4.3.7 - When a number of correction
factors applies

In some cases all the correction factors
will need to be applied because there are parts of the cable
which are subject to all of them. For example, if a mineral
insulated cable with p.v.c. sheath protected by a circuit
breaker and with a tabulated rated current of 34 A is run
within the insulated ceiling of a boiler house with an ambient
temperature of 45°C and forms part of a group of four circuits,
derating will be applied as follows:

Actual current rating (lz)

= tabulated current (It) x ambient temperature factor(Ca)
x group factor (Cg) x thermal

insulation factor (Ci)

= 34 x 0.77 x
0.65 x 0.5A =** 8.5A**

In this case, the current rating is only
one quarter of its tabulated value due to the application
of correction factors. A reduction of this sort will only
occur when all the correction factors apply at the same
time. There are many cases where this is not so. If, for
example, the cable above were clipped to the ceiling of
the boiler house and not buried in thermal insulation, the
thermal insulation factor would not apply.

Then,
Iz = It x Ca x Cg = 34 x 0.77 x 0.65 A =** 17.0 A**

The method is to calculate the overall
factor for each set of cable conditions and then to use
the lowest only. For example, if an the way to the boiler
house the cable is buried in thermal insulation in the wall
of a space where the temperature is only 20°C and runs on
its own, not grouped with other circuits, only the carrection
factor for thermal insulation would apply. However, since
the cable is then grouped with others, and is subject to
a high ambient temperature, the factors are:

Ci = 0.5

Ca x Cg = 0.77 x 0.65 = 0.5

The two factors are the same, so
either fbut not both) can be applied. Had they been different,
the smaller would have been used.