Head fuse rating

How the fitted service fuse relates to supply capacity, maximum demand, load management and a DNO load-increase decision.

The head fuse is the network operator’s protective fuse in the service cut-out. Its current rating is one important limit on a property’s electricity connection, but it is not a complete statement of available capacity.

The fuse protects the service equipment. The DNO chooses its rating to suit the cut-out, cable and network. Only the DNO, or a party acting with its authority, can confirm or change it.

Useful nominal conversions

For a single-phase supply at the UK’s nominal 230 V, apparent power is approximately voltage multiplied by current:

Confirmed fuse rating	Approximate capacity at 230 V
60 A	13.8 kVA
80 A	18.4 kVA
100 A	23.0 kVA

These figures are design references, not promises that the full amount is always available for a new appliance. A fuse has a time-current characteristic rather than opening at exactly its printed rating, and sustained overloading can heat the cut-out and cable before the fuse operates.

For three-phase supplies, current is stated per phase and load balance matters. Do not multiply a visible carrier marking by three unless the DNO has confirmed three live phases and the rating of each fitted fuse.

Confirm the fitted rating

The number moulded into a cut-out or fuse carrier may be the assembly’s maximum rating rather than the fuse inside it. Opening the sealed carrier to check is not permitted for a householder and can be dangerous.

Use the following evidence in order:

a rating already confirmed in DNO records or correspondence
an external label that the DNO confirms refers to the fitted fuse
an enquiry with clear photographs and the MPAN or address
a DNO visit where the rating or condition remains uncertain

Do not design load management around an assumed 100 A fuse because “100 A” is visible on the carrier.

Maximum demand is the relevant comparison

The installer assesses the maximum demand of the whole connection after the new equipment is added. That is not the sum of every circuit-breaker rating, and it is not a generic percentage of the service fuse.

The assessment should include:

existing peaks and sustained loads
EV charging current
heat-pump compressor, crankcase heater and auxiliary or backup heating
immersion heaters, showers, cooking and space heating
battery grid charging and other controllable loads
any simultaneous schedules created by a cheap-rate tariff
phase balance on a multiphase connection

Diversity can be applied where justified, but long-duration controlled loads cannot be dismissed simply because they are unlikely to run at the same time by chance. If an app deliberately starts the EV, battery and hot-water heating together, that coincidence is part of the design.

Current EV and heat-pump connection route

This is a live industry process. As at 22 July 2026, the ENA combined EV and heat-pump process uses 13.8 kVA per phase as a key maximum-demand boundary for an existing DNO connection.

Where the whole-connection maximum demand does not exceed that level, and the other service-equipment and connection checks pass, the connect-and-notify route may apply. Above it, or where the rating or adequacy is uncertain, the installer follows apply-to-connect and waits for the DNO decision.

Installers use ENA Connect Direct for current low-carbon-technology applications. The platform may give rapid results for straightforward cases, but no fixed response or completion time should be promised. A network study, cut-out work, unlooping, legal access or reinforcement can extend the job.

The process for solar and battery generation is separate. G98, G99 and G100 deal with generating units and export; the head-fuse assessment deals with import load and the adequacy of the service. A battery can engage both when it charges from and exports to the grid.

Load management

Dynamic load management can keep a controllable appliance below an agreed whole-property limit. A sensor measures import at the relevant boundary and the controller reduces or pauses the flexible load as other demand rises.

It can be a sound alternative to a fuse increase where:

the equipment fails safely if the sensor or communications path is lost
the measurement covers all relevant loads
the limit matches the confirmed connection capacity
the design prevents conflicting controllers from each assuming the other will reduce load
commissioning proves the response under high demand

It does not repair inadequate, damaged or unconfirmed service equipment. Nor does it remove G98, G99 or other notification requirements.

What a fuse upgrade involves

A larger fuse can be fitted only if the DNO confirms that the service cable, cut-out and upstream network are suitable. The meter, meter tails, customer tails, isolator, consumer unit, earthing and bonding must also be adequate for the proposed connection.

Possible outcomes include:

the existing rating is accepted with load management
a fuse is uprated in the existing cut-out
the cut-out or service cable is replaced
a looped service is separated
the meter operator or electrician must first upgrade other intake equipment
the DNO offers a different capacity or a three-phase connection

Charges and responsibility depend on why work is needed and on the DNO’s current policy. A safety repair, low-carbon enablement and customer-requested capacity increase are not automatically treated alike. Obtain a written scope rather than describing all fuse upgrades or unlooping as free.

Handover record

Keep the DNO confirmation of:

fitted fuse rating and phase arrangement
authorised import capacity, where stated separately
any load-management limit or condition
service-head, cable or unlooping work completed
application or notification reference

The visible fuse rating is not a substitute for those connection terms.