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Topic Title: Domestic Smoke Alarm Circuit
Topic Summary: 1.00mm2 or 1.5mm2
Created On: 05 October 2017 09:22 AM
Status: Read Only
Linear : Threading : Single : Branch
 Domestic Smoke Alarm Circuit   - sdltn - 05 October 2017 09:22 AM  
 Domestic Smoke Alarm Circuit   - broadgage - 05 October 2017 09:30 AM  
 Domestic Smoke Alarm Circuit   - AJJewsbury - 05 October 2017 09:40 AM  
 Domestic Smoke Alarm Circuit   - sdltn - 05 October 2017 10:45 AM  
 Domestic Smoke Alarm Circuit   - WiredScience - 05 October 2017 12:12 PM  
 Domestic Smoke Alarm Circuit   - AncientMariner - 05 October 2017 07:01 PM  
 Domestic Smoke Alarm Circuit   - Zoomup - 07 October 2017 09:01 AM  
 Domestic Smoke Alarm Circuit   - geoffsd - 05 October 2017 07:15 PM  
 Domestic Smoke Alarm Circuit   - geoffsd - 07 October 2017 01:03 PM  
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 05 October 2017 09:22 AM
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sdltn

Posts: 7
Joined: 12 September 2017

Hi,

I am installing a circuit for smoke alarms in my new home and have a query over cable size if you can assist please?

The alarms will be mains powered and hard wire interlinked using 3 core and earth with them being the sole devices on the circuit i.e. no lighting etc

I had originally planned to use 1.00mm2 cable but having read about it a bit some interpret the regs as requiring 1.5mm2 for any power circuit other than lighting regardless of the current requirements.

Discussed here:

Link Removed theiet.org/Forums/forum/messageview.cfm?catid=205&threadid=52339&STARTPAGE=1&FTVAR_FORUMVIEWTMP=Linear

What are peoples thoughts on this?

Thanks,

Sam

P.S. I am aware this a new circuit and subject to part P notification. I have a friend who is a qualified electrician who is happy to certify for me after discussion and inspection at 1st and 2nd fix.
 05 October 2017 09:30 AM
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broadgage

Posts: 2380
Joined: 07 August 2007

From a practical point of view, 1.0mm will be fine considering the very small load and the OCPD size.
However a pedantic interpretation of the regs would in my view require 1.5mm.
 05 October 2017 09:40 AM
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AJJewsbury

Posts: 15823
Joined: 13 August 2003

Requiring 1.5mm2 for a simple smoke alarm circuit doesn't seem sensible (the DPC for the 18th Ed tries to side-step the issue by claiming that 'small power' circuits (e.g. bathroom fans) can count as 'lighting') so personally I wouldn't have a problem with smoke alarms wired in 1.0mm2.

In terms of showing compliance with the letter of the 17th Ed, I'd suggest either putting the smokes on the same protective device as a frequently used lighting circuit (often recommended anyway as it reduced the chances of the smokes supply being interrupted for an extended length of time - either accidentally or deliberately) or (looking a bit further down table 52.3) claim that the smoke alarm circuit comes under 'signalling and control' rather than 'power' and so the 1.5mm2 limit doesn't apply (but 0.5mm2 instead).

- Andy.
 05 October 2017 10:45 AM
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sdltn

Posts: 7
Joined: 12 September 2017

Thanks for the quick replies. I was thinking along the same lines as you've said, shame there isn't a black and white, definitive answer for it in the regs. Will discuss with my electrician about what he's happy with as he'll be certifying it after all.

Yes, the circuit will be under RCD protection which also covers lighting among other readily missed things.
 05 October 2017 12:12 PM
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WiredScience

Posts: 318
Joined: 25 January 2012

The instructions for the Kidde Slick-Fit range state

"Total cable length between alarms must not exceed 250m (273yds) and be 1.0mm in diameter, 3 core and earth (BS6243Y). Wiring must conform to the IEE regulations - BS7671, or latest edition."

I don't think I've ever used anything other than 1.0mm for smokes, but I usually wire them to a lighting circuit.
 05 October 2017 07:01 PM
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AncientMariner

Posts: 709
Joined: 14 December 2004

Originally posted by: WiredScience

Total cable length between alarms must not exceed 250m (273yds) and be 1.0mm in diameter, 3 core and earth (BS6243Y).


Interesting, 1.0mm in diameter equates to 0.7854 mm2

And they say to follow the manufacturers instructions...........

Clive

-------------------------
Clive S Carver GCGI IEng MIET MITP
 07 October 2017 09:01 AM
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Zoomup

Posts: 3268
Joined: 20 February 2014

Originally posted by: AncientMariner

Originally posted by: WiredScience



Total cable length between alarms must not exceed 250m (273yds) and be 1.0mm in diameter, 3 core and earth (BS6243Y).




Interesting, 1.0mm in diameter equates to 0.7854 mm2



And they say to follow the manufacturers instructions...........



Clive


It must be a typo and should read 1.0 mm2, not 1.0mm dia.

Z.
 05 October 2017 07:15 PM
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geoffsd

Posts: 1702
Joined: 15 June 2010

If you use flexible cable you can use 0.75sq.mm. or smaller if specified in the product standard.

Apparently 'non-sheathed and sheathed flexible cables' does not come under 'non-sheathed and sheathed cables'.

Such is the nonsense of Table 52.3
 07 October 2017 01:03 PM
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geoffsd

Posts: 1702
Joined: 15 June 2010

I have come across one which specifies 1.5mm. diameter - 1.77sq.mm.

https://04646a9cf351cc0d3888-b8b406d15fe93f790abb5bf0e9ab7ab3.ssl.cf3.rackcdn.com/downloads/product/17238_c93483da07c8b602d5b0aadb5655b4db.pdf?1398865065
Copy and search. It doesn't work without the 's'.

Why does it specify T&E anyway?
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