Meter tails longer than 3 metres: what you need and why

Helpful video reference. We use eFIXX's video "METER TAILS LONGER THAN 3 METRES - Switch fuse isolators from LEWDEN" as the reference here. eFIXX - The Electricians' Channel - covers the regulation background and walks through two Lewden switch fuse isolator options, showing how they mount, how the tails connect and what the installation should look like when it is done correctly.

1. Why meter tails are limited to 3 metres

The meter tails run from the DNO's cut-out fuse (the sealed unit at the bottom of your meter cupboard, which belongs to the electricity network company) to the main switch in your consumer unit. They carry the full incoming supply current for the property.

The cut-out fuse provides overcurrent protection for this cable - but it is rated at 60A, 80A or 100A depending on your supply, and it only operates on very large fault currents. A fault on the tails that draws, say, 500A will blow the cut-out fuse quickly. But a lower-level fault on a long run - an insulation failure that allows arcing rather than a dead short - may not be large enough to operate the cut-out, and could sustain a fault current for long enough to start a fire in the cable.

BS 7671 Regulation 521.15.2 addresses this by limiting the unprotected run between the cut-out and the first overcurrent protective device (typically the consumer unit main switch) to 3 metres. Within 3 metres, the risk of a sustained fault causing a fire is considered acceptably low. Beyond 3 metres, additional protection is required.

2. What happens in practice

In most UK homes the meter and consumer unit are in the same cupboard or on the same wall, and the tails are well under 3 metres. The regulation is relevant in three common scenarios:

• Consumer unit in a different room. The DNO meter is in a hallway cupboard but the consumer unit has been placed in the garage or kitchen. The distance through the wall easily exceeds 3 metres.
• Moving the consumer unit. When a consumer unit is relocated - often to clear space or to meet the BS 7671 Regulation 132.12 requirement that it not be in a kitchen - the new position may be further from the meter than the old one.
• Large or unusual properties. In some houses the DNO entry point and the main distribution position are separated by a long route through the building.

3. The switch fuse isolator solution

The solution required by BS 7671 is to install an overcurrent protective device - in practice, a switch fuse isolator - within 3 metres of the cut-out fuse. This device provides:

• A fuse that will operate if the tails beyond it develop a fault.
• An isolating switch so that the remainder of the installation can be isolated without requiring the DNO to isolate the supply.

With a switch fuse in place, the unprotected section (from cut-out to switch fuse) is kept within 3 metres, and the protected section (from switch fuse to consumer unit) can be any length.

4. Choosing a switch fuse isolator

For a standard single-phase domestic supply, a 100A switch fuse is the correct rating. Lewden manufacture a commonly used range (the SF100 and related models) that is designed for this application. The unit is typically surface-mounted close to the meter, with the meter tails entering at the top and the onward cable to the consumer unit leaving from the bottom.

Key points when specifying the unit:

• The fuse inside must match the supply: a 100A supply needs a 100A fuse, not a smaller one.
• The enclosure must be correctly IP-rated for its location - a garage or external meter cupboard may need IP65 or better.
• The terminals must accept the cable size being used for the tails (25mm2 is standard for most domestic supplies).

5. Cable sizing for a longer run

Standard domestic meter tails are 25mm2, which is adequate for a 100A single-phase supply at typical domestic run lengths. If the longer run introduces a volt drop concern, the electrician will need to calculate the actual volt drop against the BS 7671 limit (normally 3% for lighting, 5% for power) and upsize the cable if required. For most domestic situations where the switch fuse is within a few metres of the cut-out, 25mm2 tails continue to the consumer unit without issue.

6. Labelling and notification

The completed installation must be labelled clearly. The switch fuse enclosure should show its purpose, its rating, and which consumer unit it feeds. If it is on a dual-supply or shared metering arrangement, additional labels will be needed to avoid confusion at the distribution point.

Installing a switch fuse for meter tails is notifiable work under Part P of the Building Regulations in England. The registered electrician carrying out the work must notify building control and issue an Electrical Installation Certificate on completion.

When to call us

This is not a DIY job. Meter tails and switch fuse installation requires a registered electrician, DNO notification in some cases, and a Part P completion certificate. Richard covers consumer unit moves, switch fuse installation and meter tail upgrades across east Kent.