Old Imperial "7/.029" Twin and Earth

[aesmith]

Hi,

One of our outbuildings is wired with old imperial sized twin and earth, examining it and measuring the cores show that it's the stuff described as "7/.029" with a cross sectional area just under 3 sq.mm for L and N. Looking at the information in the Wiki, the figures given in the 14th Edition show this cable as being lower rated than current 17th Edition figures for 2.5mm2 T&E in spite of the greater cross section. Is there a particular reason for this, or have I missed something? I'm speculating maybe the insulation material was rated for lower temperature back then.

The relevant methods are clipped direct, and within surface mounted conduit.

Thanks, Tony S

 

[OwainDIYer]

The 14th Edition probably assumed rewireable fuses. The 17th Edition assumes MCBs and has a derating factor for rewireable fuses - 0.725 IIRC.

There is (or used to be) also different derating factors for ambient temperature if rewireable fuses were used.

Probably if you apply the relevant derating factors to the 17th ratings you'll get figures more similar to the 14th.

Anyway, 7/.029 is nice stuff.

 

[john4703]

I grew up working with 3/.029, 7/.029 and 7/.044. They were good British made cables.

 

[JohnW2]

The 14th Edition probably assumed rewireable fuses. The 17th Edition assumes MCBs and has a derating factor for rewireable fuses - 0.725 IIRC.

Indeed. The 'fusing current' (I2) of a re-wireable fuse was taken as 2 x In, and that of a Type B MCB is taken as 1.45 x In, whence .................... (1.45 x In) / (2 x In) = 0.725.

Kind Regards, John

 

[Taylortwocities]

Also, remember that "back in the day" Imperial cable had smaller cpc than today's equivalents, making EFLI limits somwhat different.

 

[aesmith]

Thanks. I'll check that out, I've never worked on fused circuits so never needed to use those deratings. (This particular circuit will be a radial, hopefully on a 20A RCBO - hence my concern because if it rates below 20A I'll need to change my plans).

Glad to hear the cable's not rubbish by the way, I've got quite a bit of it to hand.

 

[PBC_1966]

The 14th Edition probably assumed rewireable fuses.

It did, with a note on the tables indicating that ratings can be multiplied by 1.33 where close excess current protection is used (i.e. cartridge fuses or circuit-breakers).

Some of the other ratings are difficult to compare directly, since the various methods aren't consistent between the different editions. I think "clipped direct" is about the only one directly comparable without other differences in limitations as to installation method. A better direct comparison of Imperial vs. metric cable can be had by comparing the original 1966 issue of the 14th edition with the revised 1970 metric edition.

There is (or used to be) also different derating factors for ambient temperature if rewireable fuses were used.

Yes, certainly in the 14th edition; e.g. at 40 deg. C a factor of 0.94 for course vs. 0.87 for close protection, the higher temperature derating for the latter being to compensate for the increase already applied at the nominal 30 deg. C for using close protection.

 

[JohnW2]

The 14th Edition probably assumed rewireable fuses.

It did, with a note on the tables indicating that ratings can be multiplied by 1.33 where close excess current protection is used (i.e. cartridge fuses or circuit-breakers).

That sounds about right. 1/1.33 is about 0.75, very close to the 0.725 factor we now use.

Kind Regards, John

 

[PBC_1966]

Also, remember that "back in the day" Imperial cable had smaller cpc than today's equivalents, making EFLI limits somwhat different.

From the 14th edition, 1966:

 

[333rocky333]

are you sure its not just stranded 2.5mm which was widely availalable as well as the solid stuff

 

[JohnW2]

Also, remember that "back in the day" Imperial cable had smaller cpc than today's equivalents, making EFLI limits somwhat different.

The 'EFLI limits' will obviously have been dependent on the OPD, not the cable, but whether a cable with a small CPC could achieve those limits might have been a different matter! Mind you, the same would have been true of the early 2.5mm²/1mm² T+E.

Kind Regards, John

 

[PBC_1966]

Mind you, the same would have been true of the early 2.5mm²/1mm² T+E.

And on that more general note, it's interesting that BS7671 still allows the reduced earth at all on the smaller sizes of cable. I believe that certainly for smaller sizes such as 1.5 & 2.5 sq. mm most of Europe requires a matching size earth conductor now. Here in the U.S., the reduced ground wire in smaller sizes of NM cable went out in the late 1960's, with 14, 12 & 10 AWG having full-size grounds since then.

 

[JohnW2]

And on that more general note, it's interesting that BS7671 still allows the reduced earth at all on the smaller sizes of cable. I believe that certainly for smaller sizes such as 1.5 & 2.5 sq. mm most of Europe requires a matching size earth conductor now. Here in the U.S., the reduced ground wire in smaller sizes of NM cable went out in the late 1960's, with 14, 12 & 10 AWG having full-size grounds since then.

Haven't the US, and all these other countries, heard of adiabatic calculations - or is it that they don't trust that adequate fault protection will be in place? Given acceptable fault protection (i.e. appropriate EFLI/OPD {and/or RCD} combination), adiabatic calculation will generally show that even the UK's 'reduced' CPCs have appreciably larger CSA than is actually necessary.

Kind Regards, John

 

[aesmith]

Thanks for the comments. I'm pretty sure it's imperial, I measured the strands at 0.72mm diameter - I wouldn't swear to the 1/100 of a millimetre, but to make it 2.5 sq.mm would need a diameter of 0.67. All wired up, tested, connected and working now.

Following up comments I checked out the CPC. Although there are only three strands, they are larger at just under 1mm diameter. This is consistent with the table posted above which specifies three strands of 0.036". Either way if my maths is correct this comes to around 1.9sq.mm. So actually larger than today's cable? I believe even modern 4sq.mm cable has a CPC of 1.5sq.mm, so presumably that's considered enough for a 32A circuit (assuming impedance within limits).

By the way, is there a superscript function? I'm sure I hit it by mistake once, but now I want it I can't find it.

 

[EFLImpudence]

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