Hi,

Just looked at a house with 16mm tails with a 100A 1361 fuse in the cut-out, TNCS. All the bonding is fine.

I dont thnk the load on the tails is ever likely to exceed about 80A for any length of time and if it was the previous regime of PIRs I would have thought a code 4 as the REC's used to accept 16mm tails.

The consumer unit has been fitted within the last few years as it is a dual RCD type, but the tails have not been upgraded. What are your opinions on the code?

Also if the situation was different and the consumer unit was the original, what code then?

Edited: 14 September 2012 at 09:28 AM by leckie

How do you know its a 100A fuse, they dont have code 4's anymore.

Sorry, when I said PIR, I meant if we were still under the old PIR regs, as opposed to EICR's.

I cant be certain it is a 100A, but it is labelled 100A not 80A or 60A so I am assuming that to be correct.

The label usually relates to the fuse carrier, rather than the fuse that's fitted (they're all the same size), so difficult to be sure without cutting seals. Around here (YEDL) I understand that they're now fitting 80A fuses as "standard". Black cutouts are apparently only rated to 100A for short periods - for continuous 100A they're fitting grey ones. Not that that helps you much - there could be a 100A fuse in there.


Makes no odds - you're comparing what's there now with the current regs (some guidance refers to things acceptable under previous editions, but that's really just a short-hand for how risky the non-conformity is).


My gut feel agrees, but it would be nice to be able to prove it.

Does the sum of the outgoing ways (or size of fixed loads if smaller) of the CU add up to? (if it was just a couple of lighting circuits, one ring and immersion and cooker) it's probably easy to show that there's adequate overload protection.

- Andy.

Thanks for that. I think I will have to check the actual fuse rating then to make sure.

If the fuse is 100A and assuming the max demand is 80A would that be a code 3? If the load could be 100A I presume it would be a code 2 but dont think that is likely

Just because you have a 100A fuse, doesn't mean that the installation is going to take 100A.

Whilst not in accordance with current guidance, BS 7671 does actually allow a fuse rating to exceed a cable rating where it is not required to offer overload protection.

Given that 16mm2 tails are good for at least 90A continuous and probably way over 100A in short(ish) burts I wouldn't be worrying - no house is ever going to get near that.



Careful of that one Andy - there is a non tripping factor to consider.

Take 2 x 10A MCB's - they could deliver 22.4A for ever

Add to that say 2 x 32A MCB's - could deliver 71.7A

Thats a difference between 84A and 94A

About 12% error

Regards

OMS

-------------------------
Failure is always an option

I don't see that this would be any code at all, unless you can show that the conductors are undersized.
To my mind, you have three options:
Establish the rating of the fuse,
Record it as requiring further investigation,
Record it as a limitation.

But wouldn't that also be the case if we had a single protective device upstream? 10x 10A MCBs downstream might allow 112A to flow, but so would a single 100A MCB upstream - they all have the same 1.12x factor, but nevertheless satisfy In <= Iz.

- Andy.

- Andy.

Yes, if you had the same device upstream and downsteam - the characteristics for fuses and MCB's would be different (although not by much).

I think my point though was that adding up MCB ratings to show that the tails were protected because of a limitation in the delivered current prone to an under estimation error.

Regards

OMS

-------------------------
Failure is always an option

I think that's what I'm not following....

(using 100A just to keep things simple)

If I had a 100A (In) MCB upstream of 100A tails - I think you'd agree that the tails are deemed to be protected from overload. That could allow 1.12x In = 112A to flow indefinitely - but we accept that (subject to the 1st line of 433.1 of course).

On the other hand if I had ten 10A MCBs downstream, as you say, each could allow 11.2A to flow indefinitely - which makes a total of 112A - isn't that absolutely identical to the single 100A MCB's performance?

- Andy.

Yes, but the tails can only carry say 90A (in this case) - so considering say 9 x 10A MCB's being 90A by addition (and you assume that's OK) or 100A if all are subject to simultaneous overload and neither upstream or downstream devices operate and the cable is at risk.

regards

OMS

-------------------------
Failure is always an option

If there was such a beast as a 90A MCB, would you be happy with it protecting 90A tails?
- Andy.

Ib</=In</=Iz

Of course I would, Andy - BS 7671 says I can -

I guess I'd want to know the cyclic nature (or otherwise) of the load though -

We could say it's an adjustable 100A MCCB if you like - long time pick up set at 0.9 = 90A - it would allow 100A + to flow forever, I know

Regards

OMS

-------------------------
Failure is always an option

...and it's characteristics could be identical to the sum total of nine 10A MCBs?

- Andy.

So, are we say 16mm tails, clipped direct, tncs,100a bs3631 is ok?
Are we saying that if doing a con unit change and existing tails are 16mm they don't need upgrading? All assuming likely max demand is 80A.

Leckie

A bit worrying that you mention a Code 4 on a PIR and then appear to not know the different requirements of Initial Verification and Periodic Inspection and Testing?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/

--"BS7671 does actually allow"--------

you can see why our esteemed poster is up till 11 at nights-reading guide
books that have no bearing on supply situations.
thankfully we have spinlondon to offer a correct answer, and our topic
writer should advise the legendary DNO to further investigate the main fuse size and downgrade if necessary to 60amp or change tails to 25sqmm.

Regards.

LoL -

25mm2 tails - domestic residence - predicted demand pattern - average thermal load about 2.5kW per house - right,



Have you considered a career in stand up comedy - that's priceless -


OMS

-------------------------
Failure is always an option

I have been to aproperty today where they (supply authority) installed and labelled that an 60 amp fuse is fitted.
The property is a three bed terrace.
The local authority work very shoddy, main earth connected to metal clad service termination loose, holes where you could get two fingures and a thumb in.
Whoose watching the watchers!!!!

--I really will have to invite the old boy to a day out with the guys and girls
who help to make london and the east tick over!

I will show him how, while we do work on 2.5 kw / dwelling the going system of mains, even for a 1 bedroom flat is 100amp main fuse, 25sqmm tails, 16sqmm earth lead and a 100amp main/sw feeding
rccds and mcbs.

I will show him, tongue in cheek, how his fellow consultants give commercial clients grief by specifying 200amp or so dis boards when their clients only require 100amps, and the 50sqmm tails have to be
downgraded to 35sqmm to match the mains.

i will show him what happens when contractors virtually overnight quadruple old age pensioners homes loads with n/stors and imm heaters and bathroom heaters when they change from coal parkrays on
a small housing estate in a rural village.
when the t/x volts drop to 200.
its no good waving the green book in the air.

what we do when customers in a petrol station get tingles from the pumps where electricians have given themselves PME in rural areas.

and many, many more.

and of course----------a genuine PNB system.