Hut Consumer Unit & MCB Question

The cabling to the hut is proper 3 core 2.5mm steel wired armour cable like this **LINK**

The garage/hut consumer unit is this one **LINK**

The sockets are configured as a ring main

This is the power feed I am using https://www.warco.co.uk/power-feeds/303231-wm18-milling-machine-power-feed.html

Hope this helps

Edited By petro1head on 27/09/2018 13:28:29

Slightly off-topic (but not too far, I hope) :

I read somewhere that the current rating of a twin 13 amp socket is 13 amps, TOTAL.

That is, if I'm drawing 10 amps from one socket, I mustn't draw more than 3 amps from the other one! And likewise for other share ratios.

Best regards,

Swarf, Mostly!

SM,

The power rating should reflect the supply. A single 13A outlet (only) should be connected to a 2.5mm^2 spur. Twin 13A sockets are only permissible on a 2.5mm^2 ring main (ie a 30 amp total). One should not be able to overload the supply if that line is not fused seperately.

Legally it's 13A total regardless of whether the socket is one way or two way. Three way sockets are also available but are sub fused at 13A in the faceplate.

The BS approval test on a double is 20A (with a 70/30 split) with a limit of 1 deg C rise

BS1363 sockets from reputable manufacturers will actually accommodate 3 x 2.5mm^2 or 3 x 4mm^2 or 2 x 6mm^2 conductors.

Petrolhead that cable isn't Steel Wire Armoured, this is SWA

We might possibly be into can of worms territory here...

For anyone seeking guidance here is a good place to start (although getting a bit dated now)

Electrical Installations - a supply to a detached outbuilding

Correct well spotted but its still armoured and where it runs would never be attacked by anything

As you said, can of worms. Enough said

Just seen the cable size. 2.5mm^2 SY is only rated at a maximum of 24A and will be less than that for any distance due to voltage drop. So breakers should be sized accordingly. 4mm^2 SWA would be the minimum for a 20 metre outdoor run and 32A breakers.

The 2.5mm^2 SY can be re-used for connecting individual machines to their plugs, that's what it's designed for.

A D curve breaker will not affect the cable rating.

[This posting has been removed]

I kind of wish I had never asked here but on the other hand I know I would get good info. Maybe more than I wanted but good for anyone else thinking aboiut doing the same

I think we need to get this into perspective.

I run either the lathe or the mill, never both. Not enough hands

The cable is more than capable of doing its job, its not like I am trying to run a constant 3KW!!

My earlier post crossed with Martins.

The SY cable is not suitable for this application especially if buried. A 32A breaker will not protect this. You need to change the breaker in the house to a 16A one as soon s possible. (a 20A might do but but you would need to know the loop impedance. Change the hut one to 10 or 16A. Even SWA comes in different current ratings depending on the plastic used for the insuation. I disagree with the comment that cable size is not affected by MCB type (B,C or D), it does indirectly becaus it affects the loop impedance. The loop impedance must be low enough for the MCB to trip quickly with a short circuit. https://www.electricalsafetyfirst.org.uk/mediafile/100106752/master_EARTHFAULTLOOPtable.pdf
If you have a long cable and a C or D MCB you may have to go down in MCB rating or up in wire size.

Sounds like you need to get a electrician in, trouble is finding one that knos what they are doing but won't do more than is really needed.

Robert.

Edited By Robert Atkinson 2 on 27/09/2018 17:29:33

Thats just wrong, 6mm SWA is rated for at least 50A. A workshop feed should not need more than 32A. 2.5mm would do for a short run and 4mm would be conservative. Too late now though.

Robert.

I was also thinking that but glad you said it

I don’t think a workshop supply being over cabled will be a problem, as a boy my workshop was at the top of the garden and powered by a very long extension cable, starting a motor dropped the voltage enough to induce severe flicker in the fluorescent tube light, once running the motor and light were happy. The startup surge should be considered if minimalist cabling is installed. The regs have plenty to say about cable size and installation conditions and breaker sizing to give proper discrimination. A proper design exercise should be done to determine what cable and breakers are required for every installation, guessing will result in an inadequate installation or wasting money on over cabling.

Mike

Sorry, but that isn't the case. It's rated at 50A current for a short length. You'd need 10mm^2 for 50A over 20 metres buried in earth. You have to plan on voltage drop as well.

I did 16mm^2 over 30m to the shed, with a 40A circuit to allow me to go to 63A if needed.

I'm sure there are many forum users who have machines or plant in home workshops that have a need for more than a 32A supply so I believe the advice to use 6mm SWA was sensible, even for such a short run, the cost difference was negligible.
There will also be many who experience the lights dipping or even going off when starting machines or plant, some of those will have undersize supply cables.

Perhaps the cable was routed through a boiler room with high ambient temperature and both walls where the cable passed totally enclosed the cable in insulation, there may also be any number of live circuits on part of the route.

Without knowing the exact details of the installation it is impossible to say the cable is not a good choice.

Emgee

The all important thing with any circuit is the disconnection time. If the circuit protective conductor (CPC) or in old terms the earth wire, is small then it will take a fault current longer to rise to a sufficiently high level to disconnect the circuit under fault conditions (High opposition to current flow). Whereas a CPC with a large cross sectional area will have less opposition to fault current flow and will therefore disconnect the circuit more quickly, before a dangerous situation arises. All very similar to water pipes and quantity of water flow. An earth loop impedance test should be carried out and with that information a decision will be made as to what type of circuit protection should be used. Then one has to consider volts drop, and that is determined by the size of the main conductors. milli volts per amp, per metre length of the cable, all divided by 1000, this information is obtainable from the wiring regs, which will soon be the 18^th Edition. So it’s not simply 2.5mm or 6 mm so many variables to be considered. Certainly not many home workshops will actually need 6mm cable unless it’s a long way from the main service. John

Most of the above is correct, in certain circumstances, but as Woody says, Totally irrelevant! The tripping is caused by slamming a motor into reverse while it is still running forwards, thus creating a massive torque reaction, and a current spike large enough to trip the MCB. Does it trip at any other time? NO, then that is the cause! "electricians" also please remember that the IEE/IET regulations are NOT law, only the requirements of the building regulations as pertaining to part P thereof is law. As far as all these supposed improvements in electrical safety brought in by advent of the MCB/RCD era, I would just say that we live in interesting times, and time will tell if safety has actually been increased, or in fact reduced.

John

You are only considering live to earth fault current, phase-phase and phase-neutral fault must also be considered when selecting the Circuit Protective Device to prevent temperature rise damaging the cable.

The EFLI has to be calculated in the circuit design stage to ensure correct PD and cable size are used, confirmed after installation by an EFLI test before the circuit is put into use.

As stated earlier without knowing the full details of requirements it is not possible to advise a cable type/size.

Emgee

Here here

There are two problems here: 1) the installation may not be fully in accordance with BS 7671 and Part P (depending on where in the UK it is). 2) the breaker trips when the motor is reversed.

1) Not enough information.

2) Making an assumption that the motor is connected via a 13A fused plug and the 13A fuse is not blowing then the 32A mcb is tripping in the instantaneous part of the curve. This suggests a short circuit rather than an overload so either the reversing switch is not suitable for purpose or it is faulty, creating a short when directly switched.

Roger what information do you want?

The reason the MCB trips but the fuse does not blow is because the MCB has a quicker disconnection time than the fuse so before the fuse has a chance to break the current has already been stopped.