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.
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