6242B Current Carrying Capacity and Volt Drop /A/m

[tony9i9i9i]

Hi, go easy on me please, I've just returned to the trade, well I am about to, I received my Gold Card last Thursday.

I've been gen'ing up for a couple of weeks going through the regs book, watching some fantastic youtube videos, and working through cable calcs and looking at products and materials on Wholesalers web pages etc.

I see in addition to 70C thermosetting T&E 6242Y which is covered in table 4D5 on pg 456 of Amendment 2, there is 90C thermosetting T&E 6242B, which does not have a similar table as far as I can see.

A 2.5mm 6242B would be able to carry more current and run hotter than a 2.5mm 6242Y, right? Although increased resistance from heating would increase volt drop I guess, but still might be a better option than going up a cable size in 6242Y. So why no table? Or does the increased Vd just cancel out the increased current carrying capacity?

Cheers Tony.

 

[davesparks]

A 2.5mm 6242B would be able to carry more current and run hotter than a 2.5mm 6242Y, right?

You can only use it at 90 degrees if every terminal it is connected to is also rated at 90 degrees.
If you are using it with normal switches, sockets, consumer units, light fittings, wago's etc etc then you must not exceed 70 degrees. So for all normal uses of T&E you use the table for 70 degrees.

6242B is used for it's LSF insulation, the fact that it can operate at 90 degrees is merely a property of the LSF insulation and rarely of any practical use.

 

[pc1966]

As @davesparks says you can only use the increased temperature to increase CCC if the end points are rated for that, which is practically unheard of for domestic accessories and probably not something to assume without checking even for industrial stuff either.

Where the increased temperature handling is useful is for odd occasions where you need to run T&E near a hot area, for example close to central heating pipes. There you might be looking at an ambient temperature of 65-70C and so 70C rated cable is unusable, but with 90C rating you can look at derating to around 0.53 (table 4B2 page 441) and with modest size increase you have a solution.

Assuming there is absolutely no other way to avoid a hot area...

While the regs don't offer a 90C table for T&E cables with the methods listed in 4D5, you can use the table 4E2A to get ratings for single-phase CCC that you can apply derating to, and VD values matching those in table 4E2B.

But again, don't do that as a rule, stick with the usual 70C design limits and treat 6242B as cable better suited for situations where fire impact on escaping, etc, is important.

 

[tony9i9i9i]

You can only use it at 90 degrees if every terminal it is connected to is also rated at 90 degrees.
If you are using it with normal switches, sockets, consumer units, light fittings, wago's etc etc then you must not exceed 70 degrees. So for all normal uses of T&E you use the table for 70 degrees.

6242B is used for it's LSF insulation, the fact that it can operate at 90 degrees is merely a property of the LSF insulation and rarely of any practical use.

OK, understood, thanks Dave.

 

[tony9i9i9i]

As @davesparks says you can only use the increased temperature to increase CCC if the end points are rated for that, which is practically unheard of for domestic accessories and probably not something to assume without checking even for industrial stuff either.

Where the increased temperature handling is useful is for odd occasions where you need to run T&E near a hot area, for example close to central heating pipes. There you might be looking at an ambient temperature of 65-70C and so 70C rated cable is unusable, but with 90C rating you can look at derating to around 0.53 (table 4B2 page 441) and with modest size increase you have a solution.

Assuming there is absolutely no other way to avoid a hot area...

While the regs don't offer a 90C table for T&E cables with the methods listed in 4D5, you can use the table 4E2A to get ratings for single-phase CCC that you can apply derating to, and VD values matching those in table 4E2B.

But again, don't do that as a rule, stick with the usual 70C design limits and treat 6242B as cable better suited for situations where fire impact on escaping, etc, is important.

Thanks PC.
I did see Tabels 4E2A and B, but as you say it doesn't have the same methods as 4D5, but looking again I see 1mm for example, using method C, 6242B could carry more current, 19A with (mV/A/m) of 46, while 6242Y could carry 16A with (mv/A/m) of 44.
But as you've both pointed out, it's not really practical to use.

Cheers, Tony.