- Reaction score
- 13,478
We recently had a thread or two about some cables that were almost certainly tinned copper, but people continued to suspect they were aluminium because they were solid core, not stranded, i.e. from the metric cable era when copper was invariably plain. In fact, the smallest tinned-copper lighting cable in general use in the imperial era was solid-core; read on for an explanation...
In the era of imperial cable sizes, domestic lighting circuits were usually wired with one of two sizes - 1/.044 i.e. one strand of 0.044 inches diameter hence a solid conductor, or 3/.029, i.e. a stranded one. 1/.044 is almost exactly 1.0mm² so the two cannot be told apart by measuring the conductor. There was no equivalent to 1.5mm² but 3/.029 at 1.28mm² was the usual 'next size up', also preferred for all 'high class' work regardless of loading because a stranded conductor was more flexible and thought to make better terminations. Next up was 3/.036 at nearly 2.0mm² but this is rarely seen in domestic wiring.
Sheathed flat-twin, twin-and-earth and their triple equivlalents were around long before plastic insulation. All had existed in rubber insulated and tough-rubber sheathed since the inter-war years, all of which have tinned-copper conductors because this was necessary to stop them reacting with the sulphur in the rubber. It was not customary to run an earth on a lighting circuit unless the wiring system required it (e.g. lead-sheathed, or singles in conduit) so the smaller sizes of flat insulated-and-sheathed rubber cables were more often without an earth.
Plastic insulation was actually being made before WW2 but didn't find a market until afterwards. When it began to take over from rubber, twin cables were often insulated with polyethylene, not PVC, just like modern XLPE SWA. It can be distinguished by its glossy, waxy appearance, often slightly translucent, and a waxy smell when heated in a flame. The cable sheath was invariably PVC though. I am not sure what influenced the the selection of PE vs. PVC over the years, although at first PVC was found to be a poorer insulator than natural rubber (which is still true - rubber ages badly, but when new is a superb insulator). Although plain copper in contact with PE and most PVC is fine, tinned conductors were still standard, probably because people expected it for corrosion-resistance at the terminations, and because rubber cables being produced alongside required it. As the 1960s rolled into the 1970s, various permutations emerged. Some imperial cables were made with plain copper, rubber cable manufacture having ended.
In a typical early plastic insulated lighting circuit, one might still find PE-insulated PVC sheathed 1/.044 both with and without an earth, the earthed variety looking rather like 1.0mm² T+E but in fact an imperial cable that could be 70 years old. The distinguishing features will be the glossy insulation (although not the sheath) and the tinned copper conductors. The sheath may be thinner than on a metric cable and may have stiffened slightly with age although the insulation will be fine.
The first pic shows two early examples of 1/.044 from the 50's, PE-insulated and PVC sheathed, without earth and with. The end-view of the stripped-off sheath shows the typical webs of plastic between the cores and the overall thinner profile compared to 1.0mm². Next are two samples of 3/.029; the top one is of similar age and material to the 1/.044, below which is a PVC-insulated and sheathed type by Rists, with plain copper conductors, that probably dates from near the end of imperial cable manufacture. The enlarged pic gives a clearer view of the difference between the early PE and late PVC insulations.
None of these cables would give me any cause for concern in themselves, their IR was measuring as good as new until recently removed and they may have many decades of useful life left. But where they are found, there may be non-MF joint boxes hidden away that have not been touched for over half a century, and that is more of an issue.
In the era of imperial cable sizes, domestic lighting circuits were usually wired with one of two sizes - 1/.044 i.e. one strand of 0.044 inches diameter hence a solid conductor, or 3/.029, i.e. a stranded one. 1/.044 is almost exactly 1.0mm² so the two cannot be told apart by measuring the conductor. There was no equivalent to 1.5mm² but 3/.029 at 1.28mm² was the usual 'next size up', also preferred for all 'high class' work regardless of loading because a stranded conductor was more flexible and thought to make better terminations. Next up was 3/.036 at nearly 2.0mm² but this is rarely seen in domestic wiring.
Sheathed flat-twin, twin-and-earth and their triple equivlalents were around long before plastic insulation. All had existed in rubber insulated and tough-rubber sheathed since the inter-war years, all of which have tinned-copper conductors because this was necessary to stop them reacting with the sulphur in the rubber. It was not customary to run an earth on a lighting circuit unless the wiring system required it (e.g. lead-sheathed, or singles in conduit) so the smaller sizes of flat insulated-and-sheathed rubber cables were more often without an earth.
Plastic insulation was actually being made before WW2 but didn't find a market until afterwards. When it began to take over from rubber, twin cables were often insulated with polyethylene, not PVC, just like modern XLPE SWA. It can be distinguished by its glossy, waxy appearance, often slightly translucent, and a waxy smell when heated in a flame. The cable sheath was invariably PVC though. I am not sure what influenced the the selection of PE vs. PVC over the years, although at first PVC was found to be a poorer insulator than natural rubber (which is still true - rubber ages badly, but when new is a superb insulator). Although plain copper in contact with PE and most PVC is fine, tinned conductors were still standard, probably because people expected it for corrosion-resistance at the terminations, and because rubber cables being produced alongside required it. As the 1960s rolled into the 1970s, various permutations emerged. Some imperial cables were made with plain copper, rubber cable manufacture having ended.
In a typical early plastic insulated lighting circuit, one might still find PE-insulated PVC sheathed 1/.044 both with and without an earth, the earthed variety looking rather like 1.0mm² T+E but in fact an imperial cable that could be 70 years old. The distinguishing features will be the glossy insulation (although not the sheath) and the tinned copper conductors. The sheath may be thinner than on a metric cable and may have stiffened slightly with age although the insulation will be fine.
The first pic shows two early examples of 1/.044 from the 50's, PE-insulated and PVC sheathed, without earth and with. The end-view of the stripped-off sheath shows the typical webs of plastic between the cores and the overall thinner profile compared to 1.0mm². Next are two samples of 3/.029; the top one is of similar age and material to the 1/.044, below which is a PVC-insulated and sheathed type by Rists, with plain copper conductors, that probably dates from near the end of imperial cable manufacture. The enlarged pic gives a clearer view of the difference between the early PE and late PVC insulations.
None of these cables would give me any cause for concern in themselves, their IR was measuring as good as new until recently removed and they may have many decades of useful life left. But where they are found, there may be non-MF joint boxes hidden away that have not been touched for over half a century, and that is more of an issue.
Last edited:
I'll see if I can dig it out!
