Page 215 - Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 199

Determining the continuous current for a feeder to a switchboard or to its incoming transformer
is a little more complicated. All the loads in a group need to be identified into continuous loads,
intermittent loads and de-energised standby loads.
The individual loads may be known to have diversity from their nameplate values. If this
is the case then the diversity should be included in the estimation of the consumed power. The
total load is estimated by adding together the continuous loads, a nominal proportion of the inter-
mittent loads e.g. 30 to 50%, and a small proportion of the standby loads e.g. zero to 10%. The
summation should be carried out in two parts, the first part for the active power and the second
part for the reactive power. This is necessary because not all the loads in a group have the same
power factor. Once these two totals are estimated the total volt-amperes can be found and then
the current.
A feeder to a switchboard should be sized on the basis of the known loads at the plant design
stage plus a contingency for future expansions. Oil industry plants tend to be upgraded and expanded
once or even several times during their lifetime. Hence a contingency of typically 15% to 25% should
be added to the feeder current estimated above.
See Chapter 1 for examples of loading and load flow estimation.

9.4.2 Continuous Rated Current of a Cable

A given size of a bare circular section conductor will carry a certain current when it is placed in
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still air at 25 C and allowed to have a surface temperature of say 85 C. If the same conductor is
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surrounded by insulating material and also placed in still air at 25 C, it will need to carry less current
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in order to maintain a surface temperature of 85 C. The electrical insulation will act as thermal
insulation. The more layers of thermal insulation that are added e.g. screens, sheathing, armouring,
the lower the current will need to be for the same conductor temperature.
The maximum surface temperature of the conductor is determined by the thermal and phys-
ical properties of the insulating materials. Some materials melt or deteriorate at lower temperatures
than others.

When a cable is placed in a group of cables on a rack, directly buried in the ground, or laid
in underground ducts the surroundings provide additional thermal insulation. Each situation adds a
different amount. The overall effect is to reduce the rated current of the cable when compared to its
performance in still air by itself.
A similar reduction in rated current occurs when several conductors are combined in one cable.
Single-core cables can carry more current than three or four core cables. Vertically run cables carry
less current than those run horizontally by a factor of approximately 5%, due to the convection of
heat given out by the lower part of the cable.
The above thermal insulating effects are taken into account by the manufacturers of cables,
before they publish their tables of rating data. International standards such as BS5467, BS6724,
BS7671 and IEC60364 also provide tables of rating data. Care should be taken when using or
comparing these tables of data because they are not necessarily compiled on the same basic parameters
e.g., ambient air temperature, standard ground temperature.
The following tables of current ratings are typical for the cable constructions and service
voltages given. There are many tables available in the international standards e.g. BS7671, which is