PVC vs XLPE SWA Cable: Why 90 °C Cable Is Sized at 70 °C
Two reels of armoured cable can look identical on the drum and behave differently in the ground. But the choice that actually decides your cable size is not the one you make in the wholesaler — it is whether anyone has confirmed what the terminals are rated for.
XLPE armoured cable is rated for a 90 °C conductor temperature, and the 90 °C column in the tables gives it a usefully higher current rating than the 70 °C column. So why do most competent designs — and most client specifications — size that cable on the 70 °C values, apparently throwing away capacity the cable legitimately has?
Because the cable is not the weakest link. XLPE gives you a 90 °C cable. Regulation 512.1.5 usually gives you a 70 °C circuit.
1. The terminal is the limit, not the cable
A conductor does not stop being hot where the cable ends. If it runs at 90 °C, that heat conducts straight into whatever it is bolted to — the MCCB terminal, the busbar connection, the accessory clamp. Most switchgear and accessories are designed, tested and certified on the assumption that the incoming conductor is at 70 °C or below. The device’s own temperature-rise budget — contacts, internal insulation, the calibration of its thermal trip — is built on that assumption.
Regulation 512.1.5 makes it explicit, and it is worth reading in full because the second sentence tells you exactly what to do about it:
“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. The 70 °C limit is met if the live conductor size is chosen based on the current rating for 70 °C cables of similar construction.”
Three things follow from that, and the rest of this article is built on them. It says conductors — there is no exemption for armoured cable. The limit is at the equipment, not along the run. And the only way out is the equipment manufacturer confirming suitability — a written confirmation for the actual device, not a general belief that modern switchgear copes.
2. Armoured cable is not exempt — and the table says so itself
This is the part that gets missed, because SWA feels like a special case with its own tables. It is not. Table 4E4A — the 90 °C armoured rating table you are reading when you take the higher figure — carries a note in its own margin:
“Where it is intended to connect the cables in this table to equipment or accessories designed to operate at a temperature lower than the maximum operating temperature of the cable, the cables should be rated at the maximum operating temperature of the equipment or accessory (see Regulation 512.1.5).”
The table tells you, on the table, to derate to the equipment. And NOTE 2 extends the same logic to grouping: where a cable from that table is grouped with others, it should be rated at the lowest of the maximum operating temperatures in the group. Bunch an XLPE cable with PVC ones and the XLPE is held to 70 °C too — quite separately from the ordinary grouping factor.
3. “Similar construction” — Table 4A3 names it
“70 °C cables of similar construction” sounds like a judgement call. For armoured cable it is not: Table 4A3 maps every cable specification to its rating tables and its conductor operating temperature.
| Specification | Construction | Tables | Temp |
|---|---|---|---|
| BS 5467 | Thermosetting insulated, armoured | 4E3, 4E4 | 90 °C |
| BS 6724 | Thermosetting, armoured, low smoke and fume | 4E3, 4E4 | 90 °C |
| BS 7211 | Thermosetting, non-armoured, low smoke and fume | 4E1, 4E2 | 90 °C |
| BS 6004 | PVC insulated, non-armoured | 4D1, 4D2, 4D5 | 70 °C |
| BS 6346 (withdrawn) | PVC insulated, armoured | 4D3, 4D4 | 70 °C |
BS 5467 armoured at 90 °C pairs with BS 6346 armoured at 70 °C. The regulation does the choosing for you.
There is a lovely quirk in that last row. BS 6346 is withdrawn — Table 4A3 says so, and keeps it “for historical purposes”. You cannot specify that cable any more. Its rating tables survive in BS 7671 precisely because Regulation 512.1.5 sends you to them: 4D3/4D4 are not there so you can buy the cable, they are there so you can size an XLPE one.
4. What it costs, and what it does not touch
Take one 95 mm² XLPE SWA on reference method C at 40 °C ambient and compute it both ways. With the terminals unconfirmed it is read from Table 4D4A: 269 A tabulated, 234 A after the ambient factor. With the terminals confirmed 90 °C-rated it is read from Table 4E4A: 338 A tabulated, 308 A after the ambient factor. That is about 24 % of the capacity, and it is the entire cost of the rule.
Note that two things moved, not one. The ampacity column moved, and so did the ambient rating factor — because Table 4B1’s columns are indexed by insulation operating temperature (60 °C thermosetting, 70 °C thermoplastic, 90 °C thermosetting). A cable held to the 70 °C ampacity column is consistently derated on the 70 °C column too. Rating on one basis and derating on another mixes them.
| Property | Effect | Why |
|---|---|---|
| Current-carrying capacity | moves | Sized on the 70 °C ampacity column of a cable of similar construction. |
| Ambient rating factor Ca | moves | Table 4B1 is indexed by operating temperature, so a cable rated on the 70 °C column derates on it too. |
| Fault withstand k | unchanged | 143 for 90 °C thermosetting against 115 for 70 °C thermoplastic (Table 43.1). The insulation has not changed, so neither has k. |
| Volt drop mV/A/m | unchanged | Still the thermosetting figure — and it is lower, because the conductor is not running as hot. |
| Mid-run headroom | unchanged | The 70 °C limit applies at the equipment. Along the run it is still a 90 °C cable. |
| Fire and smoke behaviour | unchanged | A property of the compound, unrelated to how you sized it. |
The rule caps the temperature the SIZING may assume. The material never changed.
5. What the two materials actually are
PVC is a thermoplastic: it softens as it heats and re-hardens as it cools. XLPE is cross-linked polyethylene, a thermosetting compound — the cross-linking means it does not soften in the same way, which is what lets it run hotter and survive more fault energy. Table 52.2 gives the maximum operating temperatures: 70 °C for thermoplastic, 90 °C for thermosetting.
That difference shows up in three places that matter. The ampacity, which the terminal rule usually takes back. The fault withstand, through k in the adiabatic equation — 143 for 90 °C thermosetting against 115 for 70 °C thermoplastic (Table 43.1), which is real and which the terminal rule does not touch. And the fire behaviour, though note that low-smoke-and-fume is a separate property: BS 6724 and BS 7211 are the LSZH thermosetting specifications.
6. When thermosetting still earns its place
- Short-circuit withstand. k = 143 against 115 is about 24 % more let-through energy for the same conductor, and the adiabatic check does not care what the terminals are rated for.
- Buried and high-ambient runs. The 90 °C column derates more gently, so where a terminal is not the binding constraint the headroom is genuinely usable.
- When the equipment IS rated. Confirm 90 °C terminals in writing and the full ampacity is yours — that is what the escape clause is for, and on the worked example it is worth a quarter of the cable.
- Low smoke and fume. BS 6724 and BS 7211 are thermosetting, so if the specification calls for LSZH the material choice is largely made for you.
- Transient overload tolerance. More thermal margin above the steady-state rating before the insulation is in trouble.
So specify XLPE for the fault performance and the fire behaviour. Just do not budget for the ampacity until someone has confirmed the terminals — and if the answer to “what am I buying the thermosetting insulation for?” is only the extra amps, PVC armoured is perfectly compliant and slightly cheaper.
Frequently asked questions
Why is 90 °C XLPE cable usually sized on the 70 °C column?
Because of what it is bolted to, not what it is. Regulation 512.1.5 says equipment shall not be connected to conductors intended to operate above 70 °C at the equipment unless the manufacturer has confirmed it is suitable. Most switchgear, accessories and busbar connections are designed, tested and certified on the assumption the incoming conductor is at 70 °C or below — their whole temperature-rise budget depends on it. A conductor does not stop being hot where the cable ends, so the heat goes straight into the terminal.
Does the 70 °C terminal rule apply to armoured cable?
Yes. The regulation says "conductors" and makes no exemption for armour. Table 4E4A — the 90 °C armoured rating table itself — carries a note saying that where the cables are connected to equipment designed to operate at a lower temperature, they should be rated at the maximum operating temperature of the equipment, and it cites Regulation 512.1.5 directly. SWA is the commonest commercial cable and it is not a special case.
Which 70 °C table do I use for an armoured cable?
Table 4A3 names it. Regulation 512.1.5 says the 70 °C limit is met if the conductor size is chosen on the current rating for 70 °C cables of "similar construction", and Table 4A3 lists BS 5467 thermosetting armoured against Tables 4E3/4E4 at 90 °C, and BS 6346 PVC armoured against Tables 4D3/4D4 at 70 °C. So the armoured equivalent is 4D3/4D4. There is a nice quirk here: BS 6346 is withdrawn and retained in Table 4A3 "for historical purposes" — you cannot buy that cable any more, but its rating tables survive precisely because this regulation sends you to them.
What do I actually lose by sizing at 70 °C?
Roughly a fifth to a quarter of the tabulated capacity, depending on size and method. A 95 mm² XLPE SWA on reference method C is 338 A at 90 °C and 269 A at 70 °C. At 40 °C ambient the gap widens, because the ambient rating factor moves too: 308 A against 234 A, which is about 24 per cent. That is the whole cost of the rule.
What do I keep?
Everything that is a property of the material rather than of the sizing. The fault withstand k stays at 143 against 115 for thermoplastic, so the adiabatic check still grants you the better short-circuit performance. The volt drop in mV/A/m is unchanged, and is actually lower than a PVC cable of the same size because the conductor is not running as hot. The mid-run temperature headroom is untouched, because the limit applies at the equipment. And the fire and smoke behaviour is a property of the compound.
So is there any point specifying XLPE?
Yes, on four counts the terminal rule does not touch. Short-circuit withstand, where k = 143 is a genuine advantage. Buried and high-ambient runs, where the 90 °C column derates more gently and a terminal may not be the binding constraint. Low smoke and fume constructions, which are thermosetting by definition — BS 6724 and BS 7211. And transient overload tolerance. What you should not do is budget for the extra ampacity before the terminals are confirmed.
How do I get the 90 °C rating?
Get the equipment manufacturer to confirm, in writing, that the specific device is suitable for connection to conductors operating above 70 °C. That is the only route the regulation offers. It is worth asking, because as the worked example shows it is worth about a quarter of the cable's capacity — but a general belief that modern switchgear is 90 °C-rated is not a confirmation, and it is not something to assume on a design you are signing.
Does grouping change anything?
It can, by the same mechanism. Table 4E4A NOTE 2 says that where a cable in that table is grouped with other cables, it should be rated at the lowest of the maximum operating temperatures of any cable in the group — and it cites Regulation 512.1.5 again. So run an XLPE cable in a bunch with PVC ones and the XLPE is held to 70 °C as well, quite apart from the ordinary grouping factor.
Is PVC SWA still a legitimate choice?
Perfectly. For a lightly loaded circuit, a cost-sensitive job, or matching an existing installation, PVC armoured is compliant and slightly cheaper. Given the terminal rule usually caps you at 70 °C anyway, the honest question is what you are buying the thermosetting insulation FOR — and if the answer is the fault performance or the fire behaviour, specify it. If the answer was the extra amps, check the terminals first.
The ampacity, volt drop and k values here are an indicative subset of BS 7671 Appendix 4 — the two-core single-phase columns of Tables 4D4A and 4E4A for armoured cable. Confirm against the full tables and the cable maker’s data for your exact construction and installation method. Where the 70 °C limit applies, this site derates on the 70 °C ambient column as well as sizing on the 70 °C ampacity column; BS 7671 does not spell out which Table 4B1 column applies once 512.1.5 has capped the sizing, and consistency is the conservative reading rather than a transcribed rule.
Where this fits
The terminal rule is one input to the current-carrying-capacity check — see cable derating factors for the rest of the factor stack, and voltage drop for the check that often sizes the cable instead. The cable sizing calculator runs the same engine as the widget above and now applies Reg 512.1.5 to armoured cable too.
Getting this right across a whole installation is exactly what outsourced design is for. Get in touch, or see ProDesign cable calculations for how it fits into a full design package.