General method for cable sizing

Possible methods of installation for different types of conductors or cables

The different admissible methods of installation are listed in Figure G8, in conjonction with the different types of conductors and cables.

Fig. G8 – Selection of wiring systems (table A.52.1 of IEC 60364-5-52)

Conductors cables		Method of installation
		Without fixings	Clipped direct	Conduit systems	Cable trunking systems (including skirting trunking, flush floor trunking)	Cable ducting systems	Cable ladder, Cable tray, Cable rackets	On insulators	Support wire
Bare conductors		-	-	-	-	-	-	+	-
Insulated conductors		-	-	+	+	+	-	+	-
Sheathed cables (including armoured and mineral insulated)	Multi-core	+	+	+	+	+	+	0	+
	Single-core	0	+	+	+	+	+	0	+

• + : Permitted.
• - : Not Permitted.
• 0 : Not applicable, or not normally used in practice.

Possible methods of installation for different situations:

Different methods of installation can be implemented in different situations. The possible combinations are presented in Figure G9.

The number given in this table refer to the different wiring systems considered.

Fig. G9 – Erection of wiring systems (table A.52.2 of IEC 60364-5-52)

Situations		Method of installation
		Without fixings	Clipped direct	Conduit Systems	Cable trunking (including skirting trunking, flush floor trunking)	Cable ducting systems	Cable ladder, cable tray, cable brackets	On insulators	Support wire
Building voids	Accessible	40	33	41, 42	6, 7, 8, 9,12	43, 44	30, 31, 32, 33, 34	-	0
	Not accessible	40	0	41, 42	0	43	0	0	0
Cable channel		­56	56	54, 55	0		30, 31, 32, 34	-	-
Buried in ground		72, 73	0	70, 71	-	70, 71	0	-	-
Embedded in structure		57, 58	3	1, 2, 59, 60	50, 51, 52, 53	46, 45	0	-	-
Surface mounted		-	20, 21, 22, 23, 33	4, 5	6, 7, 8, 9, 12	6, 7, 8, 9	30, 31, 32, 34	36	-
Overhead/free in air		-	33	0	10, 11	10, 11	30, 31, 32, 34	36	35
Window frames		16	0	16	0	0	0	-	-
Architrave		15	0	15	0	0	0	-	-
Immersed 1		+	+	+	-	+	0	-	-

• – : Not permitted.
• 0 : Not applicable or not normally used in practice.
• + : Follow manufacturer’s instructions.
• Note: The number in each box, e.g. 40, 46, refers to the number of the method of installation in Table A.52.3.

Examples of wiring systems and reference methods of installations

An illustration of some of the many different wiring systems and methods of installation is provided in Figure G10.

Several reference methods are defined (with code letters A to G), grouping installation methods having the same characteristics relative to the current-carrying capacities of the wiring systems.

Fig. G10 – Examples of methods of installation (part of table A.52.3 of IEC 60364-5-52)

Item No.	Methods of installation	Description	Reference method of installation to be used to obtain current-carrying capacity
1	Room	Insulated conductors or single-core cables in conduit in a thermally insulated wall	A1
2	Room	Multi-core cables in conduit in a thermally insulated wall	A2
4		Insulated conductors or single-core cables in conduit on a wooden, or masonry wall or spaced less than 0,3 x conduit diameter from it	B1
5		Multi-core cable in conduit on a wooden, or mansonry wall or spaced less than 0,3 x conduit diameter from it	B2
20		Single-core or multi-core cables: - fixed on, or sapced less than 0.3 x cable diameter from a wooden wall	C
30		Single-core or multi-core cables: On unperforated tray run horizontally or vertically	C
31		Single-core or multi-core cables: On perforated tray run horizontally or vertically	E or F
36		Bare or insulated conductors on insulators	G
70		Multi-core cables in conduit or in cable ducting in the ground	D1
71		Single-core cable in conduit or in cable ducting in the ground	D1

Maximum operating temperature:

The current-carrying capacities given in the subsequent tables have been determined so that the maximum insulation temperature is not exceeded for sustained periods of time.

For different type of insulation material, the maximum admissible temperature is given in Figure G11.

Fig. G11 – Maximum operating temperatures for types of insulation (table 52.1 of IEC 60364-5-52)

Type of insulation	Temperature limit °C
Polyvinyl-chloride (PVC)	70 at the conductor
Cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR)	90 at the conductor
Mineral (PVC covered or bare exposed to touch)	70 at the sheath
Mineral (bare not exposed to touch and not in contact with combustible material)	105 at the seath

Correction factors

In order to take service conditions of the installation into account, correction factors have been introduced.

The cross-sectional area of cables is determined using the current-carrying capacity of the cable I_Z, multiplied by correction factors: I'_Z = I_Z . k₁ . k₂ ...

where:

I_Z = current carrying capacity of the cable in the reference installation method
I'_Z = "corrected" current carrying capacity of the cable in real installation conditions
k₁, k₂ ... = corrections factors, detailed in the following paragraphs

The cross sectional area of cables is then chosen in order to ensure that their "corrected" current-carrying capacity I'_Z is higher than the rated load current I_B: I_B ≤ I'_Z

Ambient temperature

The current-carrying capacities of cables in the air are based on an average air temperature equal to 30 °C. For other temperatures, the correction factor is given in Figure G12 for PVC, EPR and XLPE insulation material.

The related correction factor is here noted k₁.

Fig. G12 – Correction factors for ambient air temperatures other than 30 °C to be applied to the current-carrying capacities for cables in the air (from table B.52.14 of IEC 60364-5-52)

Ambient air temperature °C	Insulation
	PVC	XLPE and EPR
10	1.22	1.15
15	1.17	1.12
20	1.12	1.08
25	1.06	1.04
30	1	1
35	0.94	0.96
40	0.87	0.91
45	0.79	0.87
50	0.71	0.82
55	0.61	0.76
60	0.50	0.71
65	-	0.65
70	-	0.58
75	-	0.50
80	-	0.41

The current-carrying capacities of cables in the ground are based on an average ground temperature equal to 20 °C. For other temperatures, the correction factor is given in Figure G13 for PVC, EPR and XLPE insulation material.

The related correction factor is here noted k₂.

Fig. G13 – Correction factors for ambient ground temperatures other than 20 °C to be applied to the current-carrying capacities for cables in ducts in the ground (from table B.52.15 of IEC 60364-5-52)

Ground temperature °C	Insulation
	PVC	XLPE and EPR
10	1.10	1.07
15	1.05	1.04
20	1	1
25	0.95	0.96
30	0.89	0.93
35	0.84	0.89
40	0.77	0.85
45	0.71	0.80
50	0.63	0.76
55	0.55	0.71
60	0.45	0.65
65	-	0.60
70	-	0.53
75	-	0.46
80	-	0.38

Soil thermal resistivity

The current-carrying capacities of cables in the ground are based on a ground resistivity equal to 2.5 K•m/W. For other values, the correction factor is given in Figure G14.

The related correction factor is here noted k3.

Fig. G14 – Correction factors for cables in buried ducts for soil thermal resistivities other than 2.5 K.m/W to be applied to the current-carrying capacities for reference method D (table B.52.16 of IEC 60364-5-52)

Thermal resistivity, K•m/W	0.5	0.7	1	1.5	2	2.5	3
Correction factor for cables in buried ducts	1.28	1.20	1.18	1.1	1.05	1	0.96
Correction factor for direct buried cables	1.88	1.62	1.5	1.28	1.12	1	0.90

• Note 1: The correction factors given have been averaged over the range of conductor sizes and types of installation included in Tables B.52.2 to B.52.5. The overall accuracy of correction factors is within ±5 %.
• Note 2: The correction factors are applicable to cables drawn into buried ducts; for cables laid direct in the ground the correction factors for thermal resistivities less than 2.5 K•m/W will be higher. Where more precise values are required they may be calculated by methods given in the IEC 60287 series.
• Note 3: The correction factors are applicable to ducts buried at depths of up to 0.8 m.
• Note 4: It is assumed that the soil properties are uniform. No allowance had been made for the possibility of moisture migration which can lead to a region of high thermal resistivity around the cable. If partial drying out of the soil is foreseen, the permissible current rating should be derived by the methods specified in the IEC 60287 series.

Based on experience, a relationship exist between the soil nature and resistivity. Then, empiric values of correction factors k3 are proposed in Figure G15, depending on the nature of soil.

Fig. G15 – Correction factor k3 depending on the nature of soil

Nature of soil	k3
Very wet soil (saturated)	1.21
Wet soil	1.13
Damp soil	1.05
Dry soil	1.00
Very dry soil (sunbaked)	0.86

Grouping of conductors or cables

The current-carrying capacities given in the subsequent tables relate to single circuits consisting of the following numbers of loaded conductors:

• Two insulated conductors or two single-core cables, or one twin-core cable (applicable to single-phase circuits);
• Three insulated conductors or three single-core cables, or one three-core cable (applicable to three-phase circuits).

Where more insulated conductors or cables are installed in the same group, a group reduction factor (here noted k4) shall be applied.

Examples are given in Figures G16 to G18 for different configurations (installation methods, in free air or in the ground).

Figure G16 gives the values of correction factor k4 for different configurations of unburied cables or conductors, grouping of more than one circuit or multi-core cables.

Fig. G16 – Reduction factors for groups of more than one circuit or of more than one multi-core cable (table B.52.17 of IEC 60364-5-52)

Arrangement (cables touching)	Number of circuits or multi-core cables												Reference methods
	1	2	3	4	5	6	7	8	9	12	16	20
Bunched in air, on a surface, embedded orenclosed	1.00	0.80	0.70	0.65	0.60	0.57	0.54	0.52	0.50	0.45	0.41	0.38	Methods A to F
Single layer on wall, floor or unperforated tray	1.00	0.85	0.79	0.75	0.73	0.72	0.72	0.71	0.70	No further reduction factor for more than nine circuits or multi-core cables			Method C
Single layer fixed directly under a wooden ceiling	0.95	0.81	0.72	0.68	0.66	0.64	0.63	0.62	0.61
Single layer on a perforated horizontal or vertical tray	1.00	0.88	0.82	0.77	0.75	0.73	0.73	0.72	0.72				Methods E and F
Single layer on ladder support or cleats etc.	1.00	0.87	0.82	0.80	0.80	0.79	0.79	0.78	0.78

Figure G17 gives the values of correction factor k₄ for different configurations of unburied cables or conductors, for groups of more than one circuit of single-core cables in free air.

Fig. G17 – Reduction factors for groups of more than one circuit of single-core cables to be applied to reference rating for one circuit of single-core cables in free air - Method of installation F. (table B.52.21 of IEC 60364-5-52)

Method of installation			Number of tray	Number of three-phase circuits			Use as a multiplier to rating for
				1	2	3
Perforated trays	31	Touching	1	0.98	0.91	0.87	Three cables in horizontal formation
			2	0.96	0.87	0.81
			3	0.95	0.85	0.78
Vertical perforated trays	31	Touching	1	0.96	0.86		Three cables in vertical formation
			2	0.95	0.84
Ladder supports, cleats, etc... formation	32	Touching	1	1.00	0.97	0.96	Three cables in horizontal formation
	33		2	0.98	0.93	0.89
	34		3	0.97	0.90	0.86
Perforated trays	31		1	1.00	0.98	0.96	Three cables in trefoil formation
			2	0.97	0.93	0.89
			3	0.96	0.92	0.86
Vertical perforated trays	31	Spaced	1	1.00	0.91	0.89
			2	1.00	0.90	0.86
Ladder supports, cleats, etc...	32		1	1.00	1.00	1.00
	33		2	0.97	0.95	0.93
	34		3	0.96	0.94	0.90

Figure G18 gives the values of correction factor k4 for different configurations of cables or conductors laid directly in the ground.

Fig. G18 – Reduction factors for more than one circuit, single-core or multi-core cables laid directly in the ground. Installation method D. (table B.52.18 of IEC 60364-5-52)

Number of circuits	Cable to cable clearance(a)
	Nil (cables touching)	One cable diameter	0.125 m	0.25 m	0.5 m
2	0.75	0.80	0.85	0.90	0.90
3	0.65	0.70	0.75	0.80	0.85
4	0.60	0.60	0.70	0.75	0.80
5	0.55	0.55	0.65	0.70	0.80
6	0.50	0.55	0.60	0.70	0.80
7	0.45	0.51	0.59	0.67	0.76
8	0.43	0.48	0.57	0.65	0.75
9	0.41	0.46	0.55	0.63	0.74
12	0.36	0.42	0.51	0.59	0.71
16	0.32	0.38	0.47	0.56	0.38
20	0.29	0.35	0.44	0.53	0.66
(a) for Multi-core cables
(a) for Single-core cables

Harmonic current

The current-carrying capacity of three-phase, 4-core or 5-core cables is based on the assumption that only 3 conductors are fully loaded.

However, when harmonic currents are circulating, the neutral current can be significant, and even higher than the phase currents. This is due to the fact that the 3^rd harmonic currents of the three phases do not cancel each other, and sum up in the neutral conductor.

This of course affects the current-carrying capacity of the cable, and a correction factor noted here k5 shall be applied.

In addition, if the 3^rd harmonic percentage h₃ is greater than 33%, the neutral current is greater than the phase current and the cable size selection is based on the neutral current. The heating effect of harmonic currents in the phase conductors has also to be taken into account.

The values of k5 depending on the 3^rd harmonic content are given in Figure G19.

Fig. G19 – Correction factors for harmonic currents in four-core and five-core cables (table E.52.1 of IEC 60364-5-52)

Third harmonic content of phase current %	Correction factor
	Size selection is based on phase current	Size selection is based on neutral current
0 ­­­­­­­- 15	1.0
15 - 33	0.86
33 - 45		0.86
> 45		1.0

Admissible current as a function of nominal cross-sectional area of conductors

IEC standard 60364-5-52 proposes extensive information in the form of tables giving the admissible currents as a function of cross-sectional area of cables. Many parameters are taken into account, such as the method of installation, type of insulation material, type of conductor material, number of loaded conductors.

As an example, Figure G20 gives the current-carrying capacities for different methods of installation of PVC insulation, three loaded copper or almunium conductors, free air or in ground.

Fig. G20 – Current-carrying capacities in amperes for different methods of installation, PVC insulation, three loaded conductors, copper or aluminium, conductor temperature: 70 °C, ambient temperature: 30 °C in air, 20 °C in ground (table B.52.4 of IEC 60364-5-52)

Nominal cross-sectional area of conductors(mm2)	Installation methods of Table B.52.1
	A1	A2	B1	B2	C	D1	D2
1	2	3	4	5	6	7	8
Copper
1.5	13.5	13	15.5	15	17.5	18	19
2.5	18	17.5	21	20	24	24	24
4	24	23	28	27	32	30	33
6	31	29	36	34	41	38	41
10	42	39	50	46	57	50	54
16	56	52	68	62	76	64	70
25	73	68	89	80	96	82	92
35	89	83	110	99	119	98	110
50	108	99	134	118	144	116	130
70	136	125	171	149	184	143	162
95	164	150	207	179	223	169	193
120	188	172	239	206	259	192	220
150	216	196	262	225	299	217	246
185	245	223	296	255	341	243	278
240	286	261	346	297	403	280	320
300	328	298	394	339	464	316	359
Aluminium
2.5	14	13.5	16.5	15.5	18.5	18.5
4	18.5	17.5	22	21	25	24
6	24	23	28	27	32	30
10	32	31	39	36	44	39
16	43	41	53	48	59	50	53
25	57	53	70	62	73	64	69
35	70	65	86	77	90	77	83
50	84	78	104	92	110	91	99
70	107	98	133	116	140	112	122
95	129	118	161	139	170	132	148
120	149	135	186	160	197	150	169
150	170	155	204	176	227	169	189
185	194	176	230	199	259	190	214
240	227	207	269	232	305	218	250
300	261	237	306	265	351	247	282

• Note: In columns 3, 5, 6, 7 and 8, circular conductors are assumed for sizes up to and including 16 mm². Values for larger sizes relate to shaped conductors and may safely be applied to circular conductors.