Fictitious Dimensions for Cable Design Calculations

Traditionally, the thickness of cable coverings has been related to nominal cable diameters using step tables. Variation in calculated nominal diameter can then cause variation in layer thickness for cables of the same basic design.

IEC 60502-2, Cables for rated voltages from 6 kV up to 30 kV, introduces the concept of fictitious calculation to avoid these inconsistencies. The method uses standardised fictitious dimensions for conductor, core and covering calculations.

Fictitious conductor diameter

The fictitious conductor diameter, d_L, is taken from the table below, irrespective of conductor shape or compactness.

CSA, mm²	d_L, mm	CSA, mm²	d_L, mm
1.5	1.4	150	13.8
2.5	1.8	185	15.3
4	2.3	240	17.5
6	2.8	300	19.5
10	3.6	400	22.6
16	4.5	500	25.2
25	5.6	630	28.3
35	6.7	800	31.9
50	8.0	1000	35.7
70	9.4	1200	39.1
95	11.0	1400	42.2
120	12.4	1600	54.1

Fictitious core diameter

For cable cores without semi-conducting layers, the fictitious core diameter, D_c, is:

$D_{c} = d_{L} + 2 t_{i}$

For cables with semi-conducting layers:

$D_{c} = d_{L} + 2 t_{i} + 3.0$

D_c	Fictitious core diameter, mm
d_L	Fictitious conductor diameter, mm
t_i	Nominal insulation thickness, mm

Diameter over laid-up cores

The fictitious diameter over laid-up cores is:

$D_{f} = k D_{c}$

Number of cores	Assembly coefficient, k
Single-core cable	1.00
Two-core cable	2.00
Three-core cable	2.16
Four-core cable	2.42
Five-core cable	2.70

Inner covering or bedding

The fictitious diameter over the inner covering or bedding, D_B, is:

$D_{B} = D_{f} + 2 t_{B}$

D_B	Fictitious diameter over inner covering or bedding, mm
t_B	Bedding thickness, mm
t_B = 0.4 mm	For D_f <= 40 mm
t_B = 0.6 mm	For D_f > 40 mm

Concentric conductors and metallic screens

The increase in diameter due to concentric conductors or metallic screens is taken from the following table. If the cross-sectional area lies between two values, use the larger value for the increase in diameter.

CSA of concentric conductor or metallic screen, mm²	Increase in diameter, mm	CSA of concentric conductor or metallic screen, mm²	Increase in diameter, mm
1.5	0.5	50	1.7
2.5	0.5	70	2.0
4	0.5	95	2.4
6	0.6	120	2.7
10	0.8	150	3.0
16	1.1	185	4.0
25	1.2	240	5.0
35	1.5	300	6.0

Tape screen

For a tape screen, the cross-sectional area is:

$cross-sectional area = n_{t} \times t_{t} \times w_{t}$

n_t	Number of tapes
t_t	Nominal thickness of individual tape, mm
w_t	Nominal width of individual tape, mm

For lapped tape with overlap, the total thickness is twice the thickness of one tape. For longitudinally applied tape, overlap below 30% gives a total thickness equal to the tape thickness; overlap of 30% or more gives a total thickness equal to twice the tape thickness.

Wire screen

For a wire screen, the cross-sectional area is:

$cross-sectional area = \frac{n_{w} \times d_{w}^{2} \times π}{4} + n_{h} \times t_{h} \times W_{h}$

n_w	Number of wires
d_w	Diameter of wire, mm
n_h	Number of counter helixes
t_h	Thickness of counter helix, mm, if greater than 3 mm
W_h	Width of counter helix, mm

Sheaths, bedding and armour

Lead sheath

$D_{p b} = D_{g} + 2 t_{p b}$

D_pb	Fictitious diameter over lead sheath, mm
D_g	Fictitious diameter under lead sheath, mm
t_pb	Thickness of lead sheath, mm

Separation sheath

$D_{s} = D_{u} + 2 t_{s}$

D_s	Fictitious diameter over separation sheath, mm
D_u	Fictitious diameter under separation sheath, mm
t_s	Thickness of separation sheath, mm

Lapped bedding

$D_{l b} = D_{u l b} + 2 t_{l b}$

D_lb	Fictitious diameter over lapped bedding, mm
D_ulb	Fictitious diameter under lapped bedding, mm
t_lb	Thickness of lapped bedding, mm

Additional bedding for tape-armoured cables

Fictitious diameter under additional bedding <= 29 mm: increase by 1.0 mm.
Fictitious diameter under additional bedding > 29 mm: increase by 1.6 mm.

Armour

For flat or round wire armour:

$D_{x} = D_{A} + 2 t_{A} + 2 t_{w}$

For double tape armour:

$D_{x} = D_{A} + 4 t_{A}$

D_x	Fictitious diameter over armour, mm
D_A	Diameter under armour, mm
t_A	Thickness or diameter of armour wire, or thickness of armour tape, mm
t_w	Thickness of counter helix, if any, mm, where greater than 3 mm

CSA of concentric conductor or metallic screen, mm²	Increase in diameter, mm	CSA of concentric conductor or metallic screen, mm²	Increase in diameter, mm
1.5	0.5	50	1.7
2.5	0.5	70	2.0
4	0.5	95	2.4
6	0.6	120	2.7
10	0.8	150	3.0
16	1.1	185	4.0
25	1.2	240	5.0
35	1.5	300	6.0

CSA of concentric conductor or metallic screen, mm²	Increase in diameter, mm	CSA of concentric conductor or metallic screen, mm²	Increase in diameter, mm
1.5	0.5	50	1.7
2.5	0.5	70	2.0
4	0.5	95	2.4
6	0.6	120	2.7
10	0.8	150	3.0
16	1.1	185	4.0
25	1.2	240	5.0
35	1.5	300	6.0