WHY are CONSUMER UNIT meter tails a maximum of 3 METERS long?
Maximum consumer unit meter tail length is defined by parts of BS7671 and local DNO (Distribution network operator) requirements.
Joe Robinson explores the topic and explains where the maximum meter tail length of 3 meters originates and the thinking behind the limits.
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== TIME STAMPS ===============================
00:00 Meter tails maximum length
00:16 Checkout our BG consumer unit review
00:44 What does BS 7671 say...?
01:44 The 3-metre regulation
02:22 DNO quote BS 7671
02:30 FREE CPD
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📹 Presented by
Joe Robinson - Technical Editor eFIXX
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I absolutely love these short, informative videos. So much information, told clearly and simply. Thanks, Efixx.
Our pleasure, thanks for watching and commenting, plenty more on the way! 😊
Agree Joe is slick and very articulate when explaining electrics
Thanks so much. 😊
Brilliant video I could watch these videos all day! Joe your an excellent presenter very knowledgeable and clear. Keep up the good work guys 👍🏻
Ah thank you, that's really kind, plenty more on the way. 😊
Joe is the most powerful brain in the Electricial forget everyone else
Ah, go on with ya! Cheers Sam. 😊
I'll remember this. Thanks Joe .👌💪✅
My pleasure!
Well explained Joe, thanks for sharing.
Thanks for watching and commenting as always Brian. 😊
@@efixx This is a quite interesting document Joe. www.northernpowergrid.com/asset/0/document/539.pdf
I'll have a look thanks Brian.
Very good explanation, many thanks.
Excellent presentation. Really appreciate the precise language.
That's really kind of you thanks. 😊
Point 1. Up to the service provider to determine if the fuse provides sufficient protection for the meter tails: If the main fuse matches the current carrying capability of the meter tails, then the length of the meter tails doesn't matter. Point 2. The 3 metre limit length on the metre tails is arbitrary as there are only two impacts: Firstly the voltage drop across the length of cable when full load current occurs, secondly, the risk of a fault occurring is greater but that is only if physical damage occurs and the cable can be protected against that. You could argue that there has to be a limit specified to ensure the Zs impedance is low enough to enable a sufficiently high current to flow to trigger the circuit breakers and I think a limit is appropriate for that. But this impedance downstream from the main fuse including meter tails can be measured,albeit not with the main fuse still in situ. It is certainly good to have limits specified so that an electrician can design an installation to those limits and be sure it meets the various R1+R2 impedance limits.
In Thailand it's normal for the metre to be in the street where the feed comes down from the overhead route then back up and into the property. In our case the metre is at the corner of our garden on the pole then the cables run overhead again to our property via a second poke almost 100mtrs to the consumer unit, only then at that location there is an isolation. For that reason I had 32mm2 cables put in as we have a 100amp supply. I need to change the CU soon and the only isolation will be by having the cables removed from the outgoing side of the metre.
I think you may have misread 434.3. It states "see item (ii) of Regulation 434.2.1" Only item (i) makes 3m a requirement.
I am doing a job at the moment and the meter tails are 14 meters long and have no switch fuse fitted . Also run in the roof thought the joists resting on the metal joist plates .. a nice find 😯
😱
Never knew why it was 3m Thanks
Would it not be the case that if you did put a fused isolator in line with the tails for distances greater than 3 meters for fault protection that you wouldn't get adequate selectivity in any case as there would still be chance of the dno's fuse operating as the reduction in current for a fuse to operate closer to the db would have to be at at ratio of 2-1 and for fuses of 60 80 or 100 amps this wouldn't be practical.
If you run the 2 DI cables and the 16mm earth wire through the building to the C U 8 metres away do you have to put in a 100 A 100 MA time delay RCD at the meter or just a double pole 100A main switch as DNO 100 fuse already there
What about an IDNO where the cut-out is just a red Henley link and the actual fuse is back at their MSDB board. The distance of the run from their fuse to their cut out will have exceeded the 3m rule already?
Use an MCCB slightly down rated from the main fuse. Then run armoured from there.
Top tip!
On existing builds where its say a flat where the meters are elsewhere in the building like a landlords meter supply cupboard/room or where the meter cabinet is the other side of the property (rear of house) to the consumer unit (front of house) I'm going to guess a KMF isolation switch with 60amp is fuse still acceptable.
That type of thing with the right sized submain. 👍
@@efixx Another situation i have seen is a large residential home building with 3 floors with just over 30 rooms/self contained flats is 3 separate landlord supply meter rooms on the ground floor ( it is a large building)with a made up retrofitted bank of 63amp mcbs cut into the 100mm metallic trunking at the landlords supply cupboard rooms each x2 supplies to each flat as there is off peak as well So about 20 x 63 amp mcbs per landlords supply cupboard with trunking lid cut to fit over the mcbs! (I'll have to take a photo one day) All looks a neat job and contained though with no access to live parts and a,separate 3 phase fused isolation ( think Mem Excel or Bill switch gear) The landlords supply power in the same cupboard is from 3 phase distribution boards for the communal power & lights etc in the same cupboards.
That is why TN-C-S is so popular and is the only earth arrangement for DNO to the meet the requirement for fault protection when popular BS1361 100A fuse fitted so the Ze@cutout@5s is 0.37 or 0.35 (Cmin) as the trip fault current is round 630A . So if meter tails is more than 3 meters or the earth arrangement is TN-S or TT, best thing to do is install MCB (TN-C-S or TN-S) or Time delay RCCB (TN-S OR TT) within 3 meters at load side of meter tail.
Great comment thanks. 😊
TNC-S is popular for one reason only, and that's cost.
If fitting a fused isolator with a 60Amp I can have 5 meters or more so if in the incoming it is a 60amp can we have the same 5 meter tails if there 25mm
Moved a consumer unit 2 months ago 5 metres to the clients kitchen thought I’d cover myself by using 25mm 3 core swa with a lewden fuse carrier. But then I’m in a new build and the tails are definitely longer than 3 metres to my board and after a look in the wall cavity they’re not protected?
Sadly not everyone follows your high standards. 😔
@@efixx thanks 😊 Any videos on the horizon about the subject of what material to mount consumer units onto? Had one last week screwed to the back fibre board of a kitchen unit🙄
Didn't there used to be something called the "CM16 conditional rating" regarding meter tails? Or have I dreamt it? It was a while ago, 1980s????
Yep, all well and good but many wood framed buildings especially built between 1984/92 have consumer units positioned in the middle of the building making the distance covered by the tails on average 4 to 5 metres if not longer, requiring armoured cable and in many cases a isolator (WY DSF100M others are available) is required too as many of these early 80s builds have no main switch. Cut outs are yanked as a matter of course🤷🏻♂️ So the intellect that changes the regs has no idea of the state of wiring in Britain or he does and thinks the whole place is a deathtrap😱
It seems, that the regulation only applies if the meter tails is of a cross sectional area which are NOT approved for the cut-out fuse or meter tail fuse. So if you use beefier meter tails which would be approved for 100A (or whatever cutout fuse is), then they can be at any length. With "approved", I mean, a fuse that is suitable for the meter tail with regards to trip time, source impedance, and short circuit protection. Looking up in tables, its 35mm^2. So running 35mm^2 meter tails should be OK for any length what I can see. But then you need to get that 35mm^2 approved by DNO what I understand. So basically, the 3m regulation allows you to "go down" in cross sectional area even if that wouldn't normally be okay for that particular fuse. Note that regulation 434.2.2 specifically says that you may install a fuse ANYWHERE before (supply side) the point of the reduction of current carrying capacity. So 434.2.1 says that a lead, that aren't approved for the supply fuse, may be protected by another fuse past the supply cable of lower cross sectional area (of lower current carrying capacity), provided that fuse is no more than 3 meter after the reduction in current capacity, and that lead have no branch to anything else.
I assume that if there is an isolator switch that the tail length from the main fuse to the isolater must be included as part of the 3m.
That's a really good question, I guess it would depend on who owned the switch, the DNO or the homeowner. They're normally right by the meter though.
As I understand it. The DNO owns the service head and tails up too but not including the meter, the surplier owns the meter, and the customer owns the tails from the meter including the isolation switch.
@@robertburrows6612 I must have had a bit of a brain fart when I made the comment... I meant meter, not fuse.
Doesn’t 434.2 concern reduction in CSA? Meter tails are usually at least the same size as the service cable.
In most instances the service cable is smaller than the tails, especially for older properties.
So if the incoming supply cable is 25mmsq and meter tails are 25mmsq, there would be no reduction in the current carrying capacity of the conductors, so no requirement for OCPD within 3m, so tails may be longer?
I think it's more to do with a change of ownership of the cables as much as it is the size. Remember the reg says that the DNO has to agree that it provides protection and they probably won't.
You got to remember the longer the tails are or infact any cable for that matter , the resistance on the tails or cables increases and there current carrying capacity decreases . Often when you exceed 3 meters you need to install 35mm2 tails or larger depending of the length of distance the CU is from there meter. Often a failing of some sparkies selecting the right cable size for the the distance the accessories are away from the CU
The ones I have seen, are not 25mm more like 16mm and they used to be aluminium
seen one on a New Build site im fairly sue they must have been longer than 3mtrs including from the meter Box to high level Consumer Unit in zone Coroner of hallway but no Mechanical protection I would have thought it would have been capped with bonding especially as it was a Rental
So what happens if it needs to be longer? - I've seen a block of flats which has many tens of metres between the meters (in a dedicated meter room) and the CUs in each flat
You would use the relevant armoured size. So a switch fuse would need to be installed at the meter end, meter tails comings in, armoured going out to the flat for mechanical protection.
What he said! 😂
@@efixx I don't think it has to be armoured unless it's buried in the wall to a depth less than 50mm or erected in one of the other ways that requires additional protection by RCD. If it's clipped to the surface, twin and earth will do, as will conduit or 6181Y cables. The use of SWA as one of the options in 522.6.204 is only necessary if your erection method is referred to by 522.6.201; 522.6.202 or 522.6.203. I would also disagree with the use of armoured cable 'Going out to the flat for mechanical protection'. My understanding of the categories listed in 522.6.204 is that SWA affords compliance with part i - 'earthed metallic covering', and not part iv - 'mechanical protection'. It's a subtle difference but the idea of the steel armour providing mechanical protection is incorrect and can lead to muddled design decisions. The steel wires will not prevent penetration of nails, screws and the like.
Yep I’ve seen this also
@@westinthewest If it's in dedicated duct work meant only to supply those flats it counts as mechanical protection on most designs signed off by building control. Which is why almost all the student accommodation and blocks of flats previously and currently being built around sussex uni's haven't complied with the reg. Also I concur with your interpretation of SWA, it does not count towards mechanical protection in any of the factories or other industrial applications I've previously or currently work in and for. I myself tend to swap to armoured via switch fuse and then back to flexi tails by the consumer unit if so required. It keeps the meter tails below the 3m limit even if the submain is 30 meters. I also tend to fit a meter tail isolator just before the consumer unit for future consideration but it allows the installation to be safely locked off in two places when one is quite a distance away.
Can meter tails run up vertically inside a cavity wall from the meter box to ceiling level (about 1500mm) where it then turns horizontal and travels between the joists then vertically down in a stud wall, 50mm from finished plaster surface at all times.
yes provided the ceiling is 0.5m high
@@mattsan70 The ceiling joists are only .25m high so could I run meter tails outside in metal conduits or below a 65mm screed in the 110mm insulation? I've not met the electrician yet but it would be helpful to know what the options are.
Interesting. NAPIT Codebreakers publication lists this as code FI with no regulation number. It just says 'ESQCR' I had a block of flats recently where the consumer tails were >3m and contained within blue alkathene pipe. Asked WPD to investigate and they installed a DP isolator, charged the landlord £216! I asked them why they did that as it is not fused and they couldn't give a definitive answer. I told them to remove it and I would install a fused isolator, all they were concerned with was me pulling the service fuse. I did it anyway and re-sealed the carrier. WPD are useless in Devon. There is nothing written in ESQCR that calls for a fused isolator.
Always good to hear some experiences from the frontline. 👍
I asked ENW about switched fuses, they said they prefer a DP isolator and then put whatever necessary after that. Again I don't get the impression they're that bothered about tail length - at the end of the day there fuse is protecting the tails and if it blows then that's what it's there for. If someone puts a nail through a tail then the wait for a DNO callout is least of their worries. The MOP guy said he doesn't like switched fuses because when they accidentally damage one - or the screws were over torque by the previous person - then the whole job comes to a halt. If an isolator needs replacing they'll grab a new one of the van.
my consumer unit is 25 metres from the meter. it's been there since 1930's. how can i re-wire this?
I am glad we don't have that restriction here in the USA. As long as its in conduit we are good to go. Our old home had aerial line about 20 meters across the back yard, down from the roof to the power meter, and then another 20M or so to the panel, which is where the 1st main breaker was, zero other protection on that line other than the high voltage 13.2Kv fuses on the pole feeding the homes on the block. That was 200A 240V split phase service.
Woah! Interesting how different countries do installs. Thanks for commenting!
Interesting, I used to install electric meters and aware of the 3m rule. But in apartments/flats where 3-phase bemcos are installed, I’ve installed as much as 20 elec meters at a time, 3-phase and single phase, and I can tell you the 25mm cable was more than 3m in length half of the time. I wonder how the designer gets around this
Was there a switch fuse closer to the meters? 🤔
@@efixx no, this would be main fuse in the bemco, then sometimes a 5m run enclosed with many other tails, inside containment, up to the meter incoming, and usually a short run thereafter to a switch fused isolator, exiting that in SWA to the in flat CU. And this wouldn’t be only 1 job. Most of the bigger flats/apartments are done exactly the same.
Yeah I did a similar one and the meters were all fed via steel trunking from a three phase head, we then took responsibility from there and did max 3m runs of tails out to an MCB in an enclosure and an armoured to the flats. Maybe in your case the designer got the DNO to agree the tails could be longer as long as certain installation conditions were met.
@@efixx possibly, our area is northern power grid (YEDL). We just installed the things tbh. I know they were over 3m because we were only supplied 3m coils of tails for that reason, and most of the time the client had to either have it pre-wired by a third party or supply us with drums of cable
It's an interesting situation isn't it, I always think it's important to look at the intent behind the regs, it can help to understand why things are done a certain way and even consider a departure. Thanks for the great comments. 👍
This doesn't really explain the science behind this. It is unclear if it's just that making the tails longer means that there is simply more chance of damage simply because of that length, or is it electrical. That is if there is a dead short from line to neutral or line to earth (on a TN-S or TN-C-S) that insufficient fault current will flow for the DNO fuse to blow within the required time due to the increased resistance of the longer tail. I can't say I find the latter particularly convincing if earth loop impedance tests have to be carried out from the consumer board. If it passes that, then sufficient fault current should pass.
Metres not Meters, unless you are talking about the machine dispays how much electricity you use.
We'll revoke biscuit rights for the relevant minion!
@@efixx can't you publicly flog them? You could then upload it.......
Blimmin' employment law, it'd turn into a whole thing!
@@efixx pesky laws....
Thank you for pointing this out, I was into the comments to add the same point. The irony is that's it correct in the timestamps in the description.
It's only complicated because of the way they write the regulations, why not say, meter tails cannot be run greater than 3m, beyond that a suitable submain cable must be used. There's no confusion then, and that's how the regulations should be, not leaving things down to interpretation.
Exactly, make the wording simple! Couldn’t agree more
Its not true that the meter tails have to be max 3m. They can be longer but must have xtra protection.
So is this about meter tails or CU tails? They are not the same thing.
Yes the same thing
@@efixx meter tails from meter to service head, CU tails from CU to meter, no? There must be thousands of modern arrangements in which the CU tails are well over 3 metres.
@@andyturner4977 Regardless of what you call them if the wires from the electricity meter or service head to the consumer unit are greater than 3 meters you need to add a switch fuse within 3 meters.
@@efixx how does this square with one of the other videos where it says you can’t put anything in the meter box? If there is no practical place to put the fused dp switch? What can you do? My place there is no where you can put a cu anywhere within 3 meters without dismantling the entire kitchen. Fuse box was on a wall behind a cupboard.
I would have thought an electrician would be live to the difference between a meter and a metre 😉
My house is fed by an overhead supply. The external cable terminates at a gable end eaves where they attach to what look like 35 sq mm individual cables (maybe aluminium) that are just draped across the rafters in my loft before descending into the meter cupboard where they terminate on a 100A service fuse and a neutral block. There is an in-line fuse (rating unknown) at the point where the external cable attaches to the internal ones, but I'm not really that happy at having these unprotected cables running through my loft.
There shouldn't be a problem so long as the cables are high up and out of reach so they are not likely to be subjected to mechanical damage. If the cables are of the correct cross sectional area to handle 100 amps and the fuse is 100 amps, the fuse should prevent the cables over heating and catching fire. However see below. I'm not keen on them simply hanging so it might be better to fasten them down. My concern would compliance against regulation 422.1.2 as they are running across what I presume to be a wooden and therefore combustible structure. Regulation 422.422.1.2 Electrical equipment shall be so selected and erected that its normal temperature rise and foreseeable temperature rise during a fault cannot cause a fire. This shall be achieved by the construction of the equipment or by additional protective measures taken during erection.
Interesting. I live in the SSE area and just had my installation inspected and a new CU plus various C2 faults to be sorted out. My meter tails are over 3 metres long plus they run through the cavity. The sparky said there is a 2 metre max distance but there is no way that can be accommodated plus the cable is undersize. He also says that because it runs through the fabric of the building, i.e the cavity that it should be armoured. This has been classified as a C3 but since there is no way short of rewiring the whole house that the meter tails can be shortened or rerouted away from the cavity or even changed for the correct size and armoured it will be left as it is and still be certified satisfactory. Comments please?
Maybe this will help www.northernpowergrid.com/asset/0/document/539.pdf
@@brianwood5220 Thanks Brian I'll have a look and see if SSE have a similar document. The problem is that the supply isn't changing. I can only rely on my sparky to advise on what can and cannot be done to produce a safe installation. The house has been standing for over 30 years now and probably has less load now than when the place was built because of the use of LED and higher efficiency appliances so I am certain it is/will be safe, just not necessarily within the regs.
@@CP-du3ci What you need fitted is one of these near the meter. amzn.to/3pGHEpr
@@brianwood5220 That's right thanks Brian, I had sussed that would take care of the excess length of the meter tails but I'm not sure about the size of the conductor. It's not specified on the report but I believe they are 16mm with a 10mm earth. Seems they should be 25mm and because it runs through the cavity (double block wall) should be armoured.
@@CP-du3ci 25mm 3 core armoured would be best and then it brings a CPC (Earth) with it. But make sure you get someone qualified to do the job. Best of luck.
Truth be told it's to do with low Ohm Measurement.
Dickie lee RAF fighter pilot
Most of these artificial limits are just a gimmick. Those rules actually think that they work. But if you look in just behind an imaginary line called "boarder" those limits do not exist. And the problems those limits solve also do not exist. So what does exist?
I think you may be looking for pFIXX, our philosophy channel. 🤔
Have you been watching the matrix 😂
So basically I can just install nine metre long tails, because nine metres is safer than ten metres.
Oh come on man . 3 meters ?? Really ?? So you are saying we have to have 3 meters ?? Isnt 1 enough ?
😂
Can't believe you still haven't changed the video title to 3 metres . You do know a meter is a measuring device and a metre is a measure of metric length ???
METRES
Guys, can you please please use the English spelling of METRE and not indoctrinate everyone with the American way - meter. You can of course use it when referring to your electric meter, gas meter etc. Rant over.