SWA 3 Core used on Single Phase - Ambiguity with current rating (UK) - Page 1
Forum section: electronics / general

author: TheBay
author_rank: Unknown
timestamp: August 16, 2025, 02:27:50 pm
content: I was specifying some cables today and while reading the wiring regs It got me thinking about something, I can't seem to get a definitive answer on this and everyone seems to have different opinions, there is a lot of ambiguity with some cable types and with SWA specifically.Historically 3 Core SWA was primarily used for three-phase no neutral and it would always come in pre/post 2004 3 phase colours. Often this was used on a single phase supply opposed to 2 core as it is better to have the CPC on it's own conductor opposed to using the metal shield as the CPC and just earthing the shield for protection of the cable. The cores would (Should) then be sleeved in the relevant colours for L/N/E. But 3 core was never really wasintendedfor single phase use.Then 3 Core SWA became commonly available in harmonised single phase colours (Brown/Blue/GY), however since the advent of this cable the tables in the regs haven't changed and there are no sub notes either and this is where there is some ambiguity in the tables.If we look at the table 4D4A for example there are 2 columns per each reference method, the first column is:1 two-core cable single-phase AC or DCand the second column is:1 three or four core cable, three-phase AC.I have gone with table 4D4A which is for PVC SWA rated at 70c opposed to 4E4A as although the majority of all SWA these days is XLPE rated at 90c this rating is only applicable when terminated in terminals that are also rated at 90c or above.So looking at the 2 columns again in 4D4A there is a significant difference between the current carrying capacities between 1 two-core cable single-phase AC or DC and 1 three or four core cable, three-phase AC.Historically this would have been very simple as the 2 Core SWA would have beenintendedfor Single Phase use with an earth rod and/or the shield as the the CPC or on some occasions a separate CPC run alongside and the 3 Core would have beenintendedto be used with three-phase no neutral.But when we use 3 Core SWA with single-phase with the cores as L/N/E there is a lot of ambiguity and the regs have not been updated for this use case, even though 3 Core SWA with the correct colours intended to be used on a single-phase supply is now fairly common place.One would assume that you would simply look at the 3 - 4 core table as it is a 3 core cable, but that does clearly state three-phase and when used on three-phase you would have 3 - 4 loaded conductors. When using 3 core on with single-phase you would only have 2 loaded conductors as the 3rd core is the CPC.Based on that then you could look at the first table which is two-core cable single-phase AC or DC, but technically it's not two-core either.The reason for derating the cable is due to heat as we know, but in column 2 that is factoring in 3 or more loaded conductors, which we are not doing in this case. You could argue that the layout of a 2 core vs 3 or more core cable is different so heat dissipation is different. Yes it is different, but usually a multicore cable will actually have the same if not higher current carrying capacity with more unloaded cores compared a cable with all cores loaded. Take any kind of flex for example, the current carrying capacity on a 3 Core cable with 2 cores loaded (Single-phase with CPC) is often higher than a 2 Core cable with 2 loaded cores.If we look back at the regs and look at Table 4B5 as one example, the derating is only applicable to loaded cores. And the same can be said for a lot of other cables of varying sizes.So what is the actual answer when using 3 Core SWA on single-phase with only 2 loaded cores? I feel thinking about it in detail that it should be the rating given in the 2 core column when using only 2 loaded cores plus CPC. And if we think about the other derating tables where multicore cables are used with unloaded cores then it could even be a bit higher, based on that factor too then that technically should give us more of a margin of safety if we do use the 2 core column.Then there is the whole fact of the cable being XLPE as well, now I mentioned earlier on about table 4E4A for XLPE SWA 90c cable and that only being applicable when terminated in terminals rated at 90c or above. This is still true, but the fact that XLPE cable has a higher temperature rating and better thermal capabilities then it really should meet the ratings in the 2 core column on 4D4A at the very least.Interested to read your replies.

author: Monkeh
author_rank: Super Contributor
timestamp: August 16, 2025, 02:37:14 pm
content: 4D4A for SWA, not 4D2A. Use the two-core ratings for single phase and don't overthink. Or if you want to overthink, go try playing with the adiabatic equation for PVC vs XLPE SWA..

author: TheBay
author_rank: Unknown
timestamp: August 16, 2025, 02:49:52 pm
content: I'm not sure why I wrote 4D2A, Yes I'm referring to 4D4A of course, I must have had it in my mind for something else. Thanks for pointing that out, I've edited my post.Yeah looking in to it in detail the two-core rating makes more sense, I'd never given it much thought before and would go with the 3 core table which has given a lot of headroom. However I feel it's not really necessary when you think about it in detail. That part of the regs does need to be updated or a footnote added."Or if you want to overthink, go try playing with the adiabatic equation for PVC vs XLPE SWA.."I may just go down that rabbit hole!

author: themadhippy
author_rank: Super Contributor
timestamp: August 16, 2025, 03:24:55 pm
content: Quote.Have you done the calculations to prove this?There does become a point were a seprate cpc is required,however from memory its somewhere around 120mm2

author: TheBay
author_rank: Unknown
timestamp: August 16, 2025, 04:23:13 pm
content: Quote from: themadhippy on August 16, 2025, 03:24:55 pmIt was more of a generalisation to why 3 core is sometimes used instead of 2 core, there can sometimes be issues when using the shield as the CPC in some not all cases.And I did say this later on "Historically this would have been very simple as the 2 Core SWA would have been intended for Single Phase use with an earth rod and/or the shield as the the CPC or on some occasions a separate CPC run alongside and the 3 Core would have been intended to be used with three-phase no neutral."Some problems can be poor termination of the shield gland, poor quality gland, water ingress in to the gland or wrong location for termination which could be effected by environmental issues. There has also been poor quality SWA that has been imported when the conductor size of the shield isn't large enough to act as a CPC or very poor quality steel. And then distance as you say.Cost wise there isn't much of a reason to go with 2 core if using the DNO earth, the price difference can be quite negligible.It's certainly safer using the 3rd core as the CPC as an issue with the shield termination could arise unknowingly and might not get noticed.

author: yngndrw
author_rank: Regular Contributor
timestamp: October 16, 2025, 03:30:05 pm
content: I've always understood it to mean "live conductors". With the neutral conductor being considered a live conductor, and the CPC not being live.