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Topic Title: Is 1mm² still a reasonable minimum for domestic lighting circuits
Topic Summary: You get a lot of light for 6 or 10 amps nowadays
Created On: 11 November 2017 04:54 PM
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 11 November 2017 04:54 PM
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tjs2

Posts: 50
Joined: 05 February 2005

With the majority of newly installed lighting now being LEDs, is 1mm² still a reasonable minimum for domestic lighting circuits?

A good 15w LED gives about the same light as a 100w incandescent. LEDs need to have a minimum power factor of 0.5.

So applying that to the domestic lighting circuit 6 amp design current (i.e. assuming the same lighting levels as the 6 amp circuit could offer with incandescent bults) would give you:

6 amps * (100 watts / 15 watts) / 0.5 = 1.8 amps.

Terminals etc. would have to be suitable for any new cable diameter which was approved I realise...

Just wondering if the powers that be had pondered it at all?
 11 November 2017 05:11 PM
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mikejumper

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I'd be reluctant to use cable with a smaller CSA purely on the grounds of mechanical robustness.
 11 November 2017 05:17 PM
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geoffsd

Posts: 1760
Joined: 15 June 2010

I think you are looking at it from the wrong position.

If, as is usual, the lighting circuit has a 6A MCB then you require a cable with a CCC of more than 6A.

1 sq.mm. has a CCC of 16A clipped direct, and even the maximum derating of 50% running through thermal insulation it is still more than adequate.
 11 November 2017 06:00 PM
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broadgage

Posts: 2435
Joined: 07 August 2007

I think that 1.0mm is a reasonable size, it is fine on a 6 amp OCPD under all normal circumstances even with grouping or de-rating for thermal insulation.
Anything smaller might need more thought/calculation/argument.
Anything smaller might be no cheaper if only produced In limited volumes.
Voltage drop would also be more significant with smaller cable.
 11 November 2017 07:11 PM
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Zoomup

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Joined: 20 February 2014

1.00 mm2 is good enough for me, and was good enough for my dad, and good enough for his dad, and even his dad before him even on a 5 Amp. fuse. So why would we need a bigger size for domestic lighting? Anything smaller...such as what? Bell wire?

Z.
 11 November 2017 07:29 PM
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leckie

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Well my grandad use a gas lamp
 11 November 2017 08:21 PM
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sparkingchip

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Generally I only use 1.5 mm for lighting circuits to reduce the number of rolls of cable I carry about.

Also most lighting circuits have cables running through considerable thicknesses of insulation in various locations, so the oversized cables resolves any issues.

Andy B.
 11 November 2017 08:23 PM
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tjs2

Posts: 50
Joined: 05 February 2005

Originally posted by: Zoomup
Anything smaller...such as what? Bell wire?
Z.


That's my point - there are no approved cables (except for signalling) smaller than 1 mm² at the moment.

In part that's because when the cable sizes were defined there was seldom a use for anything smaller than 1 mm² for power circuits. So 1 mm² CSA was a reasonable minimum size for low power final circuits.

Now with a 6-fold drop in power requirements for domestic lighting, the power capacity of 1 mm² is significantly over spec for new lighting circuits with LED fittings, so would the introduction of a smaller CSA LV insulated cable (with suitably downsized protections) be reasonable?

The regs already allow the use of 0.5 mm² CSA conductors for signalling and control, and you could reasonably use this cable diameter for domestic LED based lighting circuits (so long as handled and terminated properly) with appropriately MCBs.

About the only reason I can see is mechanical strength and extra care needed when stripping cables etc., but if 0.5 mm² is OK for "signalling and control" (and I can go out and buy BS6004 0.5 mm² singles already), and if the correct tools and terminations were used (and maybe even muti-strand conductors mandated?), it seems like it could be viable.

I'm not saying "it definitely should be done", but was wondering if it had been considered?
 11 November 2017 08:24 PM
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sparkingchip

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Bear in mind if you stock 1.0 mm and 1.5 mm it is four rolls of cable.

Similarly I only stock two way light switches, I don't carry one way switches around with me.

Andy B.
 11 November 2017 08:33 PM
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tjs2

Posts: 50
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Originally posted by: sparkingchip

Bear in mind if you stock 1.0 mm and 1.5 mm it is four rolls of cable.

Similarly I only stock two way light switches, I don't carry one way switches around with me.


Good point.

I should add, that I'm not saying it should be mandatory, but I'm just wondering if the regs should be extended to make it an allowable option in future?

As mikejumper said - it might just be deemed not robust enough, but then 0.5 mm² is already allowed for "signalling and control" - which I presume includes using it to operating things like contactors (and contactors use the same sort of power that LED lamps do).
 11 November 2017 08:58 PM
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tjs2

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Originally posted by: sparkingchip
Also most lighting circuits have cables running through considerable thicknesses of insulation in various locations, so the oversized cables resolves any issues.
Andy B.


I thought it was pretty difficult to get the capacity of 1 mm² below 6 amps, even if fully surrounded by insulation (only if grouped and or at high ambient temperatures)?
 11 November 2017 09:20 PM
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sparkingchip

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The quick reference is the On Site Guide table 7.1(ii) page 69.

1.0 mm ranges from ref C clipped direct 16 amps to ref 103 surrounded by thermal insulation including in a stud wall with thermal insulation with cable not touching wall 8 amps.

1.5 mm ranges from ref C clipped direct 20 amps to ref 103 surrounded by thermal insulation including in a stud wall with thermal insulation with cable not touching wall 10 amps.

So using 1.5 mm universally is over the top, but reduces stock and its generally a more useful size and also a requirement for emergency lighting circuits for mechanical strength.

Andy B.
 11 November 2017 09:32 PM
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sparkingchip

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Price wise including VAT 1.0 mm T&E is around 27 pence per metre and 1.5 mm is 35 pence per metre.

So the price difference is a full 30%, so the cost deserves full consideration if the is s reasonable of cable being used.

Andy B.
 12 November 2017 08:11 AM
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Zoomup

Posts: 3366
Joined: 20 February 2014

Originally posted by: sparkingchip

Generally I only use 1.5 mm for lighting circuits to reduce the number of rolls of cable I carry about.



Also most lighting circuits have cables running through considerable thicknesses of insulation in various locations, so the oversized cables resolves any issues.



Andy B.


I generally only carry 50m of 1.0mm2 for lighting on my jobbing work vehicle. It is lighter in weight and easier to trow about in lofts. 100m of 1.5 T&E would cost me more fuel to cart about.

Z.
 12 November 2017 08:15 AM
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Zoomup

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Joined: 20 February 2014

Perhaps we can wire lighting circuits in 0.75mm2 3 core flex? I have just installed a customer's 400 Watt panel heater. The heater was supplied with a 0.75mm2 integral flex and a 13 Amp. moulded on plug fitted with a 13 Amp fuse. So there you are, if we apply the same logic to fixed wiring cables as to appliance cables then we can use 0.75mm2 flex for lighting circuits fused at 13 Amp.

559.6.1.6

Z.
 12 November 2017 09:07 AM
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dustydazzler

Posts: 1449
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As above , when domesticated jobbing I only ever carried 2 rolls of twin and earth 1mm and 2.5mm. That's it.
Plus a van full of white plastic switches , sockets , surface boxes etc etc
One could argue we could now use 0.75mm flex for led lighting circuits but it's just more cable stock to carry
 12 November 2017 10:28 AM
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sparkingchip

Posts: 10104
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Three core and earth?

Potentially on the van you could have:
1.0mm 2+E
1.0mm 3+E
1.5 mm 2+E
1.5 mm 3+E

So only having the 1.5 mm reduces the load by two rolls, at a cost.

Don't forget smoke alarms, ventilation and central heating wiring jobs.

Andy B.
 12 November 2017 11:12 AM
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tomgunn

Posts: 3929
Joined: 25 May 2005

Originally posted by: leckie

Well my grandad use a gas lamp




Bet my dad could beat your dad!

-------------------------
Tom.... (The TERMINATOR).

handyTRADESMAN

Castle Builders
 12 November 2017 12:49 PM
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dustydazzler

Posts: 1449
Joined: 19 January 2016

Originally posted by: sparkingchip

Three core and earth?



Potentially on the van you could have:

1.0mm 2+E

1.0mm 3+E

1.5 mm 2+E

1.5 mm 3+E



So only having the 1.5 mm reduces the load by two rolls, at a cost.



Don't forget smoke alarms, ventilation and central heating wiring jobs.



Andy B.


never carried 1.5mm unless it was specified or I felt it was needed.
sometimes carried a 50m roll of 1mm3C+ earth if I knew I was wiring smokes or wiring a 2way circuit.
but only carried 1mm and 2.5mm twin and earth as a matter of course
 12 November 2017 01:06 PM
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Blencathra

Posts: 16
Joined: 07 November 2017

I carry 1.0, 1.5 twin & 3 core, 2.5, 4.0, 6.0, cat5 indoor & outdoor, hi tuff 1.5, HR flex 1.5, selection of singles, 2 core flex for small appliances plus other odds. My van is decked out split level with Ply and the "ground floor" is full of cable reels
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