The guide

Thermal backfill for underground power cables

Thermal backfill is any material packed around a buried power cable to carry its heat away, so the cable can run at its full rating. Cable sand is the everyday one. Cement-bound sand and gravel and sand mix do the job better when the ground dries out, and engineered backfills are used on the most demanding high voltage work.

Last checked 1 October 2026. Sources at the bottom of the page.

Why heat decides the backfill

A cable carrying current gets warm, and the hotter it runs, the less current it is allowed to carry. The ground around it is what takes the heat away. Damp, well-packed ground does that well. Dry soil and loose sand do it badly, and the heat from the cable dries out the ground right next to it, which makes things worse.

A thermal backfill is chosen to keep carrying heat away even when it has dried out completely. That is how ENA TS 97-1, the industry standard, judges them: by their thermal resistivityonce fully dry. Thermal resistivity is how hard it is for heat to pass through; the lower it is, the better.

The kinds of thermal backfill

We supply

Cable sand (selected sand)

Natural sand from a tested source, compacted around the cable. The everyday thermal backfill for most cable work.

Cement-bound sand (CBS)

Sand bound with cement. Holds together and keeps carrying heat away once it has dried out, so it is often asked for at higher voltages. As sand to mix on site or pre-mixed dry.

Gravel and sand mix

Selected sand mixed with small single-size gravel. Packs very densely, which helps it carry heat away better than sand alone.

We supply

Crushed stone dust

Crushed limestone or granite, 3 mm to dust. National Grid Electricity Distribution uses it instead of sand to bed and blind its cables in the Midlands, South West and South Wales.

Specialist

Engineered thermal backfills

Purpose-made products such as thermal concretes, flowable or fluidised thermal backfill, and engineered sands. Mostly used on big high voltage and renewables jobs, each with its own test results to ENA TS 97-1.

Which one the job needs

On network work it is set by the cable rating calculations and the network operator’s rules, so take it from your drawings or spec. Most low voltage and 11 kV work is done in cable sand. Higher voltage cables often go in cement-bound sand.

Where poor ground limits a cable’s rating, operators usually look at other answers first. Northern Powergrid’s rules (NSP/002, 2021), for example, put a bigger cable first and stabilised backfill such as cement-bound sand second, with bentonite as a last resort.

If your spec names an engineered backfill or a particular product, send it to us with your enquiry and we will tell you what we can supply.

Questions

Is cable sand a thermal backfill?

Yes. Cable sand, cement-bound sand and gravel and sand mix are the three traditional thermal backfills in ENA TS 97-1, the industry standard. "Thermal backfill" is the umbrella term for any of them, plus the engineered products.

What does thermal resistivity mean?

How hard it is for heat to pass through a material. The lower the number, the more easily heat escapes from the cable. It is measured in K·m/W, and backfills are judged at their worst, once fully dried out.

What is fluidised thermal backfill?

A flowable mix that is poured into the trench rather than laid and compacted. It fills around the cables on its own and sets, which suits long runs, deep trenches and places that are hard to compact. It is a specialist product with its own test results.

Do I need an engineered thermal backfill?

Only if your cable rating calculations or your spec call for one. Most cable work, including much higher voltage work, is done in cable sand or cement-bound sand. If your spec names a particular product, send it to us and we will tell you what we can supply.

How is a thermal backfill tested?

Under ENA TS 97-1, samples are tested in a lab for how well they carry heat, how densely they pack and how they hold together, and then checked in the trench as the job goes on. Ask for our test certificate to check our sand against your spec.

Sources

Energy Networks Association, ENA TS 97-1, Special backfill material for cable installations. The industry standard. Covers the three traditional backfills and how other materials can qualify.
Northern Powergrid, NSP/002 Policy for the Installation of Distribution Power Cables, version 5.0, October 2021. An example of an operator’s order of choice when ground conditions limit a cable’s rating.

Get a price

Thermal backfill delivered to site

Cable sand, sand for CBS or CBS pre-mixed dry, and gravel and sand mix, delivered anywhere in the UK. Pick what your spec asks for, or “Not sure, advise me”.