## Fictitious Dimensions

Last updated on 2023-03-31 5 mins. to readTraditionally the thickness of cable coverings has been related to nominal cable diameters by step tables. As there can be variations in calculated nominal diameter, this can cause variations in

## Fictitious Method

**Conductors **- them fictitious diameter, *d _{L}* (irrespective of shape or compactness) is given by:

CSA mm^{2} |
d mm _{L} |
CSA mm^{2} |
d mm _{L} |
---|---|---|---|

1.5 | 1.4 | 15 | 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 |

**Cores **- for cable cores without semi-conducting layers, the ficititious core diameter *D _{c}* is given by:

${D}_{c}={d}_{L}+2{t}_{i}$

and for cables with semi-conducting layers:

${D}_{c}={d}_{L}+2{t}_{i}+3.0$

where:

*D _{c}* - core diamter in mm

*t*- is the nomiman insulation thickness, in mm

_{i}**Diameter over laid-up Cores** - is given by:

${D}_{f}=k{D}_{c}$

where: *k* is the assembly coefficient

k = 1 for single core cables

k = 2 for two core cables

k = 2.16 for three core cables

k = 2.42 for four core cables

k = 2.70 for five core cables

**Inner Covering/ Bedding** - the fictitious diamter *D _{B}* is given by:

${D}_{B}={D}_{f}+2{t}_{B}$

where:

*D _{B}* - diamter or inner covering/bedding in mm

*t*- bedding thickness in mm

_{B}= 0.4 mm for

*D*<= 40 mm

_{f}= 0.6 mm for

*D*> 40 mm

_{f}**Concentric condcutors and metallic screens** - increase in diamter due to screens is given by:

CSA of cocentric conductor or metallic screen mm^{2} |
Increase in diamater, mm | CSA of cocentric conductor or metallic screen mm^{2} |
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 |

Note: if the cross-sectional area lies between two values, that the largest value as the increase in diameter.

- tape screen:

$\text{cross-sectionalarea}={n}_{t}\times {t}_{t}\times {w}_{t}$

where:

*n _{t}* - number of tapes

*t*- nominal thickness of individual tape in mm

_{t}*w*- nominal width of indvidual tape in mm

_{t}for lapped tape with overlap, the total thickness is twice that of one tape

for longitudinally applied type:

- overlap < 30%, total tickness = thickness of tape

- overlap >= 30%, total thichness - 2 x thickness of tape

- wire screen:

$\text{cross-sectionalarea}=\frac{n{}_{w}\times {d}_{w}^{2}\times \pi}{4}+{n}_{h}\times {t}_{h}\times {W}_{h}$

where:

*n _{w}* - number of wires

*d*- diameter of wire in mm

_{w}*n*- number of counter helix

_{h}*t*- thickness of coutner helix in mm (if greater than 3 mm)

_{h}*W*- width of counter helix in mm

_{h}**Lead sheath** - fictitious diamter of lad shearth , *D _{pb}* is given by :

${D}_{pb}={D}_{g}+2{t}_{pb}$

where:

*D _{g}* - fictitious diameter under lead sheath in mm

*t*- thickness of lead sheath in mm

_{pb}**Seperation sheath** - fictitious diameter *D _{s}* given by:

${D}_{s}={D}_{u}+2{t}_{s}$

where:

*D _{u}* - fictitious diameter under separation sheath in mm

*t*- thickness of separation sheath in mm

_{s}**Lapped bedding** - fictitious diameter *D _{lb}* given by:

${D}_{lb}={D}_{ulb}+2{t}_{lb}$

where:

*D _{ulb}* - fictitious diameter under lapped beding in mm

*t*- thickness of lapped bedding in mm

_{lb}**Addition bedding for tape-armoured cables** - provide dover inner covering:

- fictitious diameter under addition bedding <= 29mm, increase 1.0 mm

- fictitious diameter under additional beddign > 29 mm, increase 1.6 mm

**Armour **- fictitious diameter over the armour, *D _{x}* is given by:

- flat or round wire armour:

${D}_{x}={D}_{A}+2{t}_{A}+2{t}_{w}$

where:

*D _{A}* - diameter under armour in mm

*t*- thickness or diameter of armour wire, in mm

_{A}*t*- thickness of counter helix (if any) in mm (if > 3 mm)

_{w}- dobuble tape armour:

${D}_{x}={D}_{A}+4{t}_{A}$

where:

*D _{a}* - diameter under armour in mm

*t*- thickness of armour tape in mm

_{A}