The DNO fuse cut out — also called the service head or service fuse — is the grey or black box you’ll find mounted just below your electricity meter. It’s the first point at which electricity from the street enters your property, and it contains the main fuse that protects the incoming supply cable.
Most homeowners never give it a second thought. But it becomes very relevant when you’re installing an EV charger, replacing your consumer unit, or adding solar and battery storage — because it determines how much total load your incoming supply can safely handle.
Who owns the DNO fuse cut out?
The cut out belongs to your Distribution Network Operator (DNO) — not you. In Bristol and South Gloucestershire, that’s National Grid Electricity Distribution (formerly Western Power Distribution). The cut out is their equipment, sitting inside your home, and it is sealed with a tamper-evident DNO seal.
You must never open or interfere with a DNO fuse cut out. Even if your main switch is off, the supply cables feeding into the cut out remain live at all times. There is no safe way for a homeowner or even an electrician to work on it. Only the DNO can replace the fuse or the cut out itself.
The different types of DNO fuse cut out
There are several generations of cut out on UK networks. Identifying which type is in your home matters because the type gives a reliable indication of the fuse rating — and that affects what work can be done without DNO involvement.
1. Modern cut out (circa 1990s to present)
Modern cut outs are made from robust Glass Reinforced Polyester (DMC) — a tough, white or light grey moulded plastic. You’ll find these in homes built since the 1980s, and in older homes where the original cut out has been replaced by the DNO. They’re available in 60A, 80A and 100A versions.
If you see a modern DMC cut out, the guidance from the Energy Networks Association is to assume a 60A fuse unless it is clearly marked otherwise. If your total calculated load won’t exceed 60A, no DNO assessment is needed before adding an EV charger or other low carbon technology.
2. Black plastic cut out (circa 1960s to 1990s)
Black plastic cut outs are an older design found in homes built or rewired between roughly the 1960s and 1990s. They’re recognisably black in colour and slightly bulkier than the modern DMC type.
Again, assume a 60A fuse. If the calculated total load of the installation (including the new EV charger or heat pump) won’t exceed 60A at any point, the install can proceed with appropriate notification to the DNO.
3. Metal-clad cut out in good condition (circa 1940s to 1960s)
Many metal-clad cut outs remain on the network and continue to function perfectly well. These are steel-cased units that pre-date the plastic designs, and they often contain rewireable fuses rather than cartridge fuses.
If the cut out is in reasonable condition and not showing signs of damage or deterioration, assume a 60A fuse. However, check carefully — some metal-clad cut outs are marked 30A or 45A on the casing, in which case the guidance below applies instead. Never open the cut out to try to read the rating — contact the DNO if you’re unsure.
4. Metal-clad cut out in poor condition, “biscuit tin” type, or low-rated (circa 1930s to 1960s)
Some older cut outs — sometimes called “biscuit tin” cut outs because of their shape — show clear signs of age: rust, corrosion, deformation, or markings showing ratings of 30A or lower. Others may simply be very old designs that pre-date modern specifications.
If you encounter this type, assume a 30A fuse or that the cut out is unsuitable for additional load. You must not proceed with adding an EV charger or heat pump without first referring the matter to the DNO for assessment, which may result in a cut out replacement being arranged.
5. Embargoed fused neutral cut outs
These are an older design where the neutral conductor is also fused — a configuration that DNOs have been replacing for many years due to safety concerns. The Health and Safety Executive has directed DNOs to replace all fused neutral cut outs within 21 business days of discovery.
If you or your electrician identifies a fused neutral cut out, the DNO must be notified immediately for urgent replacement.
6. Looped services
In some areas, particularly older housing stock, properties are supplied by a single incoming cable that is “looped” through several properties in a daisy chain. In this arrangement, two or more homes share a single incoming cable — or even a single fuse.
Looped services can be hard to identify. The cable might enter the bottom of the cut out from the direction of the neighbouring property rather than coming from the street. If you suspect a looped service, the matter must be referred to the DNO before any additional load is connected.
Why does the cut out rating matter for EV chargers and solar installs?
A standard 7kW home EV charger draws around 32A. Add that to a home that might already be using 20–30A on appliances, heating, and lighting, and you could be approaching or exceeding a 60A fuse rating at peak times.
This is why EV charger installers are required to assess the DNO cut out type and calculate the total potential load before installing a charger. If the calculations show the total load will stay under 60A, the install can proceed. If it won’t — or if the cut out is an older type with a lower rating — the DNO must be contacted first.
The same logic applies to heat pumps, which can also add significant load to the incoming supply. The Energy Networks Association publishes guidance on this specifically for EV charge point and heat pump installers.
What to do if your cut out needs upgrading
If the DNO assessment determines that your cut out needs replacing — because it’s an old type, in poor condition, or the supply capacity needs to be increased — the DNO arranges this themselves. In most cases it’s free of charge, because it’s their equipment.
The process typically takes several weeks to a few months to arrange, depending on the DNO’s workload. Your electrician can make the notification on your behalf as part of the install process.
For Bristol and South Gloucestershire, the DNO is National Grid Electricity Distribution. You can contact them on 0800 6783 105 to report issues with DNO equipment or request an assessment.
Quick reference: cut out types and assumed ratings
| Type | Era | Assumed fuse rating | Action needed? |
|---|---|---|---|
| Modern DMC (white/grey plastic) | 1990s–present | 60A | No, if total load ≤ 60A |
| Black plastic | 1960s–1990s | 60A | No, if total load ≤ 60A |
| Metal-clad, good condition | 1940s–1960s | 60A | No, if total load ≤ 60A and not marked lower |
| Metal-clad, poor condition or “biscuit tin” | 1930s–1960s | 30A or unsuitable | Yes — refer to DNO |
| Fused neutral | Pre-1960s | Unknown — treat as unsafe | Yes — notify DNO urgently |
| Looped service | Any era | Shared — potentially limited | Yes — refer to DNO |
Source: Energy Networks Association, DNO Cut-Out Types & Ratings Guidance for EV/HP Installers
Installing an EV charger or upgrading your consumer unit?
When we carry out an EV charger installation or consumer unit upgrade, we assess the cut out type and calculate the total load as part of our standard process. If a DNO notification or referral is needed, we handle that on your behalf.
We cover Bristol, South Gloucestershire and the surrounding areas. Get in touch to discuss your installation.
