MEC sized by max demand

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Dan L
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MEC sized by max demand

Post by Dan L »

5.3.3.2b

example if you had a 35mm mains supplying a 63amp switchboard with 63amp mcb ( maximum demand by limitation)

Then you can have a 6mm main earth conductor instead of 10mm

And same in other situations we're the actual conductor of the mains is larger due to voltage drop, than You can find the smallest active required to carry the maximum demand or main overcurrent rating (maximum demand by limitation) and then from that active use table 5.1 to find the size of MEC and MEN.

The reference specifically mentions for VD. But what about if the mains were larger for another reason. Like larger for future reference . Can we still do the same

I would assume you can as the short circuit device will be what is limiting the fault current that the MEC and MEN has to carry with out being damaged. Note I am aware in a fault the fault current will go back through the PEN not MEC but the MEC is still rated as such)

I think 5.3.3.2 a, backs the theory up as the mains size seems irrelevant to determine the earth size. Just the highest fault current that can be produced / carried.

I would really appreciate some clarification on this.

Thanks.
AlecK
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Re: MEC sized by max demand

Post by AlecK »

Selection of MEC conductor size is based entirely on size of mains actives;
but with an Exception that allows the size of active(s) to be assessed as the size required to carry the max demand instead of what's actually installed. That Exception [Ex 2 to 5.3.3.2] ONLY applies where mains are over-sized to deal with volt drop.
Since there's only one condition given for the Exception to apply, we can't apply it where mains are larger for any other reason (eg future-proofing).
Dan L
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Re: MEC sized by max demand

Post by Dan L »

Thanks mate much appreciated
Dan L
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Re: MEC sized by max demand

Post by Dan L »

To add another question to this thread, (curve ball)

In regards to the exception of voltage drop limitations

Can the voltage drop reason be for fsc or sub mains

For example

So instead of increasing the size of individual submains to comply with total of 5% voltage drop the size of mains is Increased instead.

Does this meet the exception?
JamieP
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Re: MEC sized by max demand

Post by JamieP »

I'd say yes, the point is that the limiting factor is based on volt drop considerations not CCC so I don't know how that would be any different
AlecK
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Re: MEC sized by max demand

Post by AlecK »

The Exception is about volt drop overall, not limited to just volt drop within the mains.
So it doesn't matter how the overall volt drop is distributed between mains, submains, and / or final subcircuits;
the only question is: has the mains size been increased; beyond that required to carry max demand, in order to deal with volt drop?
If so, then the Exception can be applied and the MEC size can be reduced.

However 5.3.3.2 is only about size of main earthing conductor;
and Exception 2 is specifically about mains that have been increased to deal with voltage drop.
So if you are asking about using smaller MEC based on submains or FSs having increased CCC to deal with volt drop, then the answer is NO.

And if you're asking about whether a similar principle applies to PECs of submains / FSs, then
a) 5.3.3.2 is the wring clause to be looking under; and
b) the relevant clause [5.3.3.1.2] does not include any similar provision.
Slovett
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Re: MEC sized by max demand

Post by Slovett »

I’ve got a similar scenario -

The Electrician has run a 4mm as the Main Earth by mistake. On a block of Appartments. The Mains are a combination of 16mm and 25mm.

While not an ideal Solution - If a 40A MCB was installed as the Meter Isolator (Max demand via limitation) could you then class 16mm and 25mm Mains sized for voltdrop? 5.3.3.2 (b).
AlecK
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Re: MEC sized by max demand

Post by AlecK »

Basically you're saying that the MD is 40 A (I assume per phase, since it's a block of flats);
and based on that the mains size only needed to be 10 mm2 - therefore allowing use of 4 mm2 MEC.

So the question becomes: can you validly claim the mains conductors have been increased in size to deal with volt drop?

Limitation is a valid means of determining MD; however the actual load still has to be allowed for;
else there will be nuisance tripping of the over-current device.
1.6.1 requires design to allow installation to "function currently as intended", and to "minimise inconvenience in event of a fault".
Which amounts to being fit for purpose.

Artificially lowering the nominal MD can only be done within reasonable limits.
Can they really expect to operate "as intended" without exceeding 40 A?
I suspect the answer is "no"; and the fact that this is stated to be an error by the installer undermines any claim that it can be "yes".
An Inspector who issues a "pass" RoI for this needs to be able to justify that decision, if / when the occupant's complaints lead to a hearing before the Board.

And applying current limitation as meter isolator or as main switch introduces another issue:
- still assuming 3-phase - the mcb would cause additional inconvenience by operating on all 3 phases together; even if only one phase exceeded 40A.
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