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

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.

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.