Max Wattage - Lighting Circuit

[ericmark]

20 x 50w spots = 1000w
11 x 60w bulbs = 660w
Total = 1660w

20 x 35W = 700W (Next size down for 50mm spots)
11 x 22W = 242 (CFL standard size)
So swap either and under 6A.
1360W = 5.9A
1242W = 5.4A
The 6A limit on lighting is due to ceiling roses being rated at 6A.

 

[AdamCH]

The 6A limit on lighting is due to ceiling roses being rated at 6A.

Didn't realise that. What will 1mm T&E carry?

 

[JohnW2]

The 6A limit on lighting is due to ceiling roses being rated at 6A.

Didn't realise that.

It's an interesting speculation, but IIRC BS7671 allows lighting circuits upto 16A.

What will 1mm T&E carry?

If 'clipped direct' (included buried directly in walls etc.) 15A or 16A, depending upon which BS7671 table you use

Kind Regards, John

 

[ericmark]

With the exception of flex cables have a range of current carrying capacity according to how they are installed. Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) 1mm² is rated at 13A. This is often considered as the norm with domestic installation but anywhere between 8A and 16A for 70C thermoplastic insulated and sheathed flat cable. Using thermal setting the rating will be higher.

The regulations give us a limit of 16A for lighting but to use that limit all switches, junction boxes, ceiling roses must also be correctly selected.

In theroy every bulb should contain a fuse so having a BA22d bulb holder rated at 2A is OK as less then 3 meters of cable between bulb holder and the bulb. If the cable is less than 3 meters then also the switch would come under the same rule. The ceiling rose has a duel job one hold the flex to bulb holder and two act as a junction box to switch and next lamp.

Even when the ceiling rose has been removed often a junction box replaces it also rated at 5 or 6 amp so often still restricted to 6A.

In real terms a B6 MCB/RBCO will trip in 0.01 seconds if the loop impedance is below 7.6 ohms with a current of 30A and the power in that time will not cause a problem. However use a B16 and then 80A needs to flow to cause the trip to operate so impedance need to be less than 2.87 ohms if this is exceeded then from bitter experience the bulb can weld into the bulb holder so on blowing 10% chance it will need new bulb holder.

With dimmer switches or power supplies for extra low voltage lamps it could also mean these fail before the MCB opens.

As a result I would not recommend going over the 6A for lighting and clearly not without measuring the loop impedance to ensure it will trip in 0.01 seconds under fault conditions.

Saying that I have seen where 10A MCB's have been fitted. Also where C type have been fitted the latter to try and stop the trip opening when a bulb blows. One problem that has arisen with the use of RCD protection is where stair two way lights cause a shared neutral. What has often happened years ago some one wanted silly 50mm spots which have caused the 6A limit to be exceeded so they have split the lights into two circuits. However when the RCD is fitted it is found a mistake has been made and they need re-combining to one circuit.

If it were me I would be looking at the 20 spot lights and seeing if I could get power from else where. The fused connection unit with a 3A fuse is likely the best option and power some lights from the power circuit with a 3A fuse.

The legacy left specially with ELV 50mm spots is becoming a real problem. 20 - 60W power supplies were common which means less than 20W will cause the supply to auto close down. There are now some 0 - 60W versions but most the older versions would not work with below 20W. A 20W LED is rather bright. It would seem some firms are now marketing dummy loads to allow the change to LED but really this defeats the whole reason for changing.

Where 35 - 105W units were used having one 35W quartz and one LED of the same transformer would get around the problem but in real terms main reason for 12 volt was to extend the quartz bulb life and once not using quartz then may as well move to GU10 style just no point in ELV.

 

[JohnW2]

The regulations give us a limit of 16A for lighting but to use that limit all switches, junction boxes, ceiling roses must also be correctly selected.

I suppose that if one wants to be 'totally safe', then that is theoretically true. However, by analogy, I don't think you'll find a 13A socket or FCU documented as having terminals 'rated at' 32A (the 'moulded on rating' is usually "13A"), yet I can't recall having seen anyone worrying or fussing about that!

Kind Regards, John

 

[Adam_151]

The BS1363 calls for them to be suitable for connecting to a ring circuit, John
This is referenced in BS7671.

If you wanted to connect a 20A DP switch directly to the ring to supply a single spurred socket for your washing machine, then you might want to fuss that its technically non-compliant

 

[JohnW2]

The BS1363 calls for them to be suitable for connecting to a ring circuit, John ... This is referenced in BS7671.

That may be true (I don't doubt you!) but it does not seem to be mentioned in the product 'specifications/ratings' - and it also leads me to wonder whether there might be a corresponding requirement regarding what sort of circuit a ceiling rose must be 'suitable for connecting to'.

If you wanted to connect a 20A DP switch directly to the ring to supply a single spurred socket for your washing machine, then you might want to fuss that its technically non-compliant

In that case, I don't think there would be much cause for 'fuss' since, just as BS7671 allows the cable from switch to socket to be protected by the downstream fuse in the plug, I assume one could correspondingly argue that the switch was also protected by that fuse. If there were just an (unfused) direct connection from the cable to the washing machine, then there clearly would be some cause for 'fuss'!

Kind Regards, John

 

[ericmark]

If I consider my house 8 original rooms with lights then for the first 6 yes the ceiling rose being 6A limits the power to whole circuit but on second to last and last likely within the 3 meters (434.2.1) hence why I said "correctly selected" rather than say what the correct selection would be.

In the main where because of a previous error I needed to do some correction then to fit a 10A MCB would seem a reasonable way around the problem as when one finds the circuits have been previously split but in doing so it had created a borrowed neutral.

However I would not use a 10A MCB to satisfy a whim where some one wanted to use 50W down lighters which were not really required.

To my mind it's not a right or wrong but shades of grey and the person doing the job has to make up his own mind as to if OK or not. I will admit I do consider the next EICR and clearly I would not want to do any work which may in the future be labelled as non compliant.

 

[ban-all-sheds]

In that case, I don't think there would be much cause for 'fuss' since, just as BS7671 allows the cable from switch to socket to be protected by the downstream fuse in the plug..

But it does not allow anything other than BS 1363 accessories to be supplied by a ring final.

 

[JohnW2]

In that case, I don't think there would be much cause for 'fuss' since, just as BS7671 allows the cable from switch to socket to be protected by the downstream fuse in the plug..

But it does not allow anything other than BS 1363 accessories to be supplied by a ring final.

A valid point. That is almost certainly the intent of 433.1.103 (even though the word "only" does not appear in its initial sentence!), but that never seems to have stopped people using JBs in ring finals

Kind Regards, John

 

[ericmark]

1) Lights are not normally wired as a ring.
2) 433.1.103 is not in the original BS7671:2008 it's an amendment so how many are even aware it exists?

What ever circuit the main point is we expect a domestic premises to conform to common practice be this the direction cables feed a switch from or the sizes of MCB's.

Once one deviates from standard practice then there is a likelihood in the future mistakes will be made.

So although one can scrutinise the regulations and find a loop hole which allows so non standard practice this is likely in the future to cause problems.

In theroy we all should get out the previous installation and minor works certificates and see how the home is wired before we proceed with any further alterations however this is rare even if we could find the paperwork.

So the chances are even if the lighting circuits as installed have 16A junction boxes and no ceiling roses or any other item which limits the power to 6A that at some time some one will alter it and use standard lighting fittings which do have a 6A limit.

From adding a ceiling rose to swapping a light switch it is likely where a supply of over 6A is used some one will in the future do something which required a 6A even if at the moment there is no problem.

To swap lamps from 50W to 35W is so easy just can't see the point in moving to a non standard system it's just asking for problems latter.

 

[JohnW2]

1) Lights are not normally wired as a ring.

We know that, but Adam_151 introduced an analogy of a washing machine socket spurred off a ring, which is what was being discussed

2) 433.1.103 is not in the original BS7671:2008 it's an amendment so how many are even aware it exists?

All that changed in the Amendment was the regulation number. Assuming that most people undertaking electrical work are (or should be!) at least familiar with the original BS7671:2008, they will know the regulation as 433.1.5, which (IIRC) had identical wording.

What ever circuit the main point is we expect a domestic premises to conform to common practice be this the direction cables feed a switch from or the sizes of MCB's. ... Once one deviates from standard practice then there is a likelihood in the future mistakes will be made. ... So although one can scrutinise the regulations and find a loop hole which allows so non standard practice this is likely in the future to cause problems. ... So the chances are even if the lighting circuits as installed have 16A junction boxes and no ceiling roses or any other item which limits the power to 6A that at some time some one will alter it and use standard lighting fittings which do have a 6A limit.

I suppose that everything you say is correct in terms of strict compliance and ultra-caution, but I think some might wish/hope that they could apply some well-considered common sense ...

... unless very poorly designed, a lighting circuit should not really afford any opportunities for overload, so we're only really talking about possible fault currents. Those currents will usually be 'hundreds of amps' (for the brief period they flow before a fault is cleared, in a properly designed system), so some would argue that it makes little difference (to what happens to them behaviour when 'hundreds of amps' are flowing) whether accessories in the circuit are 'rated' at 6A, 10A, 16A or whatever.

Kind Regards, John

 

[AndyPRK]

Most light switches are rated at 10A.

Do you have any ceiling roses on this ground floor circuit?

 

[Risteard]

Diversity.

 

[JohnW2]

20 x 50w spots = 1000w
11 x 60w bulbs = 660w
Total = 1660w

Rather than have all these discussions about the current ratings of cables, accessories etc, I wonder if you should not be reconsidering the type of lighting you are using - even at a very modest average of 4 hours per day, that lot would, by my calculation, cost you nearly £400 per year to run.

Kind Regards, John