using 6mm as mains tails in a domestic DB

[bennettst]

Hi Everyone

Hey can someone show me where in any wiring Reg's is it ok to use a 6mm as mains tails from the main switch to the meter and then from the meter onwards in a domestic DB,
I'm told ok to have a 16mm from the gyro to the main switch and the use 6mm for the tails for the inspector to connect the meter and then continue on to the DB if its in another location in 16mm.
I know the 6mm tails in question are only about 300-400mm long but the only fuse protection it is in the gyro at 63A,
from what I see in the Reg's the max fuse for a 6mm cable is 40A, i know the 'average' house will not run a full load for a long amount of time but i would of thought the minimum size tails used by the electrician would be 10mm or am i missing something
Asking for a friend

[DougP]

The current ratings for conductors in switchboards is unenclosed in air. Normal PVC cable ratings are 3008 table 4.
So the rating of 6mm² would be 58A @ 75deg.

Almost all conductors these days are V90, so exceeding the 75deg rating is possible, and that would take the rating up to 67A.

[JamieP]

As Doug said, it's not about length it's about different installation condition and is directly related to insulation temperature limits

6mm in those conditions is suitable for the current required and the reason it changes back to a larger size is because it's installation conditions change

3008 series has all the information, just make sure you get the one for NZ conditions

[AlecK]

6 mm2 is not good enough; even if using V90 conductor.

True a 6 mm2 probably meets the requirement of 3.4.1 that the CCC be not less than the current to be carried.
But that's only one of several requirements.

Overload protection is not designed & installed on the basis of what's likely to happen; and it has absolutely nothing to do with maximum demand.
It's required to be designed and installed to absolutely prevent conductors carrying excessive loads;
by applying the relevant calculation for the type of protective device used.

So having satisfied 3.4.1; we need to consider 3.4.2; ensuring that the max conductor tempo is NEVER exceeded.
Which means dealing with short circuit protection & overload protection

Bearing in mind that while networks are required to provide short circuit protection for "mains";
if you don't tell them about the use of reduced-size conductors for part of the mains they can't deal with it.
(Truth be told; they won't anyway, they'll expect you to use adequately sized conductors for their arbitrarily-selected fuse rating).
Regardless, applying 2.5.4.5 & assuming 63 A supply fuse, 6 mm2 will be protected for SC.

Networks are not required to provide an overload protection for mains; that's our job.
Clause 2.5.3.1 requires that HRC fuses be rated not more than 0.9 x CCC of the conductor.
So for the assumed 63A supply fuse, 63/0.9 = 70 A CCC required - so 10 mm2 minimum.
Alternatively, fit an additional overcurrent device to protect the 6 mm2; eg as the meter isolating switch.

But of you're using 16 mm2, why reduce conductor size at all?
Any tiny saving on materials will be more than eaten up by the increased time faffing about; and meter terminals are generally OK for 35 mm2.

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I'd also point out that since we're discussing mains, there can't be any "DB" involved; it must be an MSB.
For a DB, the conductors would be submains.
Which makes no difference at all to the fact that just having a CCC high enough for the expected load is only one of several requirements we have to comply with.

And the switch upstream of the metering is generally not a "main switch".
It can only be a "main switch" if it's at the MSB (exception for alternative supplies connected at DBs).

Lastly; the person installing the revenue metering isn't acting as an "inspector".
The person may hold an "I" Pl, and may also do some inspection of high risk PEW while they are there;
but installing meters is not part of inspecting.
Installation of meters is general PEW (for which a CoC must be issued).

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Yes that's all rather pedantic; but when we're discussing rules it's important to use correct terminology.
Sloppy wording leads to sloppy thinking; and from there to incorrect interpretations.

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please explain "gyro" in this context.
From the fact "the only fuse protection is in the gyro"; I presume this is either a pillar or pit where the installation's supply is connected to the network.
But it's not a standard term; in fact it's one I've never heard in this context
"gyro" is usually short for gyroscope, and Google suggests it's come to mean a sandwich using meat off a rotisserie.
It also used to be what they called the dole in the UK)
But how does an electrical connection pit / pillar get to be called a 'gyro"

[PeteRig]

Hi Alec, I believe the OP when using the term "gyro" is referring to the company Gryo Plastics whom make underground distribution pillars.
Peter

[JamieP]

My comment for 6mm being suitable was based on 63A MCB used as over current protection, this because where I'm from we provide our own isolation before meter and also provide over current protection here in the for of an MCB

Also assumed V90 cable, unenclosed in air

Just to clear that up, apologises for any confusion