Secondary Consumer Unit and Cable Size for Feed


Postby Fn105Fn » Mon Jan 09, 2017 6:11 pm

Hi everyone, I am a qualified journeyman electrician; however I haven't practiced since 1974 whereupon I became a Firefighter. I used to help my mates out but have done nothing for years and I am aware of the change in Regulations. So gentlemen and ladies this is my question.

Can I use an existing fused feed from the main Consumer Unit that is currently feeding a secondary unit. The cable feeding is a 2.5 twin and earth.

The secondary unit currently has 3 skts in radial and 1 lighting circuit.

My mate wants to run a ring main instead and some lights (not more than the limit)

He also wants to run an armoured cable from this box to his shed at the bottom of the garden.

I was hoping to pick this up from the incoming supply to the secondary unit, sharing the connections and fusing it in the shed with a third 2 way consumer unit.

I am aware that I might need a heavier cable from the main to the secondary. However there is a lot of work in lifting floorboards etc. If 2.5 will do then hunky dorey. Otherwise he will have to employ a qualified electrician. The reason -- I cant work out the Regulations. It is harder understanding the Regs than the circuitry.

Best Regards
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Postby ericmark » Mon Jan 09, 2017 11:40 pm

There are two sets of what one may call regulations. BS7671 and Part P, the latter is law, but BS7671 is not law but may be used in a court of law.

I did my apprenticeship in the late 1960's early 1970's and by that time journeyman had gone, my dad born 1924 did a journeyman that was year 6 and 7 of an apprenticeship. Well he actually never did it because of the war, he ended the war as a engine room artificer/chief petty-officer in Royal navy.

So I am assuming you are looking at BS7671 well I have 2008 but not amendment 3 so I may make some errors. But to summarise I will give some pointers.

One the ring final is a special, it needs to have a reasonably even load, as if the load is too close to the origin you can overload the 2.5mm sq cable. So the regulations suggest in the appendix any fixed appliance over 2 kW should have a dedicated supply.

Now it may seem strange but a outbuilding can be considered as a fixed appliance. So if the fused connection unit is fitted 1/3 to 2/3 around the ring final then even with a 13A fuse you are unlikely to draw more than 22 amp from either leg, but if close to the consumer unit it could.

You also have the RCD problem, a RCD FCU is the simple way out where there is no RCD in the consumer unit. With a 7 amp fuse in the FCU you are within the 2 kW limit.

The Part P varies according to if in England, or Wales, and there are other rules for Scotland. The English used is very poor, so it is hard to say exactly what is allowed. In England a new circuit needs permission and fees paid to LABC. However it seems they don't use the BS7671 definition and a FCU is not considered as forming a new circuit.

The word consumer unit means a type tested distribution unit. For an ordinary person in control you must use the type tested units, however if you are able to fit a distribution unit then you are not an ordinary person, you are instructed, or skilled, the competent person class has been removed from amendment 3 so no longer exists. So if you do anything to remove the type testing approval then it's not a consumer unit.

So in real terms Part P is important for rented accommodation, but in real terms can be ignored for owner occupier. Of course officially it is valid, but one is hardly likely to take ones self to court. And if planned before 2004 Part P is not valid. You should see all the plans I made!

So the main change was move from imperial to metric, 2.5mm is thinner than 28/0.014 cable so pre metric one was not that worried if the ring was broken, or if the loop impedance was exceeded, but with metric inspection and testing becomes important.

The ELI is not that important with RCD's and the line - neutral impedance is more linked to volt drop than ability to trip a B32 MCB. 105% of 5 x 32 amp = 168 amp and ohms law 230/168 = 1.37 ohms but volt drop with a incomer of 0.35 ohms means a loop impedance of around 1 ohm.

Also the RCD must trip at 5 x 0.030 amp in less than 40 mS which means test gear costing around £750 to buy, yes and a DIY'er is really going to buy it! So use common sense, but realise you can't possibly comply with the regs. So only do it with owner occupied.

Scotland is far stricter, and Wales is also more strict than England.
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Postby Fn105Fn » Wed Jan 11, 2017 11:51 am

Hi, thank you for that. However I am a bit blinded by the tech info. I would like to know the principle points of my question. Such as can ;--

A ring main and a lighting circuit be run from a secondary consumer unit being supplied by a single 2.5 twin and earth ?

And share the incoming connections to the second CU - via a 2.5 mm armoured cable to feed another CU in a shed?

As I say the technical side is way above my recollection but well aware of the fusing and connections.

Very grateful for your assistance.
Best Regards
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Postby ericmark » Wed Jan 11, 2017 11:13 pm

There is no point in running a ring final from 2.5mm cable but in theory you could do it.

There is also no point in having a consumer unit supplied from a 2.5mm cable but in theory you could do it.

2.5mm cable has a range of amps according to type of insulation and installation method, but in general one considers it as being 22 amp cable, so maximum of 20 amp fuse/MCB/RCBO.

So with 2.5mm to outbuilding fused connection units (FCU) is enough to both limit total to 13A and reduce it further to 3A for the lights, a switched FCU for lights can be used as the light switch.

You stated "I am a qualified journeyman electrician" as such you should be answering the questions not asking them, to reach journeyman stage means 5 years apprenticeship has already been done. In days gone by before collage qualifications the apprentice had to spend 4 sets of 6 months in other firms to learn how other firms worked as well as the one who gave him the apprenticeship, that final 2 years is what is called the journeyman.

Today the collages tell the apprentice how other firms work, and instead of starting off making the tea they start off with 6 months in collage. But in all same time required, leave school at 14 + 7 years means 21 when fully qualified, leave school at 18 + 3 years means 21 when fully qualified.

Not sure which produced the best electricians? In the old system very few electricians learn imaginary numbers and power factor correction, however to be fair, very few needed to know power factor correction.

However if you say your fully qualified then expect an answer aimed at the fully qualified.
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Postby Fn105Fn » Mon Jan 16, 2017 9:47 pm

Thank you Eric for you reply. After qualifying in 1970, I went to sea as a 2nd lecky. Then became a Fiirefighter on land for the rest of my working life. And today's Firefighter is extremely technical. My memory recall had to change, keeping just the knowledge of Electricity's dangers.

Thanks again Ericmark. I am only going to connect it for him, but I wasn't sure if 2.5 mm was sufficient. The cable and a 2 way CU was already there feeding 3x13 amp skts and a light. It was only that he wanted to run the kitchen mod cons, that a ring main would be the answer. There is a nightmare to re-run the cable at a higher rating, that is something I am not into these days.

Best Regards
FN
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