310 / 319
3
the luminaires are protected
against overload current by
the characteristics of the load.
And so, answering the original
question, when overload
current protection is not
required the nominal current
of the protective device can
be greater than the current
carrying capacity of the
flexible cord, as in figure 1.
Q
So what if the luminaire is
swapped with one that has a
higher load current than that
of the flexible cord rating?
It is wrong and against the
Wiring Regulations’ guidance
to do this. Before making
any addition or alteration to
an existing installation, you
must check that the rating
and condition of any existing
equipment is adequate to
carry the additional load. This
is a fundamental requirement
for safety.
Q
How do I calculate the
conductor cross sectional
area of the flexible cord?
Appendix 4 of the Wiring
Regulations tells how to go
about this process.
I
t
≥
I
b
–––––––––
C
a
C
g
C
I
Where:
I
t
= the value of current
tabulated for the type of
cable and installation method
concerned, for a single circuit
in an ambient temperature of
30°C:
I
b
= load current;
C
a
= rating factor for ambient
temperature;
C
g
= rating factor for grouping;
C
I
= rating factor for thermal
insulation.
Having calculated I
t
, this value
can then be used to select the
appropriate cross sectional
area of flexible cord from
the relevant table in Wiring
Regulations.
Q
When calculating the load
current are there any special
factors for discharge lamps?
Discharge lamps take a higher
than normal current during
starting. This current may
be up to several times the
conductor current rating.
Generally the duration of this
starting current is considered
not long enough to cause
unacceptable overheating of
the conductors.
The important characteristics
of the starting current are the
magnitude of the current and
its duration.
However, the flexible cord
must be capable of carrying
the total steady current of the
lamp(s) and any associated
gear and also their harmonic
currents.
The IEE Guidance note 1
states: “Where more exact
information is not available,
the demand in volt-amperes is
taken as the rated lamp watts
multiplied by not less than 1.8.
This multiplier is based upon
the assumption that the circuit
is corrected to a power factor
of not less than 0.85 lagging,
and takes into account control
gear losses and harmonic
current.”
Q
Does the type of
lampholder used affect the
current rating of the circuit
protection device?
Yes. Regulation 559.6.1.6
specifies a maximum rating
of the overcurrent protective
device to be 16A where the
lighting circuits incorporate
B15, B22, E14, E27 or E40
lampholders.
Table 1 Overcurrent protection of lampholders
Type of lampholder
Cap type
Maximum rating of
overcurrent protective device
protecting the circuit (A)
BS5042
or
B15 SBC
6
BS EN 61184
B22 BC
16
Bayonet type
BS EN 60238
E14 SES
6
Edison screw
E27 ES
16
E40 GES
16
Note: Where overload protection is omitted, then a calculation must be
made to ensure that the conductors concerned are large enough to carry
the fault currents without damage until the overcurrent device operates.