Avoneer
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| posted on 15/4/06 at 07:50 PM |
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Garage Consumer Unit Questions
Planning on buying the parts to fit a small consumer unit in my garage when I move to my new house.
There is one spare slot in the main consumer unit.
So:
Wack in a 40A MCB in the house consumer unit.
Armoured cable from house to garage, 75cm under ground.
New consumer unit with 40A 30mA RCD, 1 16A MCB and 1 6A MCB.
Plug socket ring main connected to the 16A MCB.
4 X Fluorescent lights connected to the 6A MCB via a switch.
How does that sound?
Just worried as my welded can sup 16A on full wack (according to the instructions).
And what size core do I need on the Armoured cable - 4mm ?
Cheers,
Pat...
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omega 24 v6
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| posted on 15/4/06 at 07:57 PM |
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Go with a 20 amp MCB for the welder. You can get industrial motor rated ones (not entirely recommended as the fault current is quite high).
You'll need at least 6mm square cable ( depends on length of run) for the armoured and a gland and shroud kit as well.
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Peteff
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| posted on 15/4/06 at 08:04 PM |
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I used 40 amp twin and earth in a conduit underground. I got an old wylex unit from the electricians for a couple of quid and put that in.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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Gav
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| posted on 15/4/06 at 08:29 PM |
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Ive had twin strip lights, a tumble dryer, radio and plasma cutter all running off a 16a MCB and it only trips when the compressor comes on 
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billy
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| posted on 15/4/06 at 09:03 PM |
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I use a wind generator and solar pannels in mine
luego-lo-cost finished,vauxhall 16v 2.0,twin 45s de-dion rear set up
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gazza285
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| posted on 15/4/06 at 09:10 PM |
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Work out your need at this website.
http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
I may have some 2.5 three core SWA spare left over when I finish wiring my garage up.
DO NOT PUT ON KNOB OR BOLLOCKS!
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Avoneer
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| posted on 15/4/06 at 09:25 PM |
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Misread my Welder instructions.
Says use with a 16A MCB.
So therefore, I should be fine with the above mentioned Consumer Unit as only ever gonna be using the welder on its own on the sockets and the
lights.
6mm square cable is FAT - surely not required for the above ? That's about 25mm diameter is it not?
Pat...
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Avoneer
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| posted on 15/4/06 at 09:25 PM |
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Cable run will be 15M max.
Pat...
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omega 24 v6
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| posted on 15/4/06 at 09:33 PM |
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What does FAT mean pat.
Basically you want the fuse (mcb) to be the weakest link. If you've got a 40 mcb in the house then the cable needs to be capable of carrying
P=V*I
P = 240 *40
P= 9600Watts
10 KW for easy counting. Put that in the chart shown in the thread above and 6mm square it is for XLPE or 10mm square for PVC.
[Edited on 15/4/06 by omega 24 v6]
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dave1888
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| posted on 15/4/06 at 09:45 PM |
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4mm armoured should be fine the current capacity is 45amp. 6mm armoured cable is rated at 56amp.
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Avoneer
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| posted on 15/4/06 at 09:48 PM |
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So 4mm min and 6mm to be safe and fell better.
Cheers,
Pat...
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dave1888
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| posted on 15/4/06 at 09:54 PM |
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I dont know if theres much of a price difference, but if its not much then 6mm to be safe. It will pop the fuse before it burns the cable even if you
use 4mm. Please bear in mind im a dentist and not an electrician 
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Avoneer
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| posted on 15/4/06 at 10:06 PM |
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FAT as in thick chunky cable.
Not to worry though.
Pat...
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However a large number of electrons were terribly inconvenienced.
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omega 24 v6
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| posted on 15/4/06 at 10:12 PM |
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quote:
FAT as in thick chunky cable.
I thought it was an acronym for something.
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JoelP
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| posted on 15/4/06 at 10:22 PM |
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i never heard of 4mm cable that could carry 45amps? news to me. Plus the RCD needs to be inside the house really, maybe not by law but certainly by
common sense.
Id have a 40amp breaker feeding a 6mm cable, with an inline RCD. Then take full advantage of you 40 amps by having a 32amp ring main round the garage
(a radial circuit would need 16amp breaker), and a seperate 6amp circuit for the lights.
I found out the other day that my garage is wired in 4mm cable and it pissed me off, whats the point? It makes a big dent on total supply and doesnt
affect the total cost much anyway.
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gazza285
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| posted on 15/4/06 at 11:28 PM |
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quote:
Originally posted by omega 24 v6
What does FAT mean pat.
Basically you want the fuse (mcb) to be the weakest link. If you've got a 40 mcb in the house then the cable needs to be capable of carrying
P=V*I
P = 240 *40
P= 9600Watts
10 KW for easy counting. Put that in the chart shown in the thread above and 6mm square it is for XLPE or 10mm square for PVC.
[Edited on 15/4/06 by omega 24 v6]
You will never come close to using 10KW.
Your welder wants a 16A supply, that's just short of 4KW. Your lighting will be 1KW if you want blinding. While you are welding you will not be
using anything else, so I'd base your absolute maximum draw at being 5KW, if you were welding your machine flat out all the time, with 10 100
watt bulbs burning all the time and a big radio on flat out.
Now according to the website I directed you to ealier, the more than enough length of 2.5mm SWA I will have spare will supply your garage with
25amp.
One other point. Do not have your weakest point in the house as it is no fun trailing back and forth to flick the trip back on.
DO NOT PUT ON KNOB OR BOLLOCKS!
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Avoneer
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| posted on 15/4/06 at 11:35 PM |
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That makes sense to me.
Have I opened a can of worms here as I don't think anyone is going to agree.
Pat...
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Avoneer
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| posted on 15/4/06 at 11:42 PM |
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According to that link as well, their 2.5mm cable is rated at 31 Amps.
Surely this will be more than enough?
If I reduce the house MCB to 30A, that should be fine shouldn't it?
Pat...
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gazza285
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| posted on 16/4/06 at 12:00 AM |
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Yes.
Have you priced up any 6mm SWA cable yet by the way? The spare 2.5mm I have will be free as I paid nothing for it.
DO NOT PUT ON KNOB OR BOLLOCKS!
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Nick Skidmore
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| posted on 16/4/06 at 08:03 AM |
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The cable rating depends on how it is fixed and what it is fixed to. The cable is given a rating and then this has Amps knocked off for length of run
and method of fixing. E.g. if you cable passes through 150mm or more of insulating material you have to de-rate by 50%, if its clipped to thermalite
block there is a further de-ration.
I have done C+G 236 Elec. Inst. in the dim and distant and can't remember exact figures but those ratings quoted do seem high.
I'll detail my hose to garage installation for those interested.
Incoming fuse from supplier to house 100A (some houses are 60A so max fuse in consumer unit should be 32A or 40A if brave to ensure
'discrimination between fuses / MCB in event of fault)
63A type C MCB in house consumer unit, C type are for inductive (motors, coils, transformer loads) they have the same fault conditons as normal (type
B) but they have an inbuilt lag (order of seconds)to blowing to cope with in-rush currents on start up.
This breaker protects a 16mm twin an earth cable which feeds 1 x 32A ring circuit, 1 x 32A type C for welder / plasma cutter (unplug one to use
other), 1 x 6A for strip lights. I need to put the phase converter in to this board with a C type breaker as it's on the house ring main on a
B-type and knocks the telly out every now and again - not popular!!.
Just a couple of comments regarding earlier postings, 16A is enough for 1 double socket, no more, certainly not a ring and if using armoured be sure
that if using the sheath as as the sole earth that you have sufficiently low resistance as I have found that supplementary earths have to be used in a
lot of these cases.
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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JoelP
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| posted on 16/4/06 at 09:47 AM |
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i wouldnt bother adding up what you expect to use, as you're sure to forget something. What if you add an electric heater? A kettle? An oven if
ever you want to bake parts. Maybe someone running a garden appliance off the same circuit.
gazzas link does provide good estimates, but you have to bear in mind that you use the appliance(s) rating to work out demand, round up to pick a
suitable breaker, then round up again to the next wire size to carry the load. Obviously the fuse protects the wire, the rcd protects you.
But as nick says, that site doesnt include the correction factors for things like using old rewirable fuses, grouping wires or insulation, so be
careful!
My old teacher explained that you can make quick decisions from the standard circuit tables, or you can spend more time doing calculations from the
full tables to try to save a few pounds on the cable size. 
[Edited on 16/4/06 by JoelP]
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omega 24 v6
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| posted on 16/4/06 at 09:47 AM |
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The fuse is the safety device and as such should be the weakest point of the circuit. If you use 2.5 then the fault current would be higher than the
cable rated current of roughly 20amps (you've got a 40 amp breaker) so the cable in theory will heat up and possibly worse. All this is actually
irrelavent as the RCD SHOULD do it's job in a fault situation. In an averload situation the RCD will not trip until the 40 amps is reached by
which time the cable could have been running at 35 amps all day and could be damaged.
When you move house etc the new owner may use the circuit to it's full capacity (40 amps) because that is your design loading (as per the MCB)
he is not to know that the cabling is sub 40amps is he.
Look at all the mcbs in Your fuse box and you will find that they are always the weakest link.
A 6 amp one for cable of 1mm square
A 15 or 20 amp one for 2.5
A 30 amp ring mains (2 of 2.5)
A 45 or 50 for cookers and showers(10kw)
and a cable size of 6mm(cooker and 6kw shower)and 10mm for a 10kw shower.
Now compare that with a 40 amp Breaker for a 2.5 cable!!!!!!!!!!!!
Do it right for your own safety or lower the mcb in the house to 20amp.
Yes it's very very unlikely you'll ever use 10kw but the weakest link can supply it.
BE SAFE cable prices are not so different.
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Avoneer
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| posted on 16/4/06 at 10:30 AM |
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Ok.
The 40A in the house was a rough guess.
The circuit will be for me in my new house, so want to get it right.
So, when I buy my consumer unit that has the following spec:
"IP55 VCGKIT comprising 40A 30mA RCD, 1 x 6A and 1 x 16A Type B MCB. RCD provides earth leakage protection. MCBs provide individual circuit
protection. BS EN 60439-3"
What size cable and what size MCB will I need in my house consumer unit.
Cheers,
Pat...
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However a large number of electrons were terribly inconvenienced.
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Macbeast
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| posted on 16/4/06 at 10:46 AM |
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I suspect you're not allowed to do work like this anymore unless you are a qualified electrician. You're probably allowed to replace a
faulty 13A outlet, but not anything that requires new wiring. Similar to the restriction on gas installations only by Corgi - registered engineer.
Having said that, of course I've done it myself, but there may be insurance implications if something goes wrong.
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froggy
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| posted on 16/4/06 at 11:03 AM |
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the way ive done it is to split the incoming tails before the consumer unit and have a separate 80 switch so that the lights dont keep going dim in
the house every time the compressor or welder fires up
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