Cables in thermal insulation — current-carrying capacity per table B.52.21
Cables in thermal insulation — current-carrying capacity per table B.52.21
The guide on the grouping factor Cg — bundled cables covers the reduction in current-carrying capacity when several cables lie next to each other and heat one another. This article covers a different, far more severe phenomenon: what happens when a single cable comes into contact with, or is fully surrounded by, thermal insulation material — such as roof or cavity-wall insulation in a residential or commercial building.
Why insulation material drastically limits heat dissipation
A cable installed in free air (reference method A/B, see the guide on reference methods) dissipates its generated heat via convection and radiation to the surrounding air. Thermal insulation material is, by definition, specifically designed to block heat transport: the same cable, surrounded by insulation instead of free air, can shed its heat far less effectively, even though the ambient air on the outside of the insulation has not itself become any warmer. This has nothing to do with the grouping effects of tables B.52.18/B.52.19/Cg — it is a fundamentally different heat-dissipation situation, governed by its own table: B.52.21.
Two situations, with a sharply different reduction
IEC 60364-5-52 distinguishes between two ways a cable can come into contact with thermal insulation material:
- One-sided contact: the cable lies against the insulation material (for example against the back of an insulated cavity wall), but the other side of the cable remains in free air. For a short contact length, table B.52.21 gives a relatively modest correction factor for this case.
- Full enclosure: the cable is surrounded on all sides by insulation material (for example embedded in roof insulation applied perpendicular to it). For an enclosure length up to 0.5 m the table gives a graduated, length-dependent correction factor; for an enclosure length greater than 0.5 m the standard applies a fixed, conservative correction factor of roughly 0.5 — the cable then loses roughly half its free-air current-carrying capacity, regardless of the precise, actual length of the enclosure beyond that point.
The reason for that fixed 0.5 threshold above 0.5 m is a practical one: the standard does not provide a fine-grained table for every possible enclosure length beyond that point, and instead opts for a value that is simple to apply and errs on the safe side — a designer does not then need to determine the exact enclosure length of every cable section to still arrive at a sound sizing.
Note: this correction factor is entirely separate from the reference installation method (A through F) that otherwise determines which base table applies — a cable that would normally be sized via reference method B in a cable trunking system falls, as soon as it runs through insulation material, under the additional correction of table B.52.21, on top of (not instead of) its own base table.
Practical relevance
For an installation in a roof or cavity-wall construction with retrofitted insulation (for example during an energy-efficiency renovation where extra insulation is applied around existing cabling), it is important to check whether an existing cable, originally sized for free-air installation, still has sufficient current-carrying capacity after the new insulation has been applied — a cable that previously had ample margin can become thermally overloaded after full enclosure by new insulation, with a factor of roughly 0.5, at an unchanged load.
Common mistakes
- Assessing a cable that lies against insulation material as if it were still in free air, without applying the additional correction factor from table B.52.21.
- Failing to distinguish between one-sided contact and full enclosure, while the reduction in the latter case is far greater.
- Applying extra insulation around existing cabling during a renovation without reassessing the current-carrying capacity of that cabling against the new installation conditions.
- Confusing the correction factor of table B.52.21 with the grouping factor Cg of table B.52.17 — both reduce current-carrying capacity, but through a fundamentally different heat-dissipation mechanism and with a very different order of magnitude.
Related
Further reading
- IEC 60364-5-52Reference methods — determining the installation method for the current-carrying-capacity table
- IEC 60502-2 (halfgeleidende laag)Semiconducting layer in MV cables — conductor screen and insulation screen per IEC 60502-2
- IEC 60502-1 / NEN-EN 50525Cable insulation material: PVC versus XLPE/EPR — operating temperature, short-circuit temperature and the effect on ampacity
- IEC 61386 (mantelbuizen)Cable protection conduit — classification code per IEC 61386
- IEC 61914 (cable cleats)Cable cleats — short-circuit withstand per IEC 61914
- IEC 60364-5-52 Bijlage B (D1/D2)Current-carrying capacity of buried cables — soil thermal resistivity and grouping, separate from burial depth