90 °C Cable on 70 °C Terminals: Why XLPE Is Usually Sized at 70 °C

Here's a question that catches out even experienced designers: your XLPE/SWA 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?

The Cable Isn't the Weakest Link — the Terminal Is

A conductor doesn't stop being hot where the cable ends. If a conductor runs at 90 °C, that heat conducts straight into whatever it's bolted to: the MCCB terminal, the busbar connection, the accessory clamp. And 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, calibration of thermal trips — is built on that assumption.

BS 7671 makes this explicit in Regulation 512.1.5: equipment must be suitable for the design temperatures it will actually see. Feed a 90 °C conductor into a terminal engineered for 70 °C and you can overheat the device, drift its protective characteristics, and age its insulation — while every individual component is nominally "within its rating".

What This Means for Sizing

Unless the equipment manufacturer explicitly confirms its terminals accept 90 °C conductors, the design current of the circuit must be limited so the conductor runs at no more than 70 °C at the termination — which in practice means selecting the cable size from the 70 °C thermoplastic columns even though the cable itself is thermosetting. The 90 °C capacity isn't wasted so much as unavailable: the system rating is set by its coolest-running requirement.

This is a system-level decision, which is why it belongs in the calculation software settings, not in someone's head. In ProDesign the cable's operating temperature basis is part of the cable selection — get it wrong one way and the design fails at the terminals; wrong the other way and you've bought a size (or two) of copper you didn't need on every circuit in the job.

When You Can Use the 90 °C Rating

• The manufacturer states the equipment terminals are rated for 90 °C conductors (some modern switchgear is — but it must be in writing, not assumed).
• Cable-to-cable joints made with 90 °C-rated connectors, away from equipment.
• Where the size is governed by a different constraint anyway — voltage drop or fault withstand often set larger sizes on long runs, making the ampacity column academic.

And even when sized at 70 °C, thermosetting cable isn't a wasted premium: you keep the better mechanical and thermal endurance, and extra margin under fault conditions.

The One-Line Summary

The drum says 90 °C; the switchboard says 70 °C; the switchboard wins. Size XLPE on the 70 °C values unless you have the manufacturer's written confirmation otherwise — and make sure your calculation software is set to match. For the authoritative treatment, see BS 7671 Regulation 512.1.5 and the IET's guidance on terminal temperature limits in its Wiring Matters series, which covers this topic in depth.

Sizing a project's cables and not sure which basis applies? Our ProDesign calculation packs state the temperature basis explicitly, circuit by circuit — no ambiguity at design review.