Cable sizing to REBT and IEC: cross-section and circuit breaker calculator

IEC calculator

System Design current I_b (A) Insulation Installation method Ambient temperature (°C) Grouped circuits Length (m) Power factor (cos φ) Maximum voltage drop

Result 2.5 mm²

Corrected current-carrying capacity I_z: 17.11 A
Voltage drop ΔU: 0.44 %

Copper, table 1 of ITC-BT-19 (in air, 40 °C). For buried cables or aluminium, check the standard.

Standards this guide follows

Current-carrying capacities. Spanish Low Voltage Regulation (REBT), ITC-BT-19, table 1: copper in air at 40 °C. Its values come from UNE 20460-5-523, the Spanish adoption of IEC 60364-5-523, and most manufacturers follow them. For other conditions, ITC-BT-19 refers to the UNE standard (today UNE-HD 60364-5-52, equivalent to IEC 60364-5-52).

Protection and voltage drop. IEC 60364-4-43: coordination between cable and protective device. IEC 60898-1 and IEC 60947-2: circuit breaker ratings. ITC-BT-19, section 2.2.2: maximum voltage drops.

Machines. The internal wiring of a machine and its control panel follows IEC 60204-1 (UNE-EN 60204-1 in Spain), which has its own table.

Values for copper conductors. National regulations may be stricter (in Spain the REBT, in Germany DIN VDE 0298-4, in Italy CEI 64-8, in the USA the local authority having jurisdiction). This guide is for preliminary sizing: always check the calculation against the standard and regulations that apply to your installation.

Installation for the USA or Canada? Use the cable sizing guide according to UL / NEC.

Step by step

The procedure has five steps. First you need the circuit's design current, I_b: for motors, the rated current on the nameplate; if you only know the power of a load, use the kW to amps calculator.

1. Choose the cable type

What matters most is the insulation, because it sets the maximum conductor temperature and therefore the current the cable can carry:

Insulation	Maximum temperature	Examples
PVC	70 °C	H07V-K (single-core), H05VV-F (sheathed flexible)
XLPE or EPR (cross-linked)	90 °C	RV-K, RZ1-K halogen-free (Spanish designations)

With the same cross-section, an XLPE cable carries more current than a PVC one. Many regulations require halogen-free cables in buildings open to the public.

2. Where it is installed: installation method

The standard groups the ways of installing a cable into reference methods. The worse the heat is dissipated, the less current the cable can carry:

Method	Installation
A	Single-core conductors in conduit in a thermally insulating wall
A2	Multicore cable in conduit in a thermally insulating wall
B	Single-core conductors in conduit or trunking on the wall, or in a panel wiring duct
B2	Multicore cable in conduit or trunking on the wall
C	Cable clipped directly to the wall or ceiling, or on an unperforated tray
E	Multicore cable in free air: perforated tray, wire mesh tray or cable ladder
F	Single-core cables touching in free air or on a perforated tray (from 25 mm²)
G	Spaced single-core cables in free air (from 25 mm²)

A wire mesh (basket) tray counts as a perforated tray: method E for multicore cables and F for single-core cables. Buried cables (method D) have their own table and are not covered here.

3. Correct for temperature and grouping

Table 1 of ITC-BT-19 gives the current-carrying capacity I_z0 in air at 40 °C with a single circuit. If the temperature is different or several circuits run together, multiply by two factors:

I_z = I_z0 × k_T × k_G

The temperature factor is k_T = √((θ_max − θ) / (θ_max − 40)), with θ_max = 70 °C for PVC and 90 °C for XLPE.

Factor k_T for ambient temperature, referred to 40 °C

Insulation	25 °C	30 °C	35 °C	40 °C	45 °C	50 °C
PVC	1.22	1.15	1.08	1.00	0.91	0.82
XLPE or EPR	1.14	1.10	1.05	1.00	0.95	0.89

Factor k_G for the number of grouped circuits (table B.52.17)

Installation	1	2	3	4	5	6	7	8	9
In conduit, bunched or embedded (methods A and B)	1.00	0.80	0.70	0.65	0.60	0.57	0.54	0.52	0.50
Single layer on the wall (method C)	1.00	0.85	0.79	0.75	0.73	0.72	0.72	0.71	0.70
Single layer on a perforated tray (method E)	1.00	0.88	0.82	0.77	0.75	0.73	0.73	0.72	0.72

4. Choose the upstream circuit breaker

The circuit breaker protects the cable against overload. IEC 60364-4-43 requires its rating I_n to lie between the circuit current and what the cable can carry:

I_b ≤ I_n ≤ I_z

The standard's second condition, I₂ ≤ 1.45 × I_z, is met automatically by circuit breakers to IEC 60898-1, because their conventional tripping current is 1.45 × I_n. Standard ratings: 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 A.

Curve	Magnetic trip	Typical use
B	3 to 5 × I_n	Resistive loads, lighting, long lines
C	5 to 10 × I_n	General use, small motors, power supplies
D	10 to 20 × I_n	Very high inrush: transformers, large motors

Single-phase circuits use a 1-pole + neutral or 2-pole breaker; three-phase circuits, a 3- or 4-pole breaker. Motors are usually protected by a motor protection switch set to the motor's rated current.

5. Check the voltage drop

If the cable is long, the voltage reaching the load can drop too much. It is calculated like this:

u = b × (ρ × L / S × cos φ + λ × L × sin φ) × I_b ΔU (%) = 100 × u / U₀

b = 1 for three-phase and 2 for single-phase; ρ = 1/γ, with γ = 48 m/(Ω·mm²) for copper at 70 °C (PVC) and 44 for copper at 90 °C (XLPE); λ = 0.08 mΩ/m; L in metres; S in mm²; U₀ = 230 V. ITC-BT-19 limits: 3 % for lighting and 5 % for other uses (3 % in dwellings); in industrial installations with their own transformer, 4.5 % and 6.5 %. If it is not met, increase the cross-section.

Worked example

7.5 kW motor, 400 V three-phase, I_b = 14.15 A; XLPE cable on a wire mesh tray with two other circuits (method E, 3 circuits), 35 °C ambient and 80 m long.

Circuit breaker: the standard rating equal to or above 14.15 A is I_n = 16 A.
Factors: k_T = 1.05 (XLPE at 35 °C, referred to 40 °C) and k_G = 0.82 (3 circuits on a perforated tray).
1.5 mm²: column 9 of the table, I_z = 21 × 1.05 × 0.82 = 18.1 A ≥ 16 A, but the voltage drop is 6.36 %: it fails.
2.5 mm²: I_z = 29 × 1.05 × 0.82 = 25.0 A and a 3.82 % drop: it passes. 2.5 mm² cable and 16 A circuit breaker.

Current-carrying capacity table (REBT)

Table 1 of ITC-BT-19, with the same columns 1 to 11 as the regulation. The first table shows which column each method uses; the second gives the current-carrying capacity I_z0. 3× = three loaded conductors (three-phase); 2× = two loaded conductors (single-phase).

Table column for each installation method

Method	3×PVC	2×PVC	3×XLPE	2×XLPE
A	2	3	5	6
A2	1	2	4	5
B	4	5	8	9
B2	3	4	6	8
C	5	6	8	9
E	6	8	9	10
F	7	–	10	–
G	9	–	11	–

Current-carrying capacity (A) per column, copper, in air at 40 °C

Cross-section (mm²)	1	2	3	4	5	6	7	8	9	10	11
1.5	11	11.5	13	13.5	15	16	–	18	21	24	–
2.5	15	16	17.5	18.5	21	22	–	25	29	33	–
4	20	21	23	24	27	30	–	34	38	45	–
6	25	27	30	32	36	37	–	44	49	57	–
10	34	37	40	44	50	52	–	60	68	76	–
16	45	49	54	59	66	70	–	80	91	105	–
25	59	64	70	77	84	88	96	106	116	123	166
35	–	77	86	96	104	110	119	131	144	154	206
50	–	94	103	117	125	133	145	159	175	188	250
70	–	–	–	149	160	171	188	202	224	244	321
95	–	–	–	180	194	207	230	245	271	296	391
120	–	–	–	208	225	240	267	284	314	348	455
150	–	–	–	236	260	278	310	338	363	404	525
185	–	–	–	268	297	317	354	386	415	464	601
240	–	–	–	315	350	374	419	455	490	552	711
300	–	–	–	360	404	423	484	524	565	640	821

Common questions

Can I use a circuit breaker rated higher than the cable's current-carrying capacity?

No. The rating must be less than or equal to the cable's corrected current-carrying capacity (I_n ≤ I_z); otherwise the cable can overheat without the breaker tripping.

Which method applies to a cable on a wire mesh tray?

A multicore cable on a wire mesh or perforated tray falls under method E of IEC 60364-5-52. Single-core cables use method F, with its own table.

What about short-circuit protection?

The cable must also withstand the short-circuit current until the protection operates (IEC 60364-4-43), and the breaker must disconnect in time in the event of a fault (IEC 60364-4-41). Check it with your installation's data.

Cable sizing according to IEC