Temperature rating of C.B's, cables and conduits

but would you please clarify what you mean by  “that you can sometimes get a better result by taking 90 degree ratings and derating them to get a 70-degree conductor temperature than you can by using 70-degree tables directly” because I think there is a contradiction. and how one can guarantee the operating temp is 70c.

There's a formula relating the derating of a cable to its conductor temperature (I don't have it to hand but should be able to dig it out if needs be) - from memory it's similar to the one for adjusting voltage drop when a cable isn't fully loaded. I'm sure I've seen it used to justify the use of 1.5mm² MICC cable on ring circuits in one of the IET books - showing that the 70 degree terminal temperature isn't exceeded.


Anyhow I wanted to show that 4mm² BS 8436 (circular) cable was good for 32A method B with 70 degree conductor temperature - the 70 degree tabulated rating (4D2A) was only 30A, but taking the 90 degree rating from table 4E2A (40A) and applying the de-rating factor from the formula handily gave 32A. On the face of it that sounds inconsistent, but I think the difference is accounted for by (thermoplastic) PVC used in 70 degree cables having a higher thermal insulation value than (thermosetting) XLPE used in 90 degree ones - hence all else being equal the XPLE insulated conductor can produce a little more heat (by carrying a little more current) than the PVC insulated one for the same conductor temperature.


  - Andy.

Dear Andy, I share your wonders concerning that derating 90c conductor for installation condition would end up that conductor is operating at 90c but would you please clarify what you mean by  “that you can sometimes get a better result by taking 90 degree ratings and derating them to get a 70-degree conductor temperature than you can by using 70-degree tables directly” because I think there is a contradiction. and how one can guarantee the operating temp is 70c.

Question 1: All PVC conduits in market to BS:61386 are rated for 60C operating temp while CU/PVC single core wires to IEC 60228 are rated for 70C. what is the BS 7671 point of view.

BS 7671 doesn't state anything clearly, but it sounds reasonable to me - if the copper conductor is at 70 degrees and ambient is 30 degrees (or below) then they will be a decent temperature gradient across the basic insulation, any air gap and the walls of the conduit. Generally if the copper is at 70 degrees I wouldn't expect the outside of the insulation to be above 60.

Question 2: BS 7671 section 512.1.5 states that :Switchgear, protective devices, accessories and other types of equipment shall not be connected to conductors

intended to operate at a temperature exceeding 70°C at the equipment in normal service unless the equipment manufacturer has confirmed that the equipment is suitable for such conditions, or the conductor size shall be chosen based on the current ratings for 70° C cables of a similar construction. where as NEC 110.14(c): “Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment,correction, or both.” Derating factors may be required because of the number of conductors in a conduit, higher ambient temperatures, or internal design requirements for a facility. By beginning the derating process at the ampacity of the conductor based on the higher insulation value, you may not be required to upsize the conductor to compensate for the derating.is there a similar way to apply the exception of NEC using the BS 7671.

I'm not sure I follow - the idea normally is to make sure the conductor temperature doesn't exceed what the terminals can withstand (which is usually 70 degrees for UK/European products). If you need to de-rate a say 90 degree conductor due to grouping or high ambient temperatures of installation conditions (e.g. thermal insulation) then you'd still expect it to run at 90 degrees at that reduced load - so the problem with overheating the terminals would remain. (I'm not entirely sure what the yanks mean by 'ampacity adjustment' etc.)


If however the de-rating factors only apply to other parts of the run, and not where the cable meets the terminals, you could well find that a 90-degree cable has a 70-degree or less operating temperature at the terminals - which would be fine. Using the 70-degree tables for just the short length of cable that connects to the terminals should confirm that.


BS 7671 is a little inflexible in that you can sometimes get a better result by taking 90 degree ratings and derating them to get a 70-degree conductor temperature than you can by using 70-degree tables directly (apparently because the 70 degree tables presume PVC insulation while the 90 degree tables presume XLPE - which has a low thermal resistance) - but then you always have the option of using cable manufacturer's (or other similar sources of data) in preference to BS 7671's tables, so there's some flexibility there still.


  - Andy.