Tri-rated vs 6491X in trunking and conduit ?

[SimonH2]

Perhaps I'm missing something, but I can't see anything in the wiring regs that says to use 6491X singles and not tri-rated. Plenty of references to non-sheathed singles - but nothing saying any particular specification.
The only reason against tri-rated I can see is that being finer stranded it needs bootlace ferrules at the terminations - but that's hardly a difficult thing to do.
For tri-rated, they go smaller than 1.5mm² which seems to be the smallest 6491X generally available. Interestingly CPC have it in 1mm², but only in blue and black - and they charge as much (or more) as others charge for 1.5mm² 6491X. Table 52.3 allows smaller than 1.5mm².

And (for example) Doncaster Cables have reproduced tables 4D1A and 4D1B adding their own note "Tables Apply to: ST91X, 6491X, 6181Y, TRI-RATED (when installed in conduit or trunking)"

So, is this just one of those "this is what we've always used" and "it's what the wholesaler stocks" things - or is there any technical argument for 6491X / against tri-rated ?

EDIT: Add link for Doncaster Cables

 

[333rocky333]

surely the cost of Tri rated is the main reason for not using it, why pay for it if not needed

 

[SimonH2]

From TLC :
1.5mm² 6491X is £20.81/100m
1mm² tri rated is £13.80/100m
For a lighting circuit supplied by a B6 MCB, 1.5mm² is overkill*, takes more room in the conduit/trunking, and costs more.
For 1.5mm², the tri rated is £18.00 and 6491X is £20.81
Their 2.5mm² are £27.60/100m for both tri rated and 6491X

And if doing control circuits, 0.5mm² is only £8.76/100m - though I don't expect to do enough of that to justify buying a whole roll.

So, for this supplier at least, there seems to be no price penalty for using tri rated - the reverse in fact.

Checking another online supplier (http://www.discount-electrical.co.uk) there does seem to be a price penalty for tri rated - but still a saving from having 1mm² available is there. And at https://www.edwardes.co.uk their 1mm² tri rated is more than their 1.5mm² 6491X. So it looks like TLC is a bit of an oddball in this respect

This surprises me since probably the most significant proportion of the cost of a cable is the materials that go into it. Solid, coarse stranded, or fine stranded - all the same amount of copper and near enough the same amount of PVC. I suspect there's both an element of "people who want tri rated (for it's three approvals) will pay a premium for it" and economy of scale with more 6491X being used.

* Subject to limits on volt drop and circuit impedance for fault clearance.

 

[SimonH2]

Oops, thread title spelling corrected

 

[333rocky333]

Ok, tri rated used to be about 3 times dearer, it seems single normal stuff has increased a lot.

 

[JohnW2]

.... 1mm² tri rated is £13.80/100m .... This surprises me since probably the most significant proportion of the cost of a cable is the materials that go into it. Solid, coarse stranded, or fine stranded - all the same amount of copper and near enough the same amount of PVC.

I suppose it depends upon what you mean by "the most significant proportion" ...

... by my reckoning, 100 m² of a 1 mm² single contains 100 cm³ of copper, which weighs about 896 g and costs (a bit difficult to find a definitive figure, but probably about £5/kg) about £4.50.

As one moves from solid through coarse stranded to fine stranded, the complexity (hence cost) of the manufacturing equipment will presumably increase appreciably.

I suspect there's both an element of "people who want tri rated (for it's three approvals) will pay a premium for it" and economy of scale with more 6491X being used.

Primarily the latter, I would imagine - I would think that the market for tri-rated will be very much smaller.

Kind Regards, John

 

[SimonH2]

Ok, tri rated used to be about 3 times dearer, it seems single normal stuff has increased a lot.

I often find things like that. I'll make some statement based on last time I looked into something, get corrected, and find that it's all changed without me noticing

 

[SimonH2]

Well, as I thought - yes tri-rated was easier to work with than (oversized) 6491X
Just added a heat detector to an existing smoke alarm circuit (interlinked Aico stuff) in one of my rental properties, and it was much better using 3off 1mm² and 1off 0.75mm² (for the interlink - permitted by the MIs) tri-rated than 4off 1.5mm² 6491X. And it leaves a lot more room in the single conduit that's the only route I have for cables between the flat and it's garage.