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#133409 - 07/27/02 06:55 AM British cable sizes
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
The question arose recently of British wire sizes and stranded vs. solid conductors.

There is an old British standard known as SWG (Standard Wire Gauge), which works in a similar way to AWG, i.e. larger numbers are smaller wires, but there is no direct correlation between the two. Although SWG was common in radio work, it was not used for electrical cables.

The old cable sizes were specified by a number such as 7/.029 where the first part is the number of strands and the second is the diameter of each strand in inches. All but the smallest standard size were stranded and the strands were tinned.

Standard sizes for applications up to about 100A were: 1/.044, 3/.029, 3/.036, 7/.029, 7/.036, 7/.044, 7/.052, 7/.064, 19/.044, 19/.053, 19/.064. For residential the 1/.044 and 3/.029 sizes were common for lights, with 7/.029 being the usual for our ring circuits and others sized as needed. Although now considered obsolete by the IEE, many of these are still in service.

Since 1970, cables have been made to metric specifications with the size quoted as the cross-sectional area in square millimeters. Standard smaller sizes are 1, 1.5, 2.5, 4, 6, 10, 16, 25 sq. mm. and the conductor is bare copper, not tinned.

In our equivalent of NM-type cable, sizes 1, 1.5 and 2.5 are solid, 4 mm and above are stranded (7 strands for the sizes listed above). The ground wire is one size smaller than the other conductors in all but the 1 sq. mm size.

Single csbles for use in conduit (or for bonding etc.) are similar, except that the 2.5 size is available as both solid and stranded. 2.5 is the usual for our ring circuits now, and is just a fraction larger than #14 AWG.

In general, the metric cables are slightly smaller overall than their Imperial equivalents, but are much harder to work and form, especially given the tight dimensions of many British fixtures and devices.

It's interesting to note that when the change to metric cables was announced, the IEE removed all references to the older standard sizes from the Regs. In my view, this was the wrong move, because even 30 years later somebody might still need to know the ratings of one of those older cables still in service. (Fine if you have a pre-1970 copy of the Regs., of course, but otherwise you have problems.)

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#133410 - 09/23/02 12:24 PM Re: British cable sizes
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Does the UK use the current carrying values (ampacity) of cables without any safety margin? I ask this since I've seen UK netters discuss using 6 mm2 cable for 40A with a 45A fuse, which to me seems like playing with fire.

The ampacity of the cables is simply the IEC standard in all countries, I think. But the way it is interpreted must be different. It would seem a Brit or a German uses the next size up MCB, whereas the rest of us use the next size down. (Possibly only after adding a 10% margin)

E.g. 6 mm2 cable is good for something like 35A single phase. I'd put a 32A fuse. What would you use Paul?

I saw a German manufacturer recommend a minimum of 4 sqmm for 35A 3-phase. In my table 4 sqmm reads 26A, if it's run in a wall...

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#133411 - 09/24/02 02:59 PM Re: British cable sizes
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Under current IEE Regs. the absolute maximum for 6mm cable is 46A (single-phase), but that's if it's "clipped direct," i.e. run exposed on a surface and not enclosed in any way. When buried in a wall in insulation the rating drops to 32A, and that's the highest rating C/B that should then be used.
(This is ignoring any further derating for bundling or ambient temperature.)

I think there is a big problem here over cable ratings. I've mentioned this elsewhere, but almost all of the big DIY places in this country display racks of cable which includes a current rating on the tickets. Almost without exception, they always list the absolute maximum rating and make no mention of the fact that cables may need to be derated. Where, for example, in a modern house, is somebody going to surface run a cable for its entire length?

Rather than go through the figures for 4mm, here's a copy of the ampacity table according to the current IEE rules:


How does this compare with Swedish ratings?

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#133412 - 09/25/02 10:23 AM Re: British cable sizes
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
 Quote:

I think there is a big problem here over cable ratings. I've mentioned this elsewhere, but almost all of the big DIY places in this country display racks of cable which includes a current rating on the tickets. Almost without exception, they always list the absolute maximum rating and make no mention of the fact that cables may need to be derated.

This explains it, I think. Thanks! How about forcing them (by law) to display the values "In wall with thermal insulation" instead?

I only have the ratings supplied by one of the manufacturers and these are valid for single cables in walls. The values listed are slightly higher than those found in the column "In wall in thermal insulation" But then, they are for 25°C ambient. The IEE values are for 30°C ambient, right?

The rule of thumb here is 1.5 sqmm for 10A and 2.5 sqmm for 16A.

BTW. I have seen postings from a Swedish electrician in old threads. Are you still there, Kent?

I'm just an engineer and supposed to be good at engines, not electrics. I found out too late that engines were less interesting than electrics

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#133413 - 09/25/02 12:42 PM Re: British cable sizes
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
I would certainly rather see the minimum ratings posted instead of absolute maximums. The average DIYer just sees the rating on the ticket and says "Oh, it must be all right, the sticker says 46A maximum."

Yes, IEE cable ratings are at a standard ambient temperature of 30 deg. C, 86 deg. F.

I haven't heard from Kent for many months. I don't know if he still drops by to read ECN sometimes.

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#133414 - 10/13/02 08:37 AM Re: British cable sizes
Belgian Offline
Member

Registered: 10/10/02
Posts: 177
Loc: antwerp
Here thereis a code for the maximum rating of circuit breakers in residential installations:
16A 1.5mm2
20A 2.5mm2
25A 4mm2
40A 6mm2
63A 10mm2

Note that the values are different with older cartridge type fuses e.g. 16A 2.5mm2 etc..

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#133415 - 10/13/02 08:53 AM Re: British cable sizes
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
>63A 10mm2

I think I'll stay away from Belgian 63A circuits. It sounds as if it could give you burns...

(For our friends overseas: 10 mm2 is somewhat larger than #8. And 63A means 63A continuous...)

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#133416 - 10/19/02 09:19 PM Re: British cable sizes
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Personally,I would never run a 6mm2 cable on a 40A MCB,you have to take into account the heating effect of the extra current.
Normally we fuse our 6mm2 cables at 32A.
32x1.5= 48A
40x1.5= 60A.
We would normally use a 16mm2 cable for a 63A supply(1 Phase), this is regardless of the fact that the installation is protected by BS88 HRC fuses, it is always backed up by an MCB.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#133417 - 10/20/02 12:08 PM Re: British cable sizes
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
 Quote:
the installation is protected by BS88 HRC fuses

A little off the topic, but...... Are they actually known as BS88 fuses in N.Z., or is there an official Kiwi standard which is referred to in your code?

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#133418 - 10/20/02 01:43 PM Re: British cable sizes
David UK Offline
Member

Registered: 10/03/02
Posts: 134
Loc: Inverness, Scotland
Don't know about NZ, Paul, but Australia use AS 2005 fuses which are marked BS 88. I believe it is the same standard, as the fuses I saw when I worked in OZ were manufactured by our usual UK manufacturers, MEM, etc.
These standards are now being superseeded both here & down under by IEC 269.

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