Live and Neutral cable sizes - Again!?!

[shoeless1963]

First, HUGE thanks for Crossy, Forkinhades and many others for the advice and careful guidance written on these posts. We moved into the house we have built from scratch this weekend and most, if not all, the electrics are based on what has been written here - from a complete novice, many thanks. Now the complicated bit... ...please read the following in the spirit intended and if you can advise on the question(s), much appreciated. Saturday started well... I've been back in the UK for two months whilst most of the second fix was done when I was away so I was really pleased to see a 30 (100) meter installed by the PEA and its a 16mm incoming feed... all good so far. I've posted on this forum earlier regarding 'big' water heaters and in the end we down rated slightly to play safe and so again, all sound and good standard cabling thanks to the advice. Now the sad bits...

...I paid the 'extra' for a well-known Thai company to install the air-cons. The legal process in Thailand being what it is I won't name them but what follows is/was/is rather frightening. All air-cons in and working, 1 x 18 for the lounge and 4 x 12 for the bedrooms and kitchen, and the loading seemed fine even at full whack at night.

Then I decided to check more closely.

I attach exhibits 1, 2 & 3 which are in turn the way they have spliced into the main earth for the house for one a/c unit and then the wiring they elected to install for the wall-mounted breakers I elected to use. I suggest the pictures speak for themselves… and I’ll spare the details of the conversation that took place with the management of the company concerned but I have had them replace the earth completely and also sorted all the relevant breakers, which I have down-sized for the reason that follow. And not at my expense!

My problem however is that the original wiring, which I thought was going to be 2 x 6 mm2 and a 2.5 mm2 for an earth to run back to the main DB, is in fact just 1 x 6 mm2 and 1 x 2.5 mm2. Idiot electrician who installed the original cabling did me over good and proper and won’t be returning. Anyway, the ‘better’ team from the Thai company I have got in hand now re-did the wiring through as best they can and I have checked that its 6 on the Live and 2.5 on the Neutral throughout, with local earths. Now I know full well the issues over potential differences with local earths but as Crossy has said elsewhere on here, that solution (possibly temporary) is better than none.

My question – and sorry for taking so long but I needed to vent! – is what exactly is the risk associated with the two different thicknesses of cabling for the Live and Neutral? I know its not perfect, but in the hunt for aesthetics (= idiot falang) I went with embedded cabling runs in the walls so to re-run an additional set of (five) cables from the main DB to the breakers and then breakers on to the a/c units themselves is a pain and not easy. Cost is luckily not the issue, its really estimating the extent of risk/issue and how best to resolve it? On my previous post, it was stated that it was a ‘…major fire hazard…’ but I’m not skilled enough to understand why… so help needed just to understand.

Thanks in advance - Me

[Forkinhades]

Simple answer

The 'neutral' is a current carrying conductor, and needs to be sized the same as the 'phase' (live, hot).

Without this it will become hot (through heat) and the insulation will degrade rather quickly, thus causing fire hazard.

[bankruatsteve]

Are each of the units on its own breaker in the DB? If so, make sure they are 20amp and don't worry about it. The different wire size is not the way to do it but not a hazard as long as the breaker is sized to the smaller wire.

Usually a separate breaker at the A/C is not needed except for service convenience - which makes me wonder why they are on the inside. (?)

Not pretty but nothing really bad otherwise.

[jcisco]

This particular question is surely going to get technical if you want to go via whatever code you want to subscribe to.

However I'd run with, the same size is ideal for the above posters reasoning, and a few others such as being able to carry the hot load in normal operation and full fault current ( bonded ) on the neutral in fault condition.

Must be no smaller than the G. As always it depends

Edited September 29, 2014 by jcisco

[Crossy]

Size the breaker to the smaller of the live or neutral, no safety issues (just a waste of bigger cable).

With a 2.5mm² 'small' cable a 20A breaker will be fine, and that will handle most domestic size aircon units

[shoeless1963]

Thanks and its nice to see the 'Usual Suspects' (Crossy, Forkinhades, etc.) coming back to help an idiot in need immediately. Much appreciated and if you ever need advice on Transaction Cost Economics, which I just got my PhD in and lecture on in the UK, please do ask... as they say, those who can do, do and those who can't do, teach.

I thought the point regarding the heat issue was the deal-breaker (pardon the pun) and that is why I had the well-known 'amateur' company go back and re-install new, 20A breakers throughout the house. Incidentally, ugly though they are the idea of the interim breakers was solely to save the longer cabling runs in the event of a problem at the a/c unit end and these are on entry to each of the rooms, not at the unit and were a deliberate design decision.

As you say Crossy, as long as I keep to the 'minimum' specification then its just a waste of the extra cable and dare I say, mere pennies in comparison to the bliss of sitting in one's own study and watching the banana trees next door blossom.

Many thanks for the reassurance - Me

[Forkinhades]

You should check that you have the same amount of neutrals, as your live conductors in the Fuseboard, so as to make sure they have not borrowed any.

[Crossy]

You should check that you have the same amount of neutrals, as your live conductors in the Fuseboard, so as to make sure they have not borrowed any.

Our main consumer unit has MORE neutrals than lives, I've never worked out why.