cable ratings

[altec]

I have noticed some catalogues stating very different ratings for cable depending on whether the cable is surface mounted, installed in conduit, buried etc. I thought that 2.5mmsq was rated for 27A full stop but a table in this catalogue shows that it can carry up to 33A (i think) in certain conditions. Does 16th edition override the values on this table

Is there a standard rating for the following sizes of cable?

1.5mmsq
2.5mmsq
16mmsq

Does 1.5mmsq flex have the same rating as 1.5mmsq cable?

Thanking You
Declan

 

[Big_Spark]

Declan, c.k., altec, whichever you wished to be addressed by,

There are guidance tables in the rear of BS7671:2001 and in the On Site Guide which give the current carrying capacities of different types of cables in different installation methods. These table also give the volt drop rating of these cables.

Further, and one thing that should also be taken into account when designing installations, is the correction factor tables. These are introduced to calculations to account for the installation of cables in areas outside the norm for electrical installation, or where cables are routed in enclosures that allow the build up of heat etc.

They are not yet available on the net to my knowledge.

 

[altec]

Thank you for your swift response, although i am not c.k. despite the fact that he appears to be replying in my name however he/she is managing it. I don't think he knows himself; he does not come across as having a high aptitude level!

So would it be fair to say that there are no set values for cable ratings and depends on the installation circumstances

 

[Big_Spark]

Declan, there are SET ratings for given installation methods, but these are MAXIMUM permissable ratings.

To calculate the exact maximum permissable for the installation in question you must calculate this from the information in the tables of BS7671:2001

For example, Table 4D2A Multicore 70C Thermoplastic (pvc) insulating and thermosetting insulated cables, non-armoured(COPPER CONDUCTORS)....basically Twin and earth gives a number of maximum crrent ratings for 2.5, these range from 17.5A to 30A, however the higher ratings only apply on perforated tray, which you do not find in your home.

For enclosure in an insulating wall, as you find in domestic premises, the maximum is 18.5A, if this is surface, in mini-trunking, it raises to 23A.

However all current calculations are done at 20C, so if your home is hotter than this you must take that into account as the higher temperature, the lower the cables ability to carry current becomes.

 

[ban-all-sheds]

They are not yet available on the net to my knowledge.

I don't think it's exhaustive, but there's a fair bit of info here

 

[Davey]

If I understand correctly then - what you are saying is that the current carrying capacity of a cable will vary depending on whether it is surface mounted, in an insulated wall, in trunking, etc?

If we consider a ring circuit then where the cable may be surface mounted (eg in the loft) and run down insulated walls and installed in trunking (eg to feed sockets) - what is the current carrying capacity of this circuit?

Is the whole circuit simply rated at the lowest capacity (presumably that of the cable in the insulated wall) or does the length of each type of cable run have to be calculated and taken into account? A ring circuit may be 60/70 metres in length with the majority in trunking - would the capacity of the whole circuit be reduced because a couple of metres may be run in an insulated wall?)

No reason for asking - just curious

 

[Big_Spark]

Davey, in the example you give the maximum permissable rating would be 23A, however you also need to account for the correction factors for given locations, whichever gives the lowest cable rating is the one that must be applied to the entire circuit.

So if your loft is well insulated and it's average temperature is say 23C, you need to calculate the rating for the cable at this temperature, then apply this rating to the entire circuit.

 

[Big_Spark]

They are not yet available on the net to my knowledge.

I don't think it's exhaustive, but there's a fair bit of info here

It's a good and well written piece, but not as exhaustive as the regs, which you will need to do the proper design of an installation. In the past it was not too critical to calculate all the factors that may effect a cable in a domestic installation, unfortunately technology has moved forward and now our homes have more electrical equipment in them, higher rated equipment in the case of some fixed appliances, better insulated walls, external and internal, and more efficient heating systems that all mean we really need to look at Installation design very carefully now.