Yer wires must not get hot and make yer switchgear hot as well, as that could damage the switchgear etc. A temperature limit of 70 degrees C is set.  The tables referred to are for copper conductors or aluminium conductors. All of those tables referred to are for 70 degree C rated cables.

The 70 degree C limit is met if 70 deg C tables are used for a 90 degree C cable of similar construction. (Last sentence of the second paragraph).

The switch gear, and the insulation of the cable must not be overheated. If you have maximum safe temperatures of 70C switchgear and 90C cable, then the 70C figure applies to the bit of cable attached to the switch or whatever.  But further along the cable route it may be grouped or in thermal lagging and there it may run at up to 90C. 

To be sure the copper of a cable of X mm2 stays below 70C , ensure loads are below the currents tabulated for thermoplastic insulation which are a bit lower than the currents for cables allowed to run hotter.

I thought you would have an emc question ;-)

Mike.

In principle equipment such as circuit breakers must be connected to thermoplastic insulated cables because their maximum operating temprature is 70° C. But the equipment is also allowed to connect to 90° C thermosetting insulated cables provided that when deciding conductor sizes of the cables, a table from Tables 4D1 to 4D4 of Appendix 4 is refered instead of a table from 4H1 to 4H4. 

Sort of - but it's not the kind of insulation that's the constraining factor, but the temperature the conductor runs at.  Themoplastic (e.g. PVC) cables are only rated for 70 degrees anyway, so no further consideration is needed in that case. Thermosetting (e.g. XLPE) insulated cables are capable of being run at a higher temperature though, so some care is needed to make sure that the conductor temperature doesn't exceed 70 degrees at the terminal - and one easy way of achieving that is to use the 70-degree tables. There are other approaches though - there are methods of calculating conductor temperatures for given currents for a cable (often useful as a first step for accurately calculating actual voltage drop when a cable is used below capacity) which can give more accurate results in some circumstances. In principle the 70 degree requirement also applies where there is no insulation involved (thermoplastic or thermosetting) - e.g. when connecting equipment to bare bus-bars (although in type-tested DBs and the like that's usually covered by the product standard rather than BS 7671).

   - Andy.

Thank you for your reply.

I will read the last sentence of the second paragraph and condider your whole reply.

Thank you, Mike.

Your guidance is always significant. Tell me one thing. To 'ensure loads are below the currents tabulated for thermoplastic insulation which are a bit lower than the currents for cables allowed to run hotter', what should I do?

To Andy

You said "one easy way of achieving that is to use the 70-degree tables"

It's very helpful for me to understand what you mean.

Thank you.

Hmm OK.

Let us consider a real example and see if that clears the fog.

Say you have a circuit that will run for long periods at 40A, and also for ease that the air ambient in the switch gear enclosure is the 30C that the tables assume.. (30C in the UK  - never ... ;-) )

you want to know will it be Ok  wired in 4mm² 4 core SWA.

this table based  on 4d4a suggests 35 A for the free air rating based on the copper reaching 70C. (as this table is for SWa with a PVC insulation) so at 35A or below , maybe the copper core reaches 70C, it should not exceed it.

the same chunk of metal (4mm cross sectiom 3 phase etc ) also in free space

reaches 90C at 44A according to this table based on 4E4A same cable contruction, same ambient air etc.

So these two tables together tell us that for the example of a 40A load is likely to get the 4mm² core to some temp above 70C  but below  90C

so although you could have used the 90 degree cable and it would be fine for 40A ,  the copper may run hot enough to damage the switchgear. so you should use 6mm cable instead, at least for the last bit connected to the switch gear. (or more simply, for the whole length) now it runs very cool ...

The swapping between the tables works because it is the same size chunk of copper in both examples, and as it happens the layout and so the thermal behaviour of the bits of plastic are similar. It is more daring to use example  tables for round cables to estimate the current rating of flat ones and things like that, and a bit of rounding in the direction to be safe is then a good idea.

The other common approach is to assume that temperature rise scales with power, so I²R, so if you increase the current by 10% to a first cut resistance is almost unaffected and temp rise increases by 110% times 110% or about 20%.

you can turn this round to estimate how much cooler an oversized cable runs - a 10% current reduction is a 20% power reduction so for example  a 25 degree rise becomes a 20 degree rise perhaps.

Mike.

Edit you can play the thermal game both ways -
If the cable is in a boiler room  at say 50C and the cable was PVC rated for 70C, then a 20 degree rise is all that is acceptable, half the  the 40 degree rise from 30 to 70 in the reference  tables. So we have to de-rate by  sqrt 2 and 35A amps becomes 25 amps.
if the cable is in a freezer room and always below say 0 degrees when the load is in use, you can say "but my insulation is rated for 90C, I can afford an 90 degree rise, instead of a 60 degree one (from 30 to 90)", so the heating can be 1.5 times higher so the current rating can  be sqrt (1.5) ~1.21 times higher,  for the same copper temperature so instead of 44A perhaps allow more like 54amps.

And then you can buy cables with a much higher insulation temperature. Cables to EN50306 WIRE are designed to run with core temperatures of up to  +120C. so you  see what look like very high current ratings

e.g.

and then you need to read the rest of the rather long document that explains the re-rating for groups, lower core temperatures various ambients etc

Mike.

To Andy

You said "one easy way of achieving that is to use the 70-degree tables"

It's very helpful for me to understand what you mean.

Thank you.

Almost pinching Mike's example, think of a 4mm² conductor in a simple multicore cable, installation method B.

According to table 4D2A it's got a rating of 30A single phase - or in other words pushing 30A through two of the cores under standard conditions will raise the conductor temperature to 70 degrees.

Table 4E4A however says 40A - or in other words pushing 40A through it under standard conditions will raise the conductor temperature to 90 degrees.

So one approach to keeping the conductor temperature at 70 degrees or below (in standard conditions) is just to keep the design current below 30A. Nice and simple and easy to demonstrate compliance.

That's not the only approach though. Thermosetting plastics (e.g.XLPE) have somewhat better thermal conductivity than typical thermosetting (PVC) so if you did shove 30A through a XLPE insulated cable the conductor temperature would actually be a little lower than 70 degrees - so you could actually shove a little extra current through and still be within the 70-degree limit. I'd have to dig out the formula, but from memory if table the 4E4A figure and de-rate to 70 degrees it actually comes out at 32A rather than 30A. It's only a small difference, but can be quite handy where you want to demonstrate that 4mm² BS 8436 cable is actually adequate for a 32A radial! (larger sized of that particular cable type being almost unavailable in the UK).

   - Andy.

To Mike

Your explanation using specific figures makes me undersatand 512.1.5 remakably than before. I confirmed the relation between conductor operating tempperature and current-carrying capacity values.

Thank you again.

Greteful to your additional explanation.

Last part of your explanation gave me impression that you have deep knowlege on those tables.

Thank you again.