Building Services Engineering 5th Edition Handbook

Electrical installations 297
1
Line 1 red
phase
Line 2 blue
phase
Line 3 yellow
phase
Relative voltage
0
120° 120°
–1
0 45 90 135 180 225 270 315 360
Phase angle, degrees
13.5 Three-phase.
For parallel-connected resistances:
1
R = 1 R 1
+ 1 R 2
+ 1 R 3
+···
The resistance of electrical cables must be sufficiently low that the cables do not become
significant sources of heat and run at temperatures that could be a fire hazard or damage their
electrical insulation. Such heat generation would generally be wasted energy. The maximum
voltage drop in a cable permitted in the Institution of Electrical Engineers (IEE) Wiring Regulations
is 4% of the nominal supply voltage from the consumer’s intake terminal to any point in the
installation at full load current.
Cables that are grouped together, run in conduits or are covered with thermal insulation, say
in a roof space, can operate at a temperature above the 30 ◦ C ambient condition assumed in
the selection of their size and their electrical insulation. Where their temperature is likely to rise
above this value, their current-carrying capacity is reduced by appropriate rating factors during
design of the system. Care should be taken to allow natural cooling of all cable routes. The
current-carrying capacity has to be 1.33 times the design current for cables partly surrounded by
thermal insulation and twice the design current if they are wholly surrounded. This will generally
mean an increase by one or two cable sizes.
EXAMPLE 13.3
Calculate the power consumption and resistance of a 240 V filament lamp if it has 1.5 A
passing through it.