Please help - naive electrical calculations dont add up

[PingCrosby]

Can somebody please clarify my assumptions/reasoning is correct.

I am intending on replacing my existing ceiling light(s) in the hallway with recessed downlights GU10/55W

I plan on using 2 lights downstairs and 3 upstairs - the circuits are on two seperate rings. Fused with what reads as a 5 amp fuse.

So upstairs I have four rooms each with 60watt bulbs and the 3 new hall lights giving a total load of (4 * 60watts) + (3 * 55 watts) equivalent to ... 405watts.

Upstairs == 405/240 = 1.68 amps...

From the wiki //www.diynot.com/pages/el/el004.php I calculate i can get away with using 1mm/sq cable. However the current wiring all seems to be 1.5mm/sq (i have some 1mm/sq wire and comparing it to the copper in my house, the house lighting circuit is definitely thicker)

Why would the existing wiring use greater than 1mm/sq for the lighting - everything else in my house has been done on the extreme cheap so why would they have used cable they may not have needed which costs more.

I am obviously missing something in my naive calculations. I thought about voltage drop but my house is so small i doubt the difference.

Using http://www.tlc-direct.co.uk/Technic...230&length=30&submit=Calculate+Min+Cable+Size with the following parameters again gave me 1mm/sq

Cable Type Twin/Earth
Application Lighting
Circuit Type
Ambient Temperature 30°C
Number of Circuits 3
Power 405 Watts
Voltage 240 Volts
Length 30 metres

Required Cable Size 1 mm
Voltage Drop 1.55 Volts.
Percentage Drop 0.67%
Current Load 1.00 Amps
Max Cable Load* 9.10 Amps




Please can somebody help me .. or give my some pointers.

Thanks in advance for reading this far

 

[LoveRocket]

Your lights will not be on rings, they will be radials.

Your calculations are correct, 1mm^2 cable would suffice. 1.5 has been used as it was a standard circuit.

You could use 1mm for your additions, but it would be sensible to use 1.5mm to keep the circuit the same throughout.

 

[PingCrosby]

LoveRocket (felt weird typing that), thanks for your reply.

A big thanks for clarifying the fact my lights are radials not rings as i originally assumed.

One thing does confuse me; could you elaborate more on what you mean by the statement

1.5 has been used as it was a standard circuit.

Thanks for your help thus far.

PS.. I will be using 1.5mm^2 as suggested

 

[LoveRocket]

What I mean is that "1.5mm for lighting, 2.5mm for ring finals" is what many electricians have been taught back when they did their apprenticeships, and "it's always been done like that"

It's actually a good rule of thumb (for lighting circuits) to stick to, as 1.5mm on a 5/6amp OPD is compliant no matter how much thermal insulation gets dumped on to the cables.

 

[Chri5]

As mentioned 1.5mm is generic and most will instal it over 1.0mm because it doesn't cost much more and will allow a broad tolerance when and if any derating is required.

It also tends to allow for larger light circuits for if you need a 10 amp lighting circuit 1.5mm will cope ***


*** in 90% of situations

 

[holmslaw]

..

 

[PingCrosby]

Thanks for all the reply's -- much appreciated.

As an aside - is the work i plan on doing notifiable.

Basically i intend to :-

Downstairs ::

a) Change existing ceiling rose to a JB and install 2 downlights.
b) Replace one way switch with a two way and route 3 core cable to upstairs, wired into a new 2 way switch

Upstairs ::
a) Change existing ceiling rose to a JB and install 3 downlights.
b) Add new light switch (see downstairs 'b')

** upstairs light already a 2-way so now work required.

All cabling to be 1.5mm^2 to keep wiring consistent.

 

[Risteard]

100W lamps should be allowed for each light. Also, the voltage is 230V.

(4*100) + (3*55)

Ib = 565/230 = 2.46A

 

[rebuke]

As an aside - is the work i plan on doing notifiable.

Assuming that the areas downstairs/upstairs are not a kitchen or bathroom, I don't believe so.

Downstairs ::

a) Change existing ceiling rose to a JB and install 2 downlights.

Remember that any junction box that involves screw terminals needs to be accessible for inspection and maintenance - you can get ones that are designed to fit through the holes for downlights, so that might be a good choice...

 

[PingCrosby]

It is my intention to lift the floorboards and screw the JB's to sides of the joists then screw the floorboards back down. As i have to lift the floor anyway to run the new lighting circuit, so may as well just do it all in one go.

What is the definition of accessible ?, everywhere in the house is technically accessible - of course some things may need the wall knocking through to get to

Given the fact that to get at the wiring / JBs in the future i will have to empty the room of furniture lift the carpets and then remove the floorboards - i presume this is acceptably accessible !

 

[LoveRocket]

It is my intention to lift the floorboards and screw the JB's to sides of the joists then screw the floorboards back down. As i have to lift the floor anyway to run the new lighting circuit, so may as well just do it all in one go.

What is the definition of accessible ?, everywhere in the house is technically accessible - of course some things may need the wall knocking through to get to

Given the fact that to get at the wiring / JBs in the future i will have to empty the room of furniture lift the carpets and then remove the floorboards - i presume this is acceptably accessible !

Nope. That's certainly not accessible, and thus not compliant with the regs.

ANY SCREW TERMINALS MUST BE ACCESSIBLE. Having to lift floorboards means they aren't accessible, no matter how you try to dance around it.

Choc-block shoved up into the ceiling void through the downlighter holes is acceptable as it can be accessed by removing the fitting (ie downlighter).

 

[PingCrosby]

Thanks for the reply..that is bad news

So to clarify, screwing a floorboard back down over the JB is not acceptable. The confusing bit is that there are JB's screwed all over the place currently in situ; are these acceptable because the house was built before new laws came in??

I am not arguing the facts btw - but it seems odd that the only difference in accessibility in this instance is the time it takes to access the JB.

For example ::

If the Jb was be pushed up in the light void i still have to get a chair, remove the light and gain access

If the JB in the floorboard i have to pull back carpet, unscrew floorboard.

Only real difference seems to be in time/effort required.

* please note, i am not trying to start an argument here - rules are rules and i can live with it, i just feel like a grumble *

 

[LoveRocket]

No problem marra, I understand that often the regs aren't too intuitive.

What you are planning is NOT acceptable. Screw terminal JBs under floorboards are NOT acceptable. They may have been in the past, I dunno when exactly the rules changed.

Part of the reasoning is that (in general) periodic inspections don't involve clearing rooms, lifting carpets and floorboards. Thus a spark will find any loose/bad/damaged connections behind a switchplate/socket outlet or above a ceiling rose (or in an ACCESSIBLE JB) but will NOT spot any potential dangers in these hidden JBs. They can also be a major pain in the hoop when trying to fault find.

 

[electronicsuk]

Not to mention that in the future, you might put down a laminate or tiled floor above the JBs.

 

[PingCrosby]

Agreed...

I just wanted to use the existing JB's as i am mainly extending cable. My plan was to unscrew the box and replace existing switch wire with a longer one

aHHHH...rules and regs... where would i be without them

Time for a rethink!!!