Looking at the lighting circuit in my new house I have realised that if all
light fittings contained bulbs of the maximum allowable wattage and all
lights were switched on at same time then it is seriously overloaded.
There is only one lighting circuit for the whole house and on top of this a
500w PIR floodlight and bugular alarm are also on the circuit.
My long term solution is split the house into two lighting circuits and move
the 500w floodlight to its own MCB or maybe to a FCU on the upstairs sockets
ring. The bugular alarm will also be eventually wired to a 3amp unswitched
But I would like to do a quick fix in the meantime to help me sleep at
night.. So I aim to replace one of the upstairs sockets with a 6amp switched
FCU and wire the floodlight to this.
But I don't know whether it is worth re-wiring the bugular alarm. The specs
state that the power supply rating is at maximim 1A but nothing about the
So my question is: What is the power consumption of 1 amp?
Aim to work one hour less this week than last week and get paid the same.
The fuse in my alarm (the internal fuse on the mains side) is 250mA at 250
volts. So the maximum power consumption is about the same as a 60 watt bulb.
Personally I would leave the alarm on the lighting circuit and remove the
500W PIR from the lighting circuit (or better still remove the 500W light)
Power consumption will be of the order a few watts, maybe 20W max
if it takes a 7AH SLA battery and has to recharge it following a
power outage, dropping back to a lower level when charged.
I would suggest leaving it on the lighting circuit as you will
quickly be aware if that circuit fails, whereas you might not
notice if a dedicated circuit fails (depending what the alarm
does and if you are around at the time). Also, you don't want
an alarm on an RCD protected circuit, and your ring circuit is
more likely to be (now or in the future).
Look up "diversity". I suggest getting hold of a copy of the "On-site
guide", which any electrical wholesaler will have, and which explains
Obviously it depends on the voltage. Power = voltage * current, so
that's 240 VA. For a simple circuit (anything domestic except big
motors) you can ignore the "power factor" and this is equal to 240
Die Gotterspammerung - Junkmail of the Gods
But the usual rules allow no diversity within a lighting circuit: the
maximum demand to be assumed is equal to the connected load, allowing a
minimum of 100 W per lampholder - see Table 1A in the OSG. /Ipso facto/ a
lighting circuit should not be overloaded when all the lights are switched
Diversity is allowed at the consumer unit or distribution board. For
lighting the figures are 66% for domestic, 90% for shops and offices, and
75% for small hotels and guest houses.
I would imagine it's tiny, given that backup batteries are of the order of
3 amp/hour at 12 volts and last for several hours. I'd guess at maybe 10
watts or less. The fuse isn't a reliable guide as it has to handle the
switch on surge of the power supply which might be considerable for a
fraction of a second.
*Bills travel through the mail at twice the speed of cheques *
Dave Plowman email@example.com London SW 12
I believe that the maximum allowable load on a lighting circuit is 1200w. If
all my light fittings contained bulbs of the maximum allowable wattage and
all lights were switched on at same time then the power consumption is more
This is how I worked it out!
Yes, but surely only in industrial buildings would 1201w (via 5 amp fuse) of
lighting be switched on at the same time, if you plugged a 3kw electric fire
into every 13 amp socket in an average domestic ring circuit and switched
them all on at the same time you stand a chance of blowing the main company
fuse off the wall - let alone the 30 amp ring circuit fuse !..
No -- it's not uncommon to end up with every light in the house on
at the same time, unless the house is occupied only by the person
who pays the electricity bill ;-)
By the way, industrial premises would normally have 10A lighting
circuits at a minimum.
But you couldn't realistically do it even if the mains wiring could
cope -- have you any idea how hot your house would get with 100kW
of heat being given off in it?
The previous poster is in any case dead wrong about blowing the main
supply fuse: by deliberate design, the final-circuit-protection on the ring
- whether an MCB or a fuse - will pop well before the 100A/80A/60A cutout
even thinks about popping; the magic term of art being "coordination".
But one of the odder sights at our site earlier this year was indeed
a computer room full of about 60 fan heaters all going full pelt! We've
put in a fancy-pantsy Utility Data Centre ("revenge of the mainframe")
which we're using for internal server consolidation, for trials of
rent-a-cycle, and for Customer Demos. The fan heaters were indeed all
running, to put a 100kW? 120kW? heating load into the then-empty computer
room, to act as a week-or-so-long stress test on the heavy-duty air
conditioning upgrade! (Probably helped to make sure the wiring was up
to snuff, too. Oddly enough, the 11kV?-to-415V-three-phase transformers for
this building are physically rather close to the machine room with the
tightly-packed racks of servers...) The heat management of the server racks
continues to intrigue the engineering-minded among us: one of the factors
in migrating the computing load around the servers is to set up a useful
pattern of warmed-air circulation to help the aircon do its job most
Stefek, rambling again
But would it if some twerp had replaced the 30 amp fuse (or MCB) with a nail
? I suggest you re read what I actually said, or are you really suggesting
that you can exceed the 'company fuse' rating and it won't blow ?!
That's not what I said, or are you saying that there is not a 'power surge'
when a light bulb is switched on, in other words the power consumption is
That was not the point, it was about realistic loads on circuits, as you say
who would want or need more that 7kw of electric fire on in an average house
(and fitted with only one ring circuit).
But if you could, you could run a standard 6-digit electricity meter
'round the clock' in no time and avoid paying for any of the electricity
(so long as the electricity company didn't come to read the meter at an
The "1A" power supply is actually referring to the low voltage side of the
alarm. In power terms, the alarm takes very little current at all.
A 6A lighting circuit can take 1.4kW of lighting, which is a lot,
particularly if you use low energy bulbs rather than disgusting incandescent
bulbs, which should be banned for the selfish waste of resources they are.
Technically, you shouldn't have more than 14 pendant fittings on one circuit
as you should assume 100W per fitting. That still makes a big house.
However, if you use low energy lighting, then it is only a technical
compliance issue and nothing to worry about.
As for the 500W light, I would suggest binning it. They cause immense light
pollution, global warming and road safety issues. There is no sensible
reason whatsoever for having floodlights on a domestic property. If you need
to get from the driveway to the house, a 9W compact fluorescent light
fitting on a timed PIR with daylight inhibition would be fine. If you want
to sit out in the garden in bright light, do so in the daytime, rather than
be single handedly responsible for flooding several Pacific islands and
causing the failure of the Gulf stream.
I thought it was 1200w, at least my DIY book says so. Two of my light
fittings take three 60 watt bulbs and in the bathroom there are three 60
watt spots. So 540W is eaten by three rooms alone. There are 8 other
pendants. So this makes 1340W. With the floodlight this is 1840W. Way too
much. This is maximum wattage by the way, I do have some low energy bulbs
and most pendants have 60W bulbs rather than 100.
With regards to your comments about the floodlight I totally agree. But it
only comes on for 20 seconds at a time when activated. It is not on all
I would like to get rid but the other half won't hear of it.
That would be an old 240V x 5A = 1200W circuit. Current designs should be
for 230V x 6A = 1380W, even though the actual voltage will be 240V. The old
system would probably be a 5A cartridge fuse in a fuse box. The new system
would be on a 6A Type B MCB in a consumer unit. What do you have?
P.S. If the spot fittings (not just any shades, which are easily replaced!)
can only take a certain wattage (i.e. 60W max), it is reasonable to use
that, rather than 100W.
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