AS/NZS 3000:2007 - 3.4.1 A2

[Slovett]

Sorry if this has been asked before - AS/NZS 3000:2007 - 3.4.1 A2 says that in domestic installs in NZ, that we must assume that Thermal insulation will be installed in walls, ceiling and under floors. If not currently, then in the future.

So, the way I interpret this clause is according to table C5 in 3000 and Table 10 in 3008.1 - A 16mm2 1 core Neutral screen in a domestic install used as Mains, for example, has to be protected by no more than a 40A MCB or equivalent - assuming the 16mm is installed inside a wall (and not clipped to the Stud) to the MeterBoard and through to the Main SwitchBoard

What are your thoughts?

[JamieP]

We have to assume thermal insulation will be installed if not. So yes we have to consider it but that doesn't mean it's going to be completely surrounded. As you know it's all about how you choose to run the cable and you definitely need to consider how the insulation is likely to be placed in regards to this. In regards to down walls insulation is normally pushed over the cable to one side and the cable becomes wedged between the lining and insulation making it partially surrounded.

As per 3.4.3 of 3008, to be completely surrounded the cable has to be covered completely for more than 400mm

So although every case is different, normally partially surrounded can be achieved with some consideration

In terms of CCC, all the single core N/S I see is X-90 as it's XLPE so table 11 is the appropriate one to use, looking at Cu 54A for full and 86A for partially

So this is why most mains in a standard new home when given a 63A supply use a 16mm² single core N/S X-90 rated for partially surrounded

[AlecK]

Nearly but not quite.
Yes we have to assume BTI; but we don't have to assume "fully surrounded".
Adding batt / blanket types of BTI, cables will only end up partially surrounded .

So if using Table C5, the CCC will be 63 A.

But why use that table, which is based on "worst case", and with fewer options for installation conditions;
also based on the higher ambient temperatures that apply in Oz?
Better to use the relevant Table in "3008.1.2";

In this case Table 10 for 2-core sheathed, thermoplastic insulated (also applies for N/S as per clause 3.3.5).
That gives CCC 46 A for fully surrounded, unenclosed (6 A more than Table C5 !)
62 A for partially surrounded, enclosed.
and 73 A for partially surrounded, unenclosed (10 A more than C5!).



Or, if the N/S is XLPE, Table 11 (as Jamie suggested).
My understanding is that generally NS is generally PVC up to 16 mm, and XLPE above.
But can vary between manufacturers.

[TPower]

Is maximum demand also a consideration here?

On the Orion network we generally have to fit a 63amp MCB prior to the meter, as the network fuse is only providing short-circuit protection, so we have to fit the MCB for over-load.

I guess this is taking the path of max demand by limitation. Could you not do a max demand calculation also? I’m thinking a lot of domestic dwellings would be less than 60amps.

So whilst you have a 60amp MCB fitted in the mains, you’ll never reach this limit anyway, assuming your calculations suggest this is the case. So this would make the 60amp MCB pointless?

[JamieP]

Max demand is a consideration but I'd say most new installations not many people work it out and why would you when you can just size your mains to the supply capacity giving the client full use of what's provided for future

You could do a max demand calc and provide a smaller cable size and smaller over current protection but doesn't really do anything but save cost and less support for future

Remember, your max demand is just for selection of conductor size, you can't not protect a cable adequately based on max demand

Also some networks have requirements for minimum size of mains

[AlecK]

Agree MD isn't only relevant to cable selection; not to providing the cable with overcurrent protection

[Slovett]

Thanks for the reply’s - The reason I say 40A MCB protection for a 16mm2 N/S is I’m using worst case scenario. Most of the N/S I see is PVC (probably because it cheaper the XLPE), so table 10. And in the Waikato, from what I’ve seen, in the walls, it’s common practice for the thermal insulation installers to slice the batts in the middle, open up the insulation and slide it around the cables. Completely surrounding it. Because of this I can’t assume that a different installer will push the cables to one side of the insulation if installed in the future. Also, we can’t assume how the cable has been installed when the house is finished (without cutting into the wall)
The WEL Network area don’t mandate MCB protection for the Mains ie at the MeterBoard. So the majority of houses are relying on the 63A supply HRC for overload protection (70A with fusing factor). Even though the Mains are likely to be 16mm or 25mm.

The reason I’m asking, is that with the amount of Load a modern house uses these days, which will get bigger as time goes on (EV Chargers etc). 40A at peak times isn’t going to last long. (Spa heating, cooking, Charging EV). And if the 16mm N/S is only relying on the supply fuse, surly the Mains inside the wall could potentially be deteriorating, unseen.


Also Notes from tables C5 and C6 3 (d) A2 (3000) says cables passing through more than 300mm of thermal insulation may be treated as completely surrounded. Is that a mandate or a recommendation? The word “may” sounds like a recommendation?

[JamieP]

Maybe I need to double check my N/S type, maybe it's is V-90 but the logic still stands. I haven't had to work with with such in a while so was going off memory but could be wrong.

Yes, you're not wrong, assumption can cause serious issues, the ones I made were based on my own experience and what I had encountered in such situations but obviously if your situation is different you must change accordingly

Having a quick look at the Wel networks document I can't see anywhere that they offer overload protection, just short circuit and fault protection as required by ESRs

So overload protection probably falls on the person doing the installation and if you are correct in that it's completely surrounded then I guess your options are 35mm² V90, 25mm² X90, 25mm² V90 with a 50A protective device or a 40A protective device for the 16mm² mains

Don't have to install such device in the meter enclosure, can be used as the main switch

[Slovett]

That’s true, but you would be doing well if you found any overload protection for Mains in a house wired in Hamilton / Waikato. No one gets audited, so no one either knows about it, or bothers. The silly thing is, when you work it out, it’s actually cheaper to install a MCB with an enclosure in-place of the Standard HPM F180 in the MeterBoard before the Meters, which is the norm here.

EDIT - Also, it should be the Inspectors job to pick up that there is inadequate overload protection on the Mains. But again, they either don’t know about it, or just don’t care

[JamieP]

Ew, HPM F180 is gross haha I doubt they remove the main switch label either, which is misleading

You can only worry about the work you do mate, in terms of others that's theirs to certify, you can advise but if they don't listen, nothing you can do

Other than obviously start reporting if you believe the work is non-compliant or if you believe it's an immediate risk to life or property, report to WorkSafe but I don't believe this situation meets the definition

[DougP]

Just a couple of observations.
The Network's documentation will not say that they provide overload protection, but the same as any HRC fuse, they do provide overload protection. It's common in many areas not to have any additional overload protection on the installation side.

For the insulation installers slitting the insulation and wrapping it around the cables, yes this does happen, because almost none of them have been trained to install the insulation correctly, and as per the standard. It's the electricians job to make sure that can't happen. So that means clipping to a stud, or other methods to prevent the cable being fully surrounded. At least with a screened cable, you don't need to be too concerned about distances from the surfaces of the walls. Or go back to the job before the plasterboard is installed to check that your cable installation isn't surrounded (if you are installing protection as partially surrounded). Because if it happens with the mains, it will happen with all your cabling.

There are also council building inspectors that tell builders that insulation can't be compressed behind cables, or behind flush boxes. Sometimes this results in the insulation installer cutting insulation around flush boxes, which is worse IMO.

You can buy NS cable as PVC up to 50mm² at least. So just use the appropriate table from 3008.1.2 depending on the type you use. Table 10 for PVC, or 11 for XLPE

[Slovett]

The Network's documentation will not say that they provide overload protection, but the same as any HRC fuse, they do provide overload protection. It's common in many areas not to have any additional overload protection on the installation side.

Is the fact that its common for no additional over load protection on the installation side an oversight by the Electricians maybe? Most House bashers I know still rate every power point with a C20A MCB and every light Circuit with a C10A MCB regardless of installation conditions.
Even if we went off the Partially surrounded values. In my opinion (Please correct me if I am wrong) A 63A HRC (70A with fusing factor) for overload protection on the supply side is still too big for a 16mm PVC N/S.

[PeteRig]

AS/NZS 3000: 2.5.3 has the formulas for protection against overload.
16mm PVC NS using table 10 from 3008 (as mention above) is good for 73A partially surrounded, so should be ok

[DougP]

2.5.3.1 - For HRC fuses the protective device needs to be 0.9 or less of the cable CCC. So the 63A (60A?) fuse is fine for 16mm² PVC partially surrounded.

[Slovett]

AS/NZS 3000: 2.5.3 has the formulas for protection against overload.
16mm PVC NS using table 10 from 3008 (as mention above) is good for 73A partially surrounded, so should be ok

Yes your right Pete. 73A Partially surrounded. My mistake, I was looking at Partially surrounded, in a wiring enclosure.

[Slovett]

2.5.3.1 - For HRC fuses the protective device needs to be 0.9 or less of the cable CCC. So the 63A (60A?) fuse is fine for 16mm² PVC partially surrounded.

Sorry Doug, my mistake, I was looking at the wrong section of Table 10. So 16mm PVC n/s partially surrounded is rated at 73A, Which Means the HRC has to have a rating of 73A x 0.9 = 65A. So a 63A HRC at the Supply fuse is fine as long as its only Partially Surrounded

[AlecK]

Time was when it could be reasonably assumed that, with a 63A supply fuse, 16 mm2 conductors would be more than adequate for both external & internal sections; and even allow for reasonable future increase in MD
But times have changed; and there are a number of factors already in place that mean we should no longer assume 16 mm2 will be adequate.
Also worth noting that when 2018 edition is cited; we'll need to be adding at least 20 A to the MD assessment for the mains, for EV charging.
And while not mandatory yet, we should be considering it now.
We only get one chance to get the mains right, and designing to bare minimum acceptable is not good practice.

Choice of cable for mains is an installer decision, as is providing o/l protection.
Networks are required to provide s/c & fault protection; but o/l is down to installer.

Relying on the network's supply fuse to provide overload protection is also a choice made by the installer.
If we do, it's our responsibility to establish what rating will be used during the design process - not just assume.
And for both certifiers & inspectors, this needs to be checked during/after installation.
Then ESR 73A requires person connecting to check this again.

Domestic mains are typically split at the meter location, with smaller active conductors (set up as "common neutral") for controlled loads.
These smaller conductors also have to be sized not just for load, but so the supply fuse can provide s/c & fault protection for them (else require an additional protective device). In effect a reduction of CCC, so needs review of o/l & s/c protection arrangements. o/l can be at downstream end, but s/c can't.

Factor of 0.9 that we need to apply if using fuse for o/l protection [2.5.3.1]
So if the cable we choose has 73A CCC, then a fuse used for o/l protection can't be rated higher than 65.7 - effectively will be 63 A.

I've not struck BTI installers splitting batts around cable; but agree if that happens the result is "completely surrounded".
Which injected foam will also create.
3.4.1 doesn't require us to assume 'completely surrounded"; but we do need to apply a reasonable degree of forethought.
And that must include assuming relevant Standards for installation of BTI will be followed.
So if we have reason to believe "completely surrounded" is likely; the answers is either to select cable accordingly, or to ensure our cable is secured against structure so it can only ever be "p.s."

So working on c.s., CCC =46 A x 0.9 =41.4; so 40 A is max fuse rating, and same for mcb.

O/L protection doesn't have to be at origin, but with split mains has to be either upstream of the split or taken as sum of the devices for the individual branches.

Agree mcb(s) at meterbox are cheaper / easier than an isolating switch.
They also allow option of o/l protection for the upstream section of mains, and all 3 types of protection for any reduced-CCC parts of the mains.