In article . com,
"geoffr" writes:
I would like to connect a halogen up lighter with a dimmer control to a
lighting circuit. However, the up lighter has a 500W bulb (yes I know
this is expensive to run!) and I am concerned about the load on the
lighting circuit.

500W halogen is probably a K9 lamp. These are available in
300W and (harder to find) 200W. They are also available in
energy saving versions at 375W and 225W, with same light
output as 500W and 300W respectively (internal infra-red
reflective coating reflects the infra-red back onto the
filament). So pick one of the lower powered versions so
you don't need to use a dimmer.

The rating info on the dimmer switch has two sets of figures with one
set, I assume, referring to the min and max wattage of the bulb stated
as 60-500W. The other set refers to 60-300W which I assume refers to
the rating of the dimmer control itself.

This is wrong, but I don't know what the second set is for.
It might the the max rating of halogen lamp.

If this is right this would explain why the light came with a 5A fused
plug, because if the load of the whole light fitting was based just on
the bulb rating then the max load would be 2.1A (500/240) and surely a
3A fuse would have been sufficient.

Filament lamps have a significant switch-on surge. For these
high power halogen lamps, the initial current is 17 times the
running current, e.g. 35A for your 500W halogen. A 5A fuse
will generally withstand this for long enough for the halogen
lamp to warm up, but a lower current fuse may not.

However, if the dimmer switch is rated at 300W then the max load of the
light would increase to 800W (3.3A).

This makes no sense.

The lighting circuit currently has a max load of 400W (1.6A) and
therefore installing the light still leave a total max load of under
5A. As this is protected by a 6A circuit breaker would this be OK.

Others already explained how you need to work out the loading.

If this is OK is there any problem with the light not having the
protection of its own separate 5A fuse as I would cut off the plug and
wire is directly into the circuit.

If it's a portable lamp, it really needs a plug and socket.
You could fit a 5A BS546 round pin socket to the lighting circuit.
The loss of fuse doesn't matter in this case.

The switch for the light would be a standard wall switch and am I
also correct in thinking that these switches are rated at 5A.

Yes, if not more, but it will be written on the back.

--
Andrew Gabriel