Meter Tail Regulations

Meter Tail Regulations

If you are installing a new consumer unit, or completing electrical inspections, you’ll need to be familiar with the meter tail regulations in force at the current time.

Whilst there is no separate “meter tail regulations”, sections of BS7671 (18th edition wiring regulations) does deal with installation of cables, of which the meter tails must also comply.

The parts of BS7671 which apply to meter tails can be broadly summarised in the following sections.

As this guide is intended to assist with 18th edition electrical inspections, we will focus mainly on what you should be checking during an EICR.

However the sections below give some indication of what you need to be doing when installing meter tails on a new installation.

It is also worth cross referencing this with our Consumer Unit Reguations Guide 2022 which has portions relating to meter tails

Requirement for adequate support for Meter Tails/Cables

This is something which is far and away the most common issue with meter tails.

They are often hung from the meter over to the consumer unit (or even fuse board on older installations).  This is often on the fashion of a large “lazy loop” as the thick cable does not bend too easily.

Meter Tail Regulations 2022 - Adequate Support Of Cables Against Strain
Meter Tail Regulations – Adequate Support Of Cable

I have personally come across an instance where a tenant has hung an umbrella by the handle from the drooping meter tails!!

Loose connections within main switches and electric meters are a huge source of fire risk. 

The very first and obvious thing about meter tails should be that they are secured in a way that does not place strain on the terminations.

This could take the form of a meter tail gland on the consumer unit, although these are a fairly modern addition to installations.

Another acceptable method would be using clips of some kind.  Obviously on an 18th edition “installation” you would need to use heat resistant fixings.

However when inspecting older installations then pay particular attention to the cable to ensure it is secured adequately. 

The meter tails often pass from an external cabinet through a wall cavity to the consumer unit. 

In most instances like this the cable passing through the wall is providing adequate support, assuming the length of tails within the meter cabinet are not too long.

Do Buried Meter Tails Need RCD protection?

It may sound odd, but the answer is yes, meter tails buried in walls, less than 50mm from the surface will require additional protection by 30mA RCD.

It should also be noted that meter tails buried in walls which contain metal parts must be RCD protected regardless of the installation depth of the cable.

A common installation method with meter tails is to run then between an external meter cabinet, through the cavity, into the consumer unit through the rear of the board.

Following the meter tail regulations to the tee would frown on this installation method as the cables being within the cavity is not a recognised installation method as per BS7671. 

I’m not suggesting you start recording that on EICRs as the simple fact is, it presents no apparent risk to the cable.

Cables buried less than 50mm underneath the wall surface is a different matter though.  To be honest it’s not something which we see very often, BUT I can think of a very specific installation type which often has buried T&E as the meter tails/submain to the consumer unit.

This would be older buildings which have been converted to flats.  I can think of a couple of instances I have seen this in old pubs which have been converted.

The usual setup there is a switch fuse between the meter and a piece of 16mm T&E used as the submain cable.  Quite often this is run throughout the building to each individual flat.

Whilst it is impossible to follow the route of the cable through the fabric of the building, it’s hard to see how they could run it without at least part of the cable being buried inside the wall.

Because of the more serious nature of the submain cable, if you come across this then I would advise to strongly consider upgrading it.

The ideal scenario here would be to use SWA cable as it does not require RCD protection in any meter tail regulations.

Adding a 30mA RCD upfront could cause alsorts of nuisance tripping issues, and does not really meet the requirement for division of circuits to minimise inconvenience.

Meter Tail Regulations – Identification Of Conductors

When considering meter tail regulations, it should be remembered that the conductors need to be correctly identified.

On a single phase supply you will simply have L & N, however on 3 phase supplies all 3 phases will require separate identification, together with the Neutral conductor.

This would generally be achieved by colour of conductor, however externally to the meter there should be no unsheathed basic insulation.

Some meter tails are now coming with coloured sheathing in order to accurately identify the conductors.

Meter Tail Regulations 2022
Meter Tail Regulations – Identification Of Meter Tails

However plain grey sheathed meter tails are most common.  These are indistinguishable from each other without external identification, in the way of letters, numbers or colouring

This can achieved in a number of ways:

  • Using coloured bands, tape or cable ties. These could be in either set of BS7671 colours (if completing EICRs). However if you were completing a new installation then they would need to be the new harmonised colours.
  • Lettering. This can be as simple as writing on the outer sheathing with a marker pen (ie: L1,L2,L3 & N). Obviously this would need to be large enough to read and at more than one single point along the length of the tails.

Meter Tail Regulations for TT Installations

TT Earthing Arrangements have particular requirements for the meter tails over and above Terra-Neutral (TN) systems.

The low loop impedance of a TN system ensures that the main supply fuse (or other main OCPD) would disconnect were the meter tail to come into contact with the external enclosure of a metallic consumer unit / distribution board.

It used to be for this reason that solely insulated (plastic) consumer units were fitted to TT earthing arrangement systems. However the advent of AMD3:2015 to the 17th edition of the wiring regulations brought a requirement that all main switchgear in domestic properties was to be of a non-combustible type (IE: metal)

As the loop impedance is generally much lower on a TT system, this cannot be required to disconnect in the required time, leaving a metal enclosure (and everything connected to the MET, including any bonded pipework and class 1 electrical items) live in the event of the meter tails coming loose and touching the enclosure.

For this reason, there are a number of specific requirements about the meter tails to ensure that they are safe against accidental contact with a metallic enclosure, TT installations have deeper meter tail regulations than normal TN installations.

In this instance, the meter tails must be secure within the enclosure itself. This can be achieved with the use of a meter tail gland on the consumer unit body. There are also a number of consumer units available which have 2 hoop clamps which fit over the meter tails right about the main switch.

Meter Tail Regulations for TT Installations
Meter Tail Regulations For TT Installations

Not only this, but when you fit a metal consumer unit to a TT earthing arrangement system, you must ensure that the meter tails have their full ‘double insulation’ (more accurately the outer sheathing of the cable) right the way to the entry of the main switch.

This is an important consideration to ensure that there is a far reduced chance of any potential contact between the live meter tails and the metallic enclosure when installed on a TT system

Sizing Meter Tails

Domestic and small scale (>100A) three phase installations have fairly simple requirements for sizing meter tails:

  • 16mm tails are fine for 60A, 63A & 80A main supply fuses (BS88/BS1361 Type2)
  • 25mm tails are required for 100A main supply fuses

Newer BS1361 Type2 supply fuse holders have 100A written on the side on a label. This generally indicates the maximum sized fuse that can be fitted, these heads may have lower rated supply fuses.

When inspecting installations which have 16mm tails, you should not be disturbing the main fuse (whether it is sealed or not) to check for the correct rating.

Sometimes you can get lucky and there is a sticker fitted which indicates the actual rating fitted, as opposed to the max rating of the fuse carrier.

Otherwise you may have to enquire with the Distribution Network Owner, although even they rarely know for sure what is fitted in older heads.

It should be noted that when installing meter tails in a new domestic installation, I would always advise fitting 25mm. Ultimately the advent of EV (Electric Vehicles) and other modern technologies mean that expected current demands are getting higher all the time.

Most DNOs specify that new supplies require 25mm meter tails before they will connect up.

Rules on Length Of Meter Tails

These are slightly different meter tail regulations to those outlined above in that this comes from the DNOs own requirements as opposed to BS7671

Most DNOs specify that meter tails cannot be more than 3 metre long without having another method of fault protection inbetween the main service head and the tails.

The reason for this is that the DNO don’t want electricians relying on their main service fuse (usually BS1361 type2)

The usual way to resolve this issue if you are needing to fit longer tails between the meter and the consumer unit is to fit a “switch fuse” inbetween. This has its own fuse built in that provides fault protection to the tails if they are over 3M long.

Occasionally this is also done with an up front DP MCB (we have seen many 63/80A type C) which provides protection to the tails after. This method often suffers from a lack of selectivity in that any fault downstream often trips the upfront MCB

These types of arrangements are usually seen in flats/apartments where the incoming supply and meter can often be much more than the 3M maximum allowed by the DNO.

In these instances, it is common to see a room dedicated to switchgear and meters on the bottom floor. The tails then enter into a switch fuse in this room before going up the rest of the building to each individual property.

The cable from the switch fuse to the properties consumer unit is usually something other than traditional meter tails however, SWA submains are quite common.

If you are unsure of the stance of your local DNO, then you should contact them to ensure that you are not extending the tails beyond their maximum allowable length (usually 3M)

Summary Of Meter Tail Regulations 2022

If you have skipped to the end looking for a quick summary to the most important meter tail regulations then check the following list which outlines the basics:

  • Meter tails must be adequately supported so that no strain is on the connections
  • On EICR = 16mm cable upto 80A, 25mm cable upto 100A. New installations generally DNO needs to see 25mm
  • TT installations then sheathing must be continuous all the way to the main switch. Special consideration given to ensure no strain on connections, usually use hoop type cable grips
  • If meter tails are buried less than 50mm from a finished wall surface, they must be provided with 30mA RCD protection (note this does not meet requirements for selectivity between protective devices)
  • Conductors must be properly identified to ensure there is no confusion over polarity
  • Meter Tail Regulations from the DNO generally state that they cannot be more than 3M long (without some form of consumer fault protection inbetween)

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10 Responses

  1. I wonder if you could answer my question, whilst it appears that most meter tails seem to average around 3 metres long , is there a specific law that governs there length ?

    • Hello Keith, thanks for reading and posting the question.

      With regards the length of meter tails, this is actually not a regulation under BS7671 (The Wiring Regulations), however this is a requirement from the DNO (Distribution Network Owners).

      Some DNOs have differing requirements, however most require a maximum of 3M from their cutout.

      The reason behind this is that they don’t want to give fault protection to meter tails over this length with the main cutout fuse.

      You can have meter tails longer than this, however you need to have your own overcurrent protective device protecting the tails.

      Our local DNO, Northern Powergrid have this on their requirements for a new install on page 7 of this guide:

      https://www.northernpowergrid.com/asset/0/document/539.pdf

      Hope that helps?

      Regards, Electrical Assistance

  2. Is it permitted for meter tails to exit an external surface mounted meter box through the top, be secured to the wall and then passed through the wall higher up (assuming all is accomplished within the maximum distance for the tails)?

    • Hello David, thanks for posting the question.
      From what you describe, I don’t think this would be the correct way to achieve this. In the case of traditional “meter tails” being 6181Y cable, this cable is not suitable for mounting outdoors without additional mechanical protection (ie: capping, large trunking).

      Consideration must also be given to the fact that 6181Y cable is not suitable for mounting where it is subject to direct sunlight. Any amount of UV from sunlight will eventually damage the grey outer sheathing, causing it to go brittle and crack off.

      If you do need to run the tails surface mounted from the meter box, there are commercial products to resolve this issue, including exterior grade cable capping. Often seen on telephone poles covering the cable drops.

      This or some heavy duty trunking (50×100) or (100×100) leaving the meter box although you would have to ensure that any product used is suitable for use outside as exposure to weather can cause white trunking to go brittle and crack.

      Hope that helps?

      Kind regards
      electrical-assistance

  3. Good morning,
    I will be having an additional single way consumer unit fitted to supply a EV charger.
    This unit will be fitted inside the fully enclosed porch approximately 300 mm above and slightly to the right of the meter box which is sunk in to the wall.
    Ideally I would prefer the meter tails to exit in to the cavity wall and then to be glanced in to the rear of the consumer unit.
    Although this would be more work it would give a cleaner finish without the tails being on display. There would technically be about 0.5M of tails in the cavity wall.
    The existing consumer unit tails also run in the cavity wall.
    I appreciate that the new regulations prefer tails not to go in to cavity’s, but would this be acceptable.
    Thank you for any advice you can give me.

    • Hello Martin

      Thanks for posting the question. From what you have described, I honestly cannot see any problem with running the tails within the cavity the short distance between the meter enclosure and the new EV charger consumer unit as you describe.
      One thing to be aware of is if there is cavity wall insulation, this can mean you need to use the next size meter tails up, as any cable ran through insulation experiences a “derating factor”. Using 25mm tails instead of the common 16mm on EV consumer units will suffice for such a short distance. Your installing electrician should be able to perform the necessary calculations.

      Hope that helps?

      Electrical-Assistance

  4. Hi. would it be permissible to run 25mm tails straight through the back of the meter box, through a cavity wall and terminated into a 80a switch fuse? Then run 25mm swa to consumer unit. Basically meter box and switch fuse will be back to back on the cavity wall.

  5. When I had extension constructed in 2011 my electrician said I needed another phase of supply due to the total load of electrical underfloor heating. We paid DNO to have a trench dug and new supply and 3 Phase meter fitted. Although the supply head has three 100A fuses only 2 sets of 25mm meter tails run from the meter and then to my 2 consumer units. If I now want to install a 3 phase ev charger, who can connect my meter to the currently unused 3rd phase? Is it my DNO or my supplier or my EV instsaller?

    • Hello Sean

      Thanks for getting in touch. The setup you have described, if there is currently no connection between the L3 of the incoming supply and the electric meter then this would generally be the responsibility of the supplier. The DNO won’t usually touch anything ‘after’ the service head

      Hope that helps?
      electrical-assistance

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