Meter Tails, Main Bonding, and Service Head Installation — Wiring the Consumer Unit Supply for UK Electricians

The connection between the electricity meter and the consumer unit is one of the most safety-critical sections of any domestic or commercial electrical installation. Despite carrying the full supply current, this short run of cable is often poorly understood — and getting it wrong causes fires, regulatory failures, and DNO refusals. This guide covers meter tail sizing, main protective bonding conductors, and the correct termination procedure at the service head and consumer unit, in full compliance with BS 7671:2018+A2:2022 (18th Edition).

Understanding the Supply Arrangement

Before touching anything near the meter, it is essential to understand who owns what. In the UK, the electricity supply to a property is divided into distinct zones of responsibility:

• DNO (Distribution Network Operator) equipment: The service cable, service head (also called the cut-out or service fuse), and meter tails up to and including the meter. This equipment belongs to the DNO and must not be worked on by a non-DNO operative without written permission.
• Meter: Owned and maintained by the supplier or metering agent. Again, not to be interfered with by an electrician.
• Meter tails from meter to consumer unit: The section of cable from the meter's outgoing terminals to the consumer unit is the responsibility of the property owner and therefore the installing electrician. This is what you can legally work on.
• Consumer unit and all downstream wiring: Your domain as the registered electrician.

The service head contains the DNO's main service fuse — typically 60A, 80A, or 100A depending on the supply capacity. You will never open this without DNO authorisation. However, you will be working immediately downstream of it when installing or upgrading meter tails.

Meter Tail Cable Specification

Meter tails are single-core, double-insulated cables typically supplied in red (line) and black (neutral) sheaths. The term "double insulation" refers to the conductor having both inner and outer insulation, giving two layers of protection without requiring an earth conductor within the tail itself.

Standard Sizes

The correct meter tail cross-sectional area depends on the DNO's service fuse rating and the length of the run:

• 16mm² tails: Suitable for supplies up to 60A. Used in older properties with a 60A service fuse and very short tail runs (typically under 3m).
• 25mm² tails: The standard size for 80A and 100A single-phase supplies. Required for most modern domestic installations. Also used where 16mm² would be marginal due to voltage drop or length.
• 35mm² tails: Required where the service fuse is rated above 100A, or where the tail run exceeds 3m on a 100A supply. Increasingly specified by DNOs on new connections.

Always confirm the required size with the relevant DNO before ordering. National Grid, UK Power Networks, Western Power Distribution, Scottish Power, and Northern Powergrid all publish their own engineering standards — and meter tail requirements vary between them.

Insulation Colours

Meter tails manufactured post-2006 use harmonised colours:

• Brown = Line
• Blue = Neutral

Older properties may still have red (line) and black (neutral) tails. When replacing meter tails in an existing installation, you must use the new colours. If the existing tails are to be retained, add a sleeving label at each termination point indicating the new colour convention.

Double Insulation

Meter tails must be double insulated because there is no protective earth in this section of the installation. The consumer unit earth terminal is bonded to earth separately (usually via the earth electrode, PME neutral, or metallic service pipe). The tails themselves carry live and neutral only.

Calculating Voltage Drop on Meter Tails

BS 7671 requires that the total voltage drop from origin to final circuit load does not exceed 3% for lighting or 5% for power circuits. Meter tails contribute to this budget.

The voltage drop in mV/A/m for common meter tail sizes (70°C thermoplastic, touching):

• 16mm²: 2.4 mV/A/m
• 25mm²: 1.5 mV/A/m
• 35mm²: 1.10 mV/A/m

For a 100A supply with 25mm² tails over a 3m run:

Voltage drop = 100A × 3m × 1.5 mV/A/m = 450 mV = 0.45V (0.19% of 230V)

This is well within tolerance. Problems arise only with very long runs (e.g., tails routed through multiple floors to reach an under-stairs consumer unit) where you may need to upsize to 35mm².

Running and Supporting Meter Tails

Meter tails are stiff, single-core cables and must be handled differently from standard twin-and-earth or multicore wiring:

• Minimum bending radius: 6× conductor diameter for single-core cables. Forcing tails around tight bends can crack the outer sheath or damage the conductor strands.
• Segregation: Do not run meter tails in the same containment as other cables unless separated by a partition or the containment is rated for 100A fault current.
• Support: Fix at maximum 750mm intervals for horizontal runs and 1000mm for vertical runs using single-way saddle clips sized to the outer diameter of the tail.
• Protection from mechanical damage: Where tails pass through floors, walls, or anywhere exposed to physical damage, use a conduit sleeve or mechanical protection. DNOs often specify armoured tail guards near the meter.
• Length: Keep meter tails as short as practicable. Ideally the consumer unit sits within 1–2m of the meter. If a longer run is unavoidable, consider siting a switch-fuse or isolation device closer to the meter.

Isolating the Supply Before Working

You cannot isolate the supply upstream of the meter without DNO involvement. This means that when working on meter tails, the service head and live side of the meter remain live at all times. The only safe working method is:

1. Confirm the existing consumer unit is switched off and locked off at its main switch
2. Ensure the existing tails are de-energised before disconnecting at the consumer unit end only
3. Work from the consumer unit end toward the meter — never start at the meter end
4. Use appropriate PPE including insulated gloves rated to 1,000V and a face shield
5. Apply temporary sleeving/insulating boots to any live terminals you are working near
6. Never work alone — always have a second competent person present when working near live meter equipment

If the scope of work requires disconnecting at the meter terminals, contact the DNO for a temporary disconnection. This is usually a chargeable service but is non-negotiable from a safety and liability standpoint.

Terminating at the Consumer Unit

The meter tail terminates at the consumer unit's main switch or busbar. Correct termination involves:

• Stripping length: Match to the terminal depth specified by the consumer unit manufacturer. Typically 15–18mm for 25mm² tails in a standard 80/100A consumer unit.
• Ferrule or bootlace: Some consumer unit designs require a bootlace ferrule on stranded conductors to prevent strand splaying. Check the manufacturer's installation instructions.
• Torque: Use a calibrated torque screwdriver. Manufacturers specify different torque values — typically 6–12 Nm for large main switch terminals. Under-torqued connections cause resistive heating and eventually fire.
• Phase barrier: Ensure the line and neutral tails are correctly terminated in the respective line and neutral bars. Verify polarity before energising.
• Sleeving: Apply green/yellow sleeving to any exposed earth tails, and colour-identify line and neutral tails with appropriately coloured ferrule or adhesive labels at all terminations.

Main Protective Bonding

Main protective bonding (also called equipotential bonding or just "main bonding") is one of the most commonly missed or incorrectly installed elements of a domestic installation. Its purpose is to ensure that all incoming metallic services are connected to the main earthing terminal (MET) at the same potential — so that if a fault energises any one of them, they all rise to the same voltage simultaneously and a dangerous potential difference cannot exist between them.

Which Services Require Bonding?

Regulation 411.3.1.2 of BS 7671 requires main bonding to be applied to all extraneous-conductive-parts that introduce a potential, including:

• Gas installation pipework: Bond to the incoming gas meter (on the consumer side of the meter union) or as close to the entry point as practicable.
• Water installation pipework: Bond to the incoming water main as close as possible to the point of entry to the building, on the consumer side of the water authority's stopcock.
• Oil supply pipework: If an oil-fired system is installed, bond the incoming oil pipe.
• Structural steel / metallic framework: Where structural steelwork could introduce a potential (e.g., in commercial/industrial buildings with steel frame construction), this must be bonded.
• Lightning protection system: If a separate LPS is installed, its earth terminal must be bonded to the MET.

Supplementary bonding within bathrooms and other special locations is a separate requirement — do not confuse it with main bonding.

Main Bonding Conductor Size

The minimum size of main protective bonding conductors is determined by Regulation 544.1.1 and Table 54.8:

• The minimum copper bonding conductor size is half the size of the largest protective conductor in the installation, subject to a minimum of 6mm² and a maximum required size of 25mm².
• For most domestic installations with a 10mm² or 16mm² main earth conductor, this gives a minimum bonding conductor of 10mm².
• In practice, 10mm² green/yellow cable is the standard main bonding conductor size for domestic properties in the UK.

Do not use the smaller supplementary bonding conductor sizes (2.5mm² or 4mm²) for main bonding — these are for supplementary bonding within zones only.

Main Bonding Clamps

Main bonding connections to pipework must use purpose-made bonding clamps, not just cable clips or jubilee clips. A proper bonding clamp:

• Has a brass or gunmetal body to resist galvanic corrosion
• Provides a screwed, wrenchable connection that cannot vibrate loose
• Is sized to fit the pipe diameter (15mm, 22mm, 28mm are most common for gas/water mains)
• Is labelled with a durable "Safety Electrical Connection — Do Not Remove" label

The label is a legal requirement under the Electricity Safety, Quality and Continuity Regulations 2002 and must be permanently fixed in a visible position.

Routing the Main Bonding Conductor

The bonding conductor runs from the bonding clamp on each service pipe back to the main earthing terminal (MET) inside the consumer unit (or on a separate earthing block if the CU is in a remote location). Requirements for the conductor route:

• Must be identifiable along its entire length — either green/yellow insulated cable throughout or sleeved at both ends with green/yellow sleeving
• Should be as short and direct as practicable
• Should not be jointed — a single continuous conductor from clamp to MET is required. If a joint is unavoidable, use a permanent maintenance-free connector
• Must be supported at appropriate intervals — treat as you would any other cable run
• Where it passes through walls or floors, protect in a conduit sleeve

Earth Electrode and Main Earthing Arrangements

The type of earthing arrangement affects how you install the main earth conductor from the consumer unit back to earth. The main earthing system types are:

TN-C-S (PME — Protective Multiple Earthing)

The most common arrangement in the UK. The DNO provides a combined neutral and earth conductor (PEN conductor) in the service cable. The earth terminal of the consumer unit is connected back to the DNO's PEN conductor at the service head via the meter. This is why you see a separate earth tail running from the service head earth terminal to the consumer unit earth bar.

PME has a potential hazard: if the PEN conductor is broken between the substation and the property, all PME-earthed metalwork in the property rises to line voltage. For this reason, PME earth connections must not be used for mobile or transportable equipment or where there is a risk of the supply being fed via a generator without a proper PEN conductor.

TN-S

The earth conductor is a separate metallic sheath of the service cable (often lead-sheathed cables in older properties). These are increasingly rare. Identify by checking: if the earth connection at the service head is made to the metallic sheath of the incoming cable (rather than to a combined neutral/earth terminal), it is TN-S.

TT

Where no reliable DNO earth connection is available (common in rural areas fed by overhead lines), the installation must use its own earth electrode — typically copper-clad steel earth rods driven into the ground. An RCD must be used to protect all circuits in a TT installation because fault loop impedance values will be too high for overcurrent protection alone to operate quickly enough.

Verification Testing After Installation

After installing or replacing meter tails and main bonding, the following tests are required before energising the installation:

1. Continuity of main bonding conductors: Using a low-resistance ohmmeter (test current >200mA), verify that each bonding conductor has a resistance not exceeding 0.05Ω. Higher readings suggest a poor connection at the clamp or MET.
2. External earth fault loop impedance (Ze): Measured at the incoming terminals of the consumer unit (with all bonding connected). Record the Ze value — it will be needed for Zs calculations on all downstream circuits.
3. Prospective fault current (PFC/PSCC): Measure the prospective short-circuit current and prospective earth fault current. Verify that the consumer unit and all its components are rated to interrupt this fault current.
4. Insulation resistance: With the consumer unit main switch open, measure IR between line and neutral, and between live conductors and earth, on the meter tail cable. Minimum acceptable value is 1MΩ.
5. Polarity: Confirm that line and neutral are connected to their correct terminals throughout.

Record all test results on an Electrical Installation Certificate (EIC) or a Minor Electrical Installation Works Certificate as appropriate. Notify Building Control (or self-certify under a competent person scheme) for any consumer unit replacement.

Metering and Smart Meter Considerations

Smart meters (SMETS2 type, now being rolled out by all UK suppliers) have specific installation requirements that affect meter tail work:

• Smart meters require a Communications Hub Device (CHD) installed on or near the meter — ensure sufficient space and that meter tails are not obstructing its installation.
• In-Home Displays (IHDs) connect wirelessly and do not affect the meter tail installation.
• Where a consumer unit upgrade is required alongside a smart meter installation, coordinate with the energy supplier — they will often want to install the smart meter at the same time to avoid two separate de-energisations.
• Check whether the existing meter tails are the DNO's property or the customer's — some older installations have the DNO's meter tails running directly from the cut-out to the meter and the meter to the CU, in which case you need DNO authorisation before touching them.

Notifiable Work and Certification

Consumer unit replacement, including replacing or rerouting meter tails, is always notifiable under Part P of the Building Regulations in England and Wales. This means:

• Either notify the Local Authority Building Control (LABC) before starting work and have them inspect, or
• Use a competent person scheme (NICEIC, NAPIT, Elecsa, etc.) where you self-certify and the scheme notifies LABC on your behalf

Failure to notify is a criminal offence. On completion, the scheme issues a Building Regulations Compliance Certificate which the property owner should retain — it will be required on sale of the property.

Common Errors and How to Avoid Them

• Under-sized meter tails: Using 16mm² tails on a 100A supply. Always check the DNO's service fuse rating and your calculation.
• Missing or loose bonding clamps: The gas and water bonding conductors are the first thing an NICEIC inspector checks. Loose clamps are a major fail.
• Bonding clamp on the wrong side of the meter: The bonding must be on the consumer side of the gas/water meter, not between the utility's meter and the street.
• No labelling: Missing "Safety Electrical Connection — Do Not Remove" labels on bonding clamps is a BS 7671 violation.
• Over-tightening or under-torquing terminal screws: Use a torque screwdriver and follow the manufacturer's specification. Over-tightening can crack ceramic terminal blocks in older consumer units.
• Working single-handed near live meter equipment: Always bring a second person. This is a non-negotiable safety requirement.

Meter tails and main bonding are unglamorous work — but they sit at the heart of the installation's safety and fault protection. Getting this section right protects the property, the occupants, and your professional indemnity.

Related Guides

• Surge Protection Devices and AFDDs (#96)
• Electrical Installation Testing: Continuity, IR, Loop Impedance (#122)
• Junction Boxes and Electrical Enclosures (#131)
• RCD Tripping: Diagnosing Causes and Fault-Finding (#133)
• Building Regulations Part P (#162)
• UK Electrical Earthing Systems: TN-S, TN-C-S, PME and TT (#169)
• Consumer Units, MCBs, and RCBOs (#173)
• SWA Armoured Cable: Glanding, Termination, and Underground Installation (#177)

Products from APM Plumbing & Electrical

Get it from APM Electricals, Acton

We stock what UK trades need — same-day collection from our trade counter in Acton. No minimum order. Trade accounts welcome.

Browse our cables and wiring range, or visit us at 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 to check stock.