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.

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

Cable brackets
+ Permitted.
­– Not permitted.
0 Not applicable, or not normally used in practice.

Fig.G8: Selection of wiring systems (table 52-1 of IEC 60364-5-52)

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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.
(see also Fig. G10)

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Situations	Method of installation
	Without fixings	With fixings	Conduit	Cable trunking (including skirting trunking,flush floor trunking)	Cable ducting	Cable ladder, cable tray,cable brackets	On insulators	Support wire
Building voids	40, 46,15, 16	0	15, 16,41, 42	-	43	30, 31, 32, 33, 34	-	-
Cable channel	­56	56	54, 55	0	44, 45	30, 31, 32, 33, 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	44, 45	0	-	-
Surface mounted	-	20,21	4, 5	6, 7, 8, 9, 12, 13, 14, 22, 23	6, 7, 8, 9	30, 31, 32, 33, 34	36	-
Overhead	-	-	0	10, 11	-	30, 31, 32, 33, 34	36	35
Immersed	80	80	0	-	0	0	-	-

– Not permitted.
0 Not applicable, or not normally used in practice.

Fig.G9: Erection of wiring systems (table 52-2 of IEC 60364-5-52)

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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.

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Item No.	Methods of installation	Description	Reference method of installation to be used to obtain current-carrying capacity
1		Insulated conductors or single-core cables in conduit in a thermally insulated wall	A1
2		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		On unperforated tray	C
31		On perforated tray	E or F
36		Bare or insulated conductors on insulators	G
70		Multi-core cables in conduit or in cable ducting in the ground	D
71		Single-core cable in conduit or in cable ducting in the ground	D

Fig.G10: Examples of methods of installation (part of table 52-3 of IEC 60364-5-52)

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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.

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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

Fig.G11: Maximum operating temperatures for types of insulation (table 52-4 of IEC 60364-5-52)

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Correction factors

In order to take environnement or special conditions of installation into account, correction factors have been introduced.
The cross sectional area of cables is determined using the rated load current IB divided by different correction factors, k₁, k₂, ...: [math]\displaystyle{ I^{'}{_B}=\frac{I_B}{k_1.k_2...} }[/math]

I’_B is the corrected load current, to be compared to the current-carrying capacity of the considered cable.

• 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.

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Ambient 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
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

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 A.52-14 of IEC 60364-5-52)

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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.

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Ground temperature °C	Insulation
	PVC	XLPE and EPR
10	1.10	1.07
15	1.05	1.04
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

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 A.52-15 of IEC 60364-5-52)

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• 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.

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Thermal resistivity, K.m/W	1	1.5	2	2.5	3
Correction factor	1.18	1.1	1.05	1	0.96

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 A52.16 of IEC 60364-5-52)

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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.

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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

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

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• 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.

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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

Fig. G16: Reduction factors for groups of more than one circuit or of more than one multi-core cable (table A.52-17 of IEC 60364-5-52)

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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.

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Method of installation			Number of tray	Number of three-phase circuits			Use as a multiplier to rating for
				1	2	3
Perforated trays	31		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		1	0.96	0.86		Three cables in vertical formation
			2	0.95	0.84
Ladder supports, cleats, etc... formation	32		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		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

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 A.52.21 of IEC 60364-5-52)

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

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Number of circuits	Cable to cable clearance (a)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

^aMulti-core cables

^aSingle-core cables

Fig. G18: Reduction factors for more than one circuit, single-core or multi-core cables laid directly in the ground. Installation method D. (table 52-18 of IEC 60364-5-52)

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• 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.

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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 (1)

(1) If the neutral current is more than 135 % of the phase current and the cable size is selected on the basis of the neutral current then the three phase conductors will not be fully loaded. The reduction in heat generated by the phase conductors offsets the heat generated by the neutral conductor to the extent that it is not necessary to apply any reduction factor to the current carrying capacity for three loaded conductors.

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

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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 aluminium conductors, free air or in ground.

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Nominal cross-sectional area of conductors (mm2)	Installation methods
	A1	A2	B1	B2	C	D
1	2	3	4	5	6	7
Copper
1.5	13.5	13	15.5	15	17.5	18
2.5	18	17.5	21	20	24	24
4	24	23	28	27	32	31
6	31	29	36	34	41	39
10	42	39	50	46	57	52
16	56	52	68	62	76	67
25	73	68	89	80	96	86
35	89	83	110	99	119	103
50	108	99	134	118	144	122
70	136	125	171	149	184	151
95	164	150	207	179	223	179
120	188	172	239	206	259	203
150	216	196	-	-	299	230
185	245	223	-	-	341	258
240	286	261	-	-	403	297
300	328	298	-	-	464	336
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	40
16	43	41	53	48	59	52
25	57	53	70	62	73	66
35	70	65	86	77	90	80
50	84	78	104	92	110	94
70	107	98	133	116	140	117
95	129	118	161	139	170	138
120	149	135	186	160	197	157
150	170	155	-	-	227	178
185	194	176	-	-	259	200
240	227	207	-	-	305	230
300	261	237	-	-	351	260

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 A.52.4 of IEC 60364-5-52)

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