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Joined: Sep 2002
Posts: 1,498
C
C-H Offline OP
Member
Look at this cable:
http://www.idh.ie/product_detail_guardian.htm

The cable isn't very interesting, but the ampacity tables are. According to this, 2.5 mm2 will do fine with a 32A breaker if it is clipped directly. [Linked Image] [Linked Image]

Joined: Aug 2001
Posts: 7,520
P
Member
We can have a 2.5 sq. mm spur cable from a 30/32A ring circuit, but in that situation there will be a 13A max. fuse at the distant end.

I don't like the high ratings that are being given to cables these days. This is definitely one area where the rules have changed for the worse.

Joined: Dec 2002
Posts: 1,250
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djk Offline
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That cable's aimed at both UK and Irish markets btw. (the core colours displayed on the picture are for the UK market).

Also bare in mind that there are other regulations regarding cable sizes and that the manufacturer's higher declaired ratings are not necessarily taken into consideration as they are only giving the extreme upper limits.

However, I would agree with paul there is a growing tendency to keep costs low by reducing safety margins.

That being said, the vast majority of fires caused by electrical installations here are not caused by under-rated cable. They're usually caused by by-passed / incorrect fusing in old installations where cable is either overloaded or subjected to a continious fault.

However, in general electricans don't design their installations to run at the maximum temp.s sustanable by the cable! I can't see the 2.5mm cable being used for 32A circuits under the current regs here even if in theory it could carry it it would be VERY warm.

Also their foot note:

The above current ratings are based on a ‘single circuit’ in accordance with the IEE Wiring Regulations BS7671, Table 4E2A.

Where a conductor operates at a temperature exceeding 70C it shall be ascertained that the equipment connected to the conductors is suitable for the conductor operating temperature. (BS7671 reg 512-02).

The above-tabulated current ratings should be multiplied by the rating factor (0.8) when conductor-operating temperature has not to exceed a recommended terminal temperature of 70C.
---

When designing a system using ring mains of a normal diameter with that cable at least you could be reasonably sure that if someone did manage to break the ring integrity by botching a DIY job that the cable would just run very warm rather than bursting into flames.

----

I really don't think that those tables are meant for the selection of cable for installation. They are the upper safety limits.

Unless you're planning on really cutting corners and having the electrical installation double as an underfloor / burried in plaster heating system [Linked Image] [Linked Image]

[This message has been edited by djk (edited 03-03-2004).]


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