Garage / workshop electrics

wrote:

Most crop growers do :-))
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C time curve for all MCBs involved in feeding the welder (and potentially a compressor). They don't take as much current as you might think, but they do take it with one hell of an inrush spike.
Do sensible diversity calculations for the real load in the garage, then rate the supply to match. If that involves uprating the cable itself, then you're going to have to do it (but chances are you won't, as diversity with only a single person working in there can be fairly generous).
My worst case isn't welding, it's plasma cutting. The plasma cutter takes a moderate current, then the compressor kicks in too and bang goes the fuse to the workshop feed...
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Upgrade the SWA as necessary. - 10mm 6943X (3 core SWA) for a 10m length and workshop. - Run the SWA right to each CU, terminating into adaptable box - Piranha earthing nut if box is plastic (TLC Direct) - 45-50A MCB in CU
Replace the garage CU - Dual RCD system - RCD-1 Lights & Garage Opener, Non-Maintained EmLight - RCD-2 Sockets & fixed appliances
RCD-1 has MCB 6A for Lights & EmLight, 6A for Garage Opener. RCD-2 has MCB 20A for 1-2-3 etc dedicated 16A sockets, 20A radial or 32A ring final for sockets.
The only other alternative is another run of SWA if the existing is concreted in and another CU way used.
An issue is that the garage may be damp, have incoming services etc. In which case you would not export a PME/TN-C-S house earth from the house, you would make the garage a TT install with earth rod. That is achieved by exporting the house earth ONLY as far as the end SWA adaptable box which would be plastic with the SWA cable terminated by a plastic SWA gland (CCG off Ebay or Sitebox) or plastic IP68 gland. Then the garage earth is provided by a local 2x 4ft Furse earth rod driven into the ground, earth rod pit cover & 6491X run to the garage earth busbar, EFLI tested to be <200ohm. The 6491X gr/ye earth could be 6mm if run in 20mm conduit or 16mm if not (check tables).
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"js.b1" wrote A Very Comprehensive Reply... which prompts me (in the design and planning stage of a New Big Shed) to ask:

I believe that if the OP uses XLPE insulated 2-core or 3-core 10mm SWA he can go up to a 63A MCB at a push - assuming he needs that much current?
I suspect I may - 10HP lathe with a VFD may be in the plan... plus a Serious Welder (up to 45A on 230 volts), both as fixed installations with lockable isolating switches, not plugged in! I like bright halogen lights, too...

Is it necessary to have a separate RCD for the lighting? Not completely up to date with the regs myself, but if he has emergency lighting would there need to be a maintained supply in the event that a fault disconnected the primary lighting ?

Again, not familiar with the current regs, is a belt-and-braces install permitted with a local earth supplementing the TN-C-S earth, or does that cause issues with earth currents in the SWA? I was anticipating bonding the local earth rods to the local CU and hence the SWA - happy to make it off unterminated at the workshop end, though!
Cheers, Dave H.
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(The engineer formerly known as Homeless)

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On Mar 23, 2:12pm, "Dave H."

Subject to... #1 - Cable Current Carrying Capacity being limited (derated) to 70oC PVC BS6004 unless wiring accessories can handle the higher temperature of 90oC XLPE BS7211. #2 - CU support for 63A MCB - some ranges were limited to 50A.

Wiring must be RCD protected unless... #1 - Cable compliant with 526-06-06 eg, SWA, MICC, BS8436 or buried at

#2 - Cable is surface run
So you could surface run the lighting cable and avoid RCD protection.

No, maintained refers to light output - not the supply.
Maintained EmLight = Fire Exit Sign. - Light is always "on" and is maintained "on" during power loss for 3hrs.
Non-Maintained EmLight = Emergency Lighting. - Light is always "off", but is turned on during power loss for 3hrs.

#1 - DNO would not like it #2 - DNO network loses its earth your rod just became its replacement :-)
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More to the point, maintained lighting can be used as ordinary switched lighting in addition to its backup purpose, non maintained can't.
--
Cheers,

John.

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Dave H. wrote:

You could with 2 core PVC sheathed as well (or more to the point using XLPE sheathed SWA but within the 70 deg C temperature budget of the accessories that you are likely to terminate it in)
Table here:
http://wiki.diyfaq.org.uk/index.php?title bles#SWA

The RCD requirement comes from the 17th edition regs on cable protection. If the cable is otherwise protected, or surface run for example then it sidesteps the need for RCD protection.
The RCD requirement is motivated by shock protection (via contact with a damaged cable - say when fixing things to the wall etc). Its separate (and in some ways obstructive to) the desire to keep the lights on in the event of another fault.
Non split CUs and individual RCBOs for each circuit are often worthwhile in these situations.

You can have a local earth rod connected with PME - it just becomes another of the multiple earths. However I would be reluctant to export a PME (TN-C-S) earth to an outbuilding anyway, since you would also need to ensure the equipotential zone that goes with it is also exported and maintained. Depending on the nature of the outbuilding this can be difficult.
More info here:
http://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside#Exporting_an_earth

If you make the workshop a TT install in its own right then the supply earth need only protect the SWA. You could terminate at a plastic box for example and use the local earth from there on.
--
Cheers,

John.

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Hi all
Thanks for all the replies they're very informative. Just got a bit more info to clarify things
1. The 20A MCB is on the RCD side of a split load CU, but I could switch it to the non-RCD side if needed 2. The SWA is 15mm in diameter if that's any help. If this is of a size that would allow for a higher rated MCB, are all MCBs the same or would I need to get one of the same make? 3. This is strictly for hobby use, lathe and milling machine use less than 1hp motors (albeit 3 phase running through inverters) I think the biggest current draw would actually be the 3hp compressor on start up 4. Ignore my comment about the door opener. It wasn't that I was worried about getting trapped inside more that I didn't want to be locked out, but I can get an 'emergency release' for the opener so will be fitting that. 5. I'll be surface running all the wiring in the garage cum workshop 6. Garage is dry so presumably no need for separate earth
Any further comments or suggestions appreciated
Cheers
Jim
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JimM wrote:

Non RCD side makes it easier, since you can then use a split load CU in the workshop/

Regards capacity - the short answer is yes you probably can upsize the MCB, however there is stuff to check first.
As to MCB brands etc, its easiest to get the same type if you can. However there is some level of interchangeability. See the early draft of the interchangeability table here:
http://wiki.diyfaq.org.uk/index.php?title=Consumer_Units
Regarding your SWA:
How many cores?
According to my table here:
http://wiki.diyfaq.org.uk/index.php?title bles#SWA
2 core 4mm^2 is 14.7mm diameter, and 3 core is 15.3mm diameter. So that seems possible.
The smallest 6mm^2 is 15.9, and the largest 2.5 (4 core) is 15mm - so probably not those.
So 4mm^2 seems possible...
How long is the cable run in total?
Does the SWA go right up to the house CU or does it switch to T&E first?
What earthing system does you house have?
http://www.diyfaq.org.uk/electrical/electrical.html#system

Yup, a dedicated circuit with a type C MCB may be required for that.

Which means you can forgo RCD protection on the lighting circuits then if you wish. (or use a RCBO anyway)

Again, it partly comes back to what earthing system you have. If you have a split load CU with only one RCD then it must be TN-C-S or TN-S. The former can normally be spotted by the way the main earth connection will come from the cable head end unit adjacent to the neutral tail. (TN-S will usually be a connection to the cable armour before the termination to tails). Post a photo somewhere we can see it if in doubt.
Any other services into the garrage like mains water, gas etc?
--
Cheers,

John.

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JimM brought next idea :

I would suggest having at least two socket circuits, such that you can have sockets which remain live all the time (for battery chargers, garage door etc.) and another for sockets which need to be isolated as you walk out the door, such as your machine tools soldering irons etc..
Rather than light switches, I use separate MCB circuits to power lighting in different areas. I also have one light on a PIR, so it comes on when ever I enter the door or drive the car in or out.
It might also be worth including an emergency ALL STOP system in the circuit used for the machine tools.
--
Regards,
Harry (M1BYT) (L)
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