Cable Size

[presta]

Does anyone know why the actual conductor size in electrical flex is a third smaller than the size marked on the sheath?

I've just bought some 0.75mm^2 flex that's actually 0.5, and some 1.5 that's actually 1.0. I've also measured some old cables, and they're a lot smaller than I thought they were as well. Co-pilot's suggesting tolerance, and since it's stranded cable, the packing density of circles, but I don't think that cuts the mustard. I've checked my calliper using feeler gauges, and it's accurate.

 

[JohnHughes307]

I believe that the measurement relates to the area rather than the diameter.

 

[newts]

I believe that the measurement relates to the area rather than the diameter.

Correct.

 

[chris-suffolk]

I believe that the measurement relates to the area rather than the diameter.

Im guessing that the OP knows it's the area rather than the diameter, as they state that

"I've just bought some 0.75mm^2 flex"

So guessing that's not the reason.

 

[mistyoptic]

Im guessing that the OP knows it's the area rather than the diameter, as they state that

"I've just bought some 0.75mm^2 flex"

So guessing that's not the reason.

But he’s said he’s using calipers to measure

 

[oxoman]

If the conductor's diameter is 2 mm, the CSA would be:
CSA = π * (2 mm / 2)² = π * (1 mm)² ≈ 3.14 mm².

 

[CXRAndy]

Just get the power/amperage and voltage ratings for said job

 

[MontyVeda]

But he’s said he’s using calipers to measure

digital calipers have a circumference setting

 

[presta]

The 1.5mm^2 flex has 48 strands of 0.16mm diameter, so:

0.16/2 = 0.08mm radius
0.08^2 = 0.0064
0.0064π = 0.02011
48x0.02011 =0.965mm^2

IE: It's two thirds of the size it's supposed to be, similarly for the other cable, which has 24 strands the same size. I didn't spell it out because I thought it seemed obvious that cross sectional area is calculated from diameter.

Anyhow, the flex is made to BS EN 50525-2-11 which refers to 'nominal' size, then references EN 60228 for the conductors. That spec lists a maximum resistance of 26Ω/km for 0.75mm^2, and 13.3Ω/km for the 1.5. From that, if I calculate the implied minimum sizes for copper conductors (ρ=16.8nΩm) they're 0.6mm^2 and 1.3mm^2 respectively, so the cables still don't meet the specification, unless the spec has been relaxed since 2004, which is the newest copy I can get free access to.

BS EN50525-2-11
EN 60228

This chart suggests that the maximum resistance hasn't changed on the latest (2023) spec.

I've just measured a few sizes of T&E (2.5, 6, & 10mm^2), and they're all the size they claim to be, so whatever's going on, they seem to reserve it just for flex.

 

[oxoman]

Twin and earth cables are solid cores where as flex and tri-rated are multiple small strands hence various different calculations.

 

[presta]

Twin and earth cables are solid cores where as flex and tri-rated are multiple small strands hence various different calculations.

1) The 6mm & 10mm T&E cables are stranded conductors.
2) The calculation of cross sectional area is the same regardless: the area of one strand multiplied by the number of strands.

 

[oxoman]

1) The 6mm & 10mm T&E cables are stranded conductors.
2) The calculation of cross sectional area is the same regardless: the area of one strand multiplied by the number of strands.

TBH i only expect dryers to use up to 2.5mm. The thought of anything bigger in diy hands is worrying. Luckily I dont have to worry about diyers much nowadays.

 

[Regular.Cyclist]

We received a medical device last week that was fitted with a 0.75mm2, two core, mains cable. It’s a class 2 device so doesn’t require an earth. Inspection of the mains plug during acceptance testing found that the manufacturer, an Italian company, had fitted a 13A fuse. This was taken up with the UK supplier who contacted the manufacturer. Their response was basically “so what?” and they are refusing to take corrective action or issue a field safety notice.

It’s been reported to the UK regulator.

 

[oxoman]

TBH i see your issue and technically / legally its wrong to be supplied like that or for a supplier to respond in said fashion. However in practice we would just fit the correct sized fuse. Used to hate PAT testing with the factory so ditched it ASAP, standard practice was to carry a hand full of different fuses to change as required.

 

[Regular.Cyclist]

TBH i see your issue and technically / legally its wrong to be supplied like that or for a supplier to respond in said fashion. However in practice we would just fit the correct sized fuse. Used to hate PAT testing with the factory so ditched it ASAP, standard practice was to carry a hand full of different fuses to change as required.

In a domestic situation I would probably fit a new fuse and be done with it but, in my professional life, it’s very different. The supplier of these devices sell to hospitals, care homes and even members of the public themselves. I would like to think hospitals would pick up the issue and deal with it correctly. Care homes and members of the public I am a lot less sure about so this is something that the regulator is required to act on. I hope to see a safety recall on all devices sold.