The class number is a test load, nothing else
Cover classes come from EN 124, and the number is simply the load the cover survived in a test rig, in kilonewtons. B125 took 125 kN. D400 took 400 kN. There is no safety story hidden in the letter — the letter is a category name and the number is the whole specification.
That makes the decision simpler than it looks. You are not choosing a quality grade, you are matching a wheel load. A cover that is one class too low does not wear out early; it fails, usually under the one vehicle nobody accounted for.
Where the line actually falls
The awkward cases are all car parks. A bay is B125 and an aisle is D400, because the aisle is where a delivery van, a refuse truck or a fire appliance travels. Draw that boundary on the drainage layout before you order, not after, because the two covers are different money and different lead times.
The trap is the route nobody calls a road. A basement car park where the refuse truck reverses to a bin store once a week is a D400 route for its whole length. So is the fire tender path across a podium. If a vehicle over about three tonnes can reach it, it is not a B125 position.
- Landscape and planted areas with no vehicle access — A15 is enough and cheaper.
- Footpath and pedestrian plaza — B125, unless a maintenance vehicle is in the access strategy.
- Car park bays — B125. Car park aisles and ramps — D400.
- Anything inside the carriageway or hard shoulder of a public road — D400 minimum, and the authority may name a higher class.
- Loading yards, container areas and plant hardstanding — E600.
The frame beats the class
Most covers we see replaced early were the right class, badly seated. A cover only carries its rated load when the frame is bedded on a full mortar or resin bed with no voids, at the finished level of the surface around it, and with the chamber wall taking the frame load rather than the cover taking the surfacing load.
Two symptoms tell you it went wrong. A cover that rocks under a wheel is sitting on high spots and will crack at the seating, whatever its class. A cover standing proud of the asphalt takes an impact load every time a wheel crosses it, which is a far bigger number than the static rating suggests.
This is worth more attention than the class debate. Getting the class right costs you a purchase decision; getting the bedding right costs you a supervision decision, and it is the one that determines whether the cover is still sound in five years.
Set the frame to the final surface level, not the base course. Coming back to raise a frame after surfacing costs more than the cover.
What to send us
The clear opening size, the class, and where the chamber sits on the layout. If you tell us the position rather than the class we will tell you the class — that conversation takes a minute and has saved a lot of re-orders.
The estimator quoting your job also delivers it
Send the drawing or a photograph. You get a delivered rate, a date and a straight answer on whether the spec is right.
Is B125 enough for a car park?
For parking bays used by cars, usually yes. For aisles, ramps and any route a refuse or fire vehicle takes, no — those positions need D400. The boundary between bay and aisle is where the decision lives.
What is the difference between B125 and D400?
The tested load: 125 kN against 400 kN. B125 belongs in footpaths and car park bays; D400 belongs in carriageways, service roads and car park aisles. They are not interchangeable and the price difference reflects the reinforcement and frame section.
Do you supply the cover with the chamber?
Yes. Covers are matched to the duty the authority specifies and supplied with the chamber so the frame and the chamber top are made for each other.