Imported: Maximum demand in final subcircuits

This includes types of overcurrent, Short Circuit and RCD protection
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DougP
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Imported: Maximum demand in final subcircuits

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DougP
Sep 25 2018 13:17


I know this has been discussed many times..

But why does C2.5 seem to say that there is only 2 methods for determining the maximum demand of a final subcircuit?

Now I have guys arguing that you can\'t have multiple powerpoints on 3x16A circuits, on a 40A MCB, because C2.5.1(b) says it must be by limitation. (for example)

I know 2.2.2 is the actual methods. And in the 2018 version there\'s an additional note on C2.5.1 that says:
\"NOTE: Table C9 provides guidance on the loading of points per final subcircuit.\"

I don\'t know why the C2.5.1 clause would be so out of sync with the generally acceptable methods of diversity as well as other clauses like 2.2.2 as well as C5.2 and table C8 (C9 in 2018 book) etc...

Any thoughts?

toyoto
Sep 25 2018 14:10


They are just Australians arguing you should have everything on a rcbo.

AlecK
Sep 25 2018 14:56


I believe the apparent discrepancy doesn\'t matter. The process has to be:
1 establish the current to be carried (ie maximum demand);
2 select an appropriate cable;
3 protect the cable, absolutely, against overload.
This is clearly stated in 2.5 (in particular 2.5.3.1), and that\'s pointed to in C2.1.

How any one with more than half a brain can read C2.5.1 as forbidding multiple sockets is beyond me. For starters, all of App C is labelled as \'informative\" which means it explains stuff but sets NO requirements.

For the case of 3 x 16 A circuits, all three provided with RCD protection by common RCD; there are several relevant ratings.
There\'s the CCC of the cable for each subcircuit, which must be sufficient to carry the MD of that subcircuit.

There\'s the 16 A rating of the mcbs, which limits the current that can be carried in any subcircuit to a value less than the CCC of the circuit cabling. This can be, but isn\'t necessarily, an example of max demand by limitation. In fact, the max demand in any or all of the subcircuits may be much less than their CCC. It\'s really that third step of the three outlined above; protecting the conductors.

Then there\'s the rating of the RCD; which is NOT part of any of the subcircuits (they commence at the respective mcb and at the N-bar). This is covered by 2.6.2.1, and basically does not have to be capable of supplying the 3 x 16 = 48 A that the subcircuits could carry if ever fully loaded.
We have to consider 2 values, the first of which is the MD of the part of the installation protected by the RCD. And as noted already, the actual MD may be less than 48 A. The 2nd value to consider is the highest current rating of the the overload protective devices for the part-installation protected by the RCD. In this case, all are the same at 16 A. So the absolute minimum rating for the RCD will be 16 A; but if the MB for the 3 subcircuits is higher than 16 A we have to use the MD instead and use a higher-rated RCD. Effectively this system applies the principles of diversity to selection of RCDs.

Lastly - and most often ignored - is the current rating of the conductors feeding the line side of the RCD and also from the RCD to the line side of the mcbs.
We have to apply the same 3-step process to these as for any other conductor:
1 establish MD
2 select appropriate conductor
3 protect the conductor from ever being overloaded.
Typically the relevant protection will be either upstream (the supply fuse, generally 63 A) or downstream ( the sum of the mcbs, ie 3 x 16 = 40A) so to this scenario, the cables need to have a CCC of 48 A minimum.
However if the line side of the RCD is looped to another RCD, those conductors need to be sized for both sets of mcbs, which could be 3 x 16 + 3 x 16. Or even 3 x 16 + 3 x 20. But if the supply fuse is 63A, we don\'t need to go past a CCC that it can protect.

Absolutely NONE of this prohibits the circuits supplying multiple sockets. Whether they supply sockets, and if so how many, is relevant to establishing MD. If the circuit cabling isn\'t capable of carrying that MD, then 2.5.3.1 has not been complied with (Iz not at least equal to In). But given there are several acceptable methods for establishing MD, there\'s some wriggle-room


What it might mean is that an existing socket circuit wired in 1.5 mm2, or in 2.5 mm2 de-rated for BTI, may not be capable of having more sockets added, due to the increased MD. Especially if BTI has been installed, reducing the CCC of the cable.

Simply relying on MD by limitation to allow load to be added would be wrong. This is really a matter of \"fit for purpose\", not a safety issue.

Another argument often advanced is thast Table C8 forbids socket circuits being wired in (for example01.5 mm2, or protected at 16 A.
That\'s not true, because - as already noted - the entire appendix is guidance, NOT rules.
The values used in and derived from all the tables are conservative. We can sail closer to the rules if we want to - but in doing so we need to be fully aware of exactly how close, and we have to accept responsibility for our choices.






f how you because

DougP
Sep 25 2018 15:57


Thanks for the detailed explanation Alec...

Sorry, part of my question might have been misleading.

My reference to multiple power points, referred to the difference between C2.5.2(a) single item, and C2.5.2(b) multiple items.
(b) - inferring that when there\'s multiple items, the MD of the subcircuit must be by limitation.

I almost thought that where you say that the RCD is not part of the subcircuit, may have decided it.. But 2.6.2.1(a) says the load current rating shall be not less than the MD of the \"portion of the installation\" - i.e. all three circuits.

So the argument was, that C2.5.1(b) states that for multiple items (p/points) it must be by limitation of the current rating of a circuit breaker. Which doesn\'t agree with 2.2.2 (or C5.2 but that isn\'t strictly for MD calcs).

So the process should be that 2.2.2 sets the requirements, and appendix C provides the guidance?

Which part of the appendix C should be used to determine the MD for final subcircuits supplying muiltiple power points?

Table C1 is for mains/submains
Table C8 is titled guidance on the loading, but doesn\'t mention maximum demand.

I feel like I\'m missing a piece of the puzzle.

AlecK
Sep 25 2018 18:30


I think the key is that because (all of) App C is \"informative\"; no interpretation that it sets any rules can be correct.
Your summary, that requirements are set in the main part of the Standard (in this case 2.2.2 and 2.5.3), and additional guidance is provided in the Appendix, is correct.

I had another long post here but just deleted it to take a different approach.
The real question is, why do we want to know the MD of an individual final subcircuit?

One answer, for purposes of 2.5.3, is to establish what size cable to use.

The other thing we want an MD for is for volt drop calcs. And for that, using limitation means you can never be wrong.

Yes we have 4 methods for establishing an MD, but they are not all equal and sometimes one will be better or more appropriate. For anything yet to be installed, we simply can\'t use \"measurement\". The figures for calculation may work OK for all of (mains) or a significant part of(submains) an installation, and at that level they\'ll generally be conservative; but they\'ll be too low for an individual final subcircuit because diversity works a bit differently at subcircuit level. Assessment is only any use for particular known circumstances; and it simply doesn\'t work for a circuit full of sockets where we can have no real idea of what\'s going to be used when, or for how long. So looked at that way; for a multiple-point subcircuit that we haven\'t yet installed, limitation is the only method that we can realistically use.

So C 2.5.3 isn\'t saying we \"shall\" use that method, it\'s just providing appropriate guidance.

I suspect most of us use some sort of rule-of- thumb guestimate when determining load to be carried; but from there on limitation is the way to go.

For mains, I suspect few routinely do MD calcs, we just \"know\" that 63A is the standard domestic supply (for instance); so we size our mains to carry that.
If we did an MD, we might get away with smaller - but we\'d have to provide overload protection. And we\'d have less margin for later additional load to be added.

Same as we generally just allocate half the available volt drop to mains and half to subcircuits.

But for an unusual case, or a very long run, it can be worth a bit of calculation, and beefing up subcircuit cabling in order to have more VD available in mains (or vice versa). And even calculating an actual MD rather than simply applying limitation and calling it 63 A. The difference between say 54 A and 63 A matters when you\'re pushing the limits; and the cable size will likely be so damn big that protecting it isn\'t going to be an issue.



DougP
Sep 25 2018 19:13


Thanks Again Alec. Of course I agree with everything you\'ve outlined there..

But, where does that leave our 40A RCD with 3x16A MCBs attached?

Which method do we use to know that the MD of those three circuit breakers is no more than 40A and we have complied with 2.6.2.1(a)?
Which guidance do we use for power point MD?
1-20 points is 10A? table C1
Or 1A each up to the CB rating on C8?

And why does table C8 state that \"sum must not exceed rating of circuit-breaker\"?
Aren\'t we allowed to work on limitation at that level?
Or is this additional \"guidance\" to help us avoid unnecessary tripping?

Some have commented (to this question) why don\'t you just use a 63A RCD - well according to my price list, they seem to be at least double the price.

But why should you need to have 3-4 RCDs, all rated at the full installation capacity.
I\'m sure that like me, many NZ electricians are using the 40A RCDs with more than 40A worth of MCBs. And I have also put only 2x20A, or 16, 16 + 6A in some installations where they might be more heavily loaded.


peter
Sep 25 2018 19:55


40 amp rcd for lighting.
63 amp for power circuits.
the 63 amp ones last time i got some were more $ but not much more

DougP
Sep 25 2018 20:18


How do you arrange that when you have to split lighting across more than one RCD?

peter
Sep 25 2018 21:24


2 times 40 amps rcd\'s

DougP
Sep 25 2018 21:44


Oh... how many light circuits are you putting in a house?


peter
Sep 25 2018 22:02


Hi . I haven\'t done many houses ...... so im not the person to ask . Recently upgraded my own switchboard .cost not a big issue . had 6 circuit breakers 3 on each rcd . so plenty of circuits . Can see it would not be good if trying to win a quote on a group house .

AlecK
Sep 26 2018 09:11


I think I finally see the reason for your question. I\'ve been concentrating on MD for subcircuits, while you\'re concerned about establishing MD for selection of RCDs under 2.6.2.1, for the \"portion of the installation\" made up by the subcircuits protected by the RCD - which, for residential, is limited to 3 subcircuits.

As per my previous, the RCD isn\'t part of any of the subcircuits, so even if C2.5.1were mandatory it couldn\'t apply to the RCD. Nor to the conductors supplying the RCD and feeding from it to a group (however large) of mcbs.

The MD for the \"portion\" can be treated ass a submain. So if you can show that the MD of the portion is less than the sum of the circuit breakers being supplied; you can use the lower figure. which could easily allow use of a 40 A RCD for a group of 3 x 16 A mcbs. Or even for a group of 3 x 20 A.

We can use the tables - C1 or C2 as appropriate - to establish MD for the group. Count up the points, do the calc.

Eg if each circuit has 5 socket points; that\'s 15 total. In a single domestic, that\'s an MD of on;y 10 A. If they have 10 points each, that\'s an MD of 15 A.

This is way less than you get by adding the MDs of the individual subcircuits established by limitation. There\'s nothing anywhere that says that the MD of a portion is the sum of the MDs of the subcircuits making up the portion. If that were the rule, then we\'d be in trouble with mains in most new houses. Try adding up the sum-of-mcb-ratings for a house and you\'ll soon get to several times the 63 A supply that we all know an average house can run happily on.

So that allows us to use a lower-rated RCD for the group. It also allows us to use a similarly-rated conductor to feed from RCD to mcbs. That would allow, for group supplying 30 points, use of a cable rated for a mere 20 A. Perfectly OK,but only as long as we then protect that conductor at 20 A, as required by 2.5.3.
------------------

Table C8 isn\'t there for purposes of establishing MD; it\'s concerned about the relationship between numbers of points and cable size. We all know of the people who will try to keep costs down by putting too many points on each circuit. and using \"limitation\" for MD would set no limit at all. So for the cheats, and the terminally thick, the guidance establishes a practicable limit. It\'s not a requirement, it\'s just advice; so we can bend the \"rule\" a little.

Each part of the guidance in App C is aimed at one aspect. By simplifying things, discrepancies are introduced; so you can\'t expect everything to fit together seamlessly.
It might be nice if C2.5.1 referred to table C8 as an alternative, but that would introduce an entirely new method for establishing MD of a subcircuit that isn\'t in 2.2.2.

The reason for the price difference between 40 A & 63 S RCDs has little to do with the rating, and lots to do with numbers sold. If more people used 63A, the price would come down.
In Oz, the price of RCBOs has dropped. Probably their Govt\'s drive for installation of \"safety switches\" helped. And now almost nobody uses the RCCB + 3 x MCB configuration, most put an RCBO on every subcircuit.




So for establishing MD of a portion, it\'s OK to use the calculation Tables.


DougP
Sep 26 2018 09:23


Perfect thank you Alec.
From the RCD to the 3 MCBs is not a final sub-circuit and is more like a submain.
That\'s the answer that I couldn\'t think of.

And I don\'t think anyone has expressed it that way on here either.

Well done!

AlecK
Sep 26 2018 18:17


Definition of \"final subcircuit\".

toyoto
Sep 26 2018 18:56


A semi related question, are type AC rcbos cheaper to produce than type A?

AlecK
Sep 27 2018 08:23


Not my field; but one would have to assume so since Type A have to be able to operate on pulsing d.c. as well as on \"clean\" sinusoidal a.c.

More functionality generally means a more complex product; and complexity costs.

However that doesn\'t explain the predominance in Oz of RCBOs for each subcircuit. They may only be required to use Type AC, but are often using Type a by default.
That\'s because suppliers don\'t like having to keep different stock for the two markets, so at least some don\'t stock Type AC any more.
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