG to mm² converter uses that geometry and shows the nearest IEC nominal size.
14 AWG is 2.081 mm², with a bare diameter of 1.628 mm:
Is 14 AWG the same as 2.5 mm²?
No. The nearest IEC size is 2.5 mm², but 14 AWG has 17% less copper and about 20% more resistance. 2.5 mm² falls close to 13 AWG (2.62 mm²), a size that is rarely stocked. When you replace a 2.5 mm² conductor with an AWG size, 12 AWG (3.31 mm²) is the smallest AWG size that is at least as large. Large US conductors use kcmil: 1 kcmil is 0.5067 mm², so 250 kcmil is 126.7 mm², between the 120 and 150 mm² IEC sizes.
IEC 60228:2023 defines a nominal size by its maximum resistance at 20 °C, not by a measured area. A class 1 or class 2 plain copper 2.5 mm² conductor must not exceed 7.41 Ω/km. A true 2.5 mm² of annealed copper (0.017241 Ω·mm²/m) gives 6.90 Ω/km. A conductor that just meets the limit therefore behaves like about 2.33 mm² of copper. When you compare a cable from one standard with a cable from another, compare the datasheet resistance, not the size label.
Conductor diameter and cable diameter
Can finished cable diameter be calculated from conductor area?
No. For a solid round conductor, A = πd²/4, so a 95 mm² conductor has a bare equivalent diameter of 11.0 mm. The finished cable is much larger. Stranding leaves gaps, and insulation and sheath add several millimetres. Nexans Australia lists 17.2 mm for its 95 mm² XLPE/PVC single-core 0.6/1 kV cable:
| Nominal area | Bare solid equivalent | Finished outside diameter (Nexans XLPE/PVC single core) |
|---|---|---|
| 16 mm² | 4.5 mm | 9.3 mm |
| 25 mm² | 5.6 mm | 10.5 mm |
| 35 mm² | 6.7 mm | 11.5 mm |
| 50 mm² | 8.0 mm | 12.9 mm |
| 70 mm² | 9.4 mm | 14.7 mm |
| 95 mm² | 11.0 mm | 17.2 mm |
| 120 mm² | 12.4 mm | 19.6 mm |
| 185 mm² | 15.3 mm | 23.5 mm |
| 240 mm² | 17.5 mm | 25.6 mm |
Use this table to judge scale only. Armour, a low-smoke sheath, a higher voltage rating, a different manufacturer or a multicore construction all change the outside diameter. Take the figure from the datasheet for the installed cable, then measure it.
Conductor resistance and temperature
DC resistance is R = ρL/A. Annealed copper is 0.017241 Ω·mm²/m at 20 °C, and the conductor resistance calculator uses 61% of that conductivity for aluminium. Say whether the length is one conductor or the full loop out and back, because the loop doubles the result.
Resistance rises with temperature. For copper, R_T = R_20 × (234.5 + T) / 254.5, which is about 0.39% per °C. For aluminium, the constants are 228 and 248, about 0.40% per °C. A PVC-insulated conductor at its 70 °C limit has 1.20 times its 20 °C resistance. An XLPE conductor at 90 °C has 1.28 times.
A 20 m run of 2.5 mm² copper, out and back, has a loop resistance of 0.28 Ω at 20 °C. At 70 °C it is 0.33 Ω:
Use the datasheet resistance when you have it. The calculator does not know the alloy, strand construction or tolerance. For AC, the skin depth in copper at 50 Hz is about 9.3 mm. Skin effect grows as the conductor radius approaches that figure, which is a conductor of about 270 mm². For large conductors, use the datasheet AC resistance, which includes skin and proximity effects.
Calculate voltage drop
The voltage drop calculator covers DC two-wire, single-phase AC and balanced three-phase AC. For a balanced three-phase circuit it uses the steady-load formula from the Schneider Electric Electrical Installation Guide:
ΔU = √3 × I × L × (R cos φ + X sin φ)
Take a 100 A load at 0.8 power factor on a 400 V supply, fed by 100 m of 25 mm² copper XLPE cable. IEC 60228 allows 0.727 Ω/km at 20 °C, which becomes 0.927 Ω/km at 90 °C. With a typical low-voltage cable reactance of 0.08 Ω/km, the drop is 13.68 V, or 3.42%:
With the 20 °C resistance, the same calculation gives 10.90 V (2.73%). That underestimates the drop by a fifth.
Annex G of IEC 60364-5-52 (Table G.52.1, informative) suggests a maximum drop from the origin of the installation to the load. For an installation supplied from the public low-voltage network, the figures are 3% for lighting and 5% for other uses. For an installation with its own transformer, they are 6% and 8%. BS 7671 Appendix 4 uses the same four figures. The example therefore passes for a power circuit and fails for lighting on a public supply.
Double the route to 200 m and the 25 mm² cable drops 6.84%. 35 mm² gives 5.04%, which is still over the limit. 50 mm² gives 3.83%. The designer selects 50 mm² for a 100 A load, and that decision matters for the current sensor.
Does a voltage-drop result select a safe cable size?
No. Voltage drop is one design check. The cable must also carry the design current in its installation method and let its protective device clear a fault in time. Those come from the wiring rules, and this guide gives no ampacity table.
Check the CT aperture against the cable
The 50 mm² cable from the example is 12.9 mm across the sheath. The ZEM-65 current-sensor options are:
- 120 A split-core CTs, 14 mm aperture, 2 m leads;
- 300 A Rogowski coils, 25 cm long, about 70 mm across when closed, 3 m leads;
- 1000 A and 3000 A Rogowski coils, 120 mm across, 2 m leads.
Each set is calibrated with its meter before it ships. Current transformer selection first builds a generic selection brief, including an installation allowance on each side. A 12.9 mm cable with a chosen 2 mm allowance requires a 16.9 mm round opening. A candidate 120 A sensor with a 14 mm opening covers the 100 A maximum current but fails this mechanical allowance:
The bare diameter difference is 1.1 mm, or only 0.55 mm on each side of a centred round cable. Sheath tolerance, an oval cable or access to the latch can make that impractical. The 2 mm allowance here is an example planning choice, not a universal electrical clearance rule. Check the sensor drawing and the installation before choosing an allowance.
Upsizing for voltage drop can leave a low-current circuit with a large cable. A 95 mm² cable that measures 17.2 mm outside does not pass through a 14 mm aperture even at 60 A. The tool’s optional EpiSensor matching can identify a larger published option to investigate; meter compatibility, low-current accuracy, coil closure and installation access still need confirmation.
How should cable size be checked against a CT aperture?
Measure the outside diameter with calipers at the point where the sensor will sit. Measure clear of glands, lugs and cleats, and take two readings at right angles if the cable is oval. Use the highest current the circuit actually carries per phase, from a clamp-meter reading, the load schedule or an existing meter. The breaker rating is only an upper limit, and it can push you to a larger sensor than the load needs.
Five problems come up on site:
- A sensor around a whole multicore cable measures the sum of the currents in it. With the neutral inside, that sum is close to zero. Without it, the sensor reads the neutral current. The symptom is a phase current far below a clamp-meter reading on one core. Put the sensor around one core, past the gland where the cores separate.
- Where a phase has parallel cables, put every cable of that phase through one coil, all in the same direction. Do not measure one cable and multiply by the number of cables. Parallel cables of different length or arrangement do not share current equally.
- A busbar in a round aperture: a rigid aperture must clear the diagonal of the bar section. A 40 × 10 mm bar needs 41.2 mm, so it needs a Rogowski coil. Keep the coil's clip away from the adjacent phase bars.
- A reversed sensor or a swapped phase makes per-phase power read negative, or gives a power factor that does not suit the load. Check each phase at commissioning, with the load running.
- Split-core CTs and Rogowski coils go on without disconnecting the conductor. The work is still work near live conductors, and the site's safe-working rules apply. In Great Britain this is Regulation 14 of the Electricity at Work Regulations 1989. Isolate the circuit where you can.
Also check that the lead reaches the ZEM without an extension or a tight bend, and that the sensor has clearance from other phases and the enclosure. The current-transformer selection guide covers output type, burden and CT secondary safety. The Rogowski-coil guide covers coil positioning and position error.
Project workflow
Keep one record for each monitored circuit:
- Record the circuit, the load it serves and the cable designation from the schedule, including the manufacturer and cable type.
- Record the maximum current per phase from a measurement or the load schedule.
- Measure the outside diameter or busbar section at the sensor position, and photograph it where site rules allow.
- Run the aperture selector, and choose the larger option if the clearance is under 3 mm.
- At commissioning, compare each phase current with a clamp meter and check that per-phase power and power factor match the load.
The transformer and distribution board guide turns this record into a monitoring plan, and the ZEM electricity monitor page lists the sensor options. If the cable schedule or the installation is uncertain, talk to an engineer. Send the per-phase current, the conductor dimensions and photographs of the panel before you order.