Help with specs!

[Shocker1]

Hi folks,

Just finishing up a garage conversion into a wood workshop for myself. I’ve trenched in a new 6mm SWA (60 metres in hard clay and rock!), and I’ve wired up all my sockets and lighting circuits on the workshop end, earth rod is in, gland made off garage side, consumer unit all ready for final wiring and wiring on the house side is done.

I’m not an electrician by trade but, I’m about to say it, I have a decent idea of what I’m doing. I’ve wired up plenty 12V, am comfortable enough with 240V and 3-phase to get me close to the finish line, but as I always want things signed off and to be safe, I bring in the sparky for the final wiring.

I’m running all 240V for now, all the usual: planer thicknessers, lathe, chop saw, table saw, extraction system, all the regular power tools etc etc. I figure that at any one time I could be running 4200W lets say; that’s a planer or table saw with the extraction system and a vacuum.

I’ve two 16A radial circuits for the sockets, one side is just chop saw, vacuum and stuff like chargers etc. The other circuit is where the good stuff happens, all the bigger machines.

So here’s where I need some advice. The sparky and his apprentice are not the best.. Messy, slow.. I’ll just say it.. incompetent. All I wanted them to do was make off the gland house side and wire it up, and install the RCBOs or MCBs garage side and wire that up. I figured a guy and his apprentice, 1 day max.

The main guy had to go to Screwfix 3 times as he forgot stuff each time, and took a 2 hour lunch. He did nothing else. The apprentice was left with no tools as they were in the car of the main sparky, so he had to use all of mine, then he proceeded to drill two wrong holes in the cupboard housing the electrical system; one in the wrong place the other too small for the gland that he’d already made off… I had to then drill the correct hole myself. He spent almost 5 hours drilling those wrong holes, making off the gland and then wiring it up. I don’t know enough about wiring up to the consumer unit to check their work and be sure it is all fine, but I’m not left with confidence for the safety of my expensive woodworking machines!

Ranting aside, onto the specs. I originally spec’s Type C RCBO 20A for the machine side, and Type B for the other side. Plus a Type B 6A for the lights. As well as an SPD. They said they couldn’t make the RCBOs work due to age and state of the house side consumer unit - it is a mess in there so fair enough, probably.

I’ve sent them back and replaced them with MCBs instead and an RCD which will go garage side rather than the house. Does this sound correct? And do those specs sound correct for the purpose?

- Type C 20A MCB for machines socket circuit
- Type B 20A MCB for other socket circuit
- Type B 6A for the lights (I’m not fussed about this)

I’m mostly looking for confirmation on the Type C, as I’ve only ever been in workshops with existing power, where I may have added a circuit but I’ve never done all of this from scratch.

If it helps, here’s what I’m running:

• Table Saw 2100W
• Beam Saw 2000W
• Surface Planer 1800W
• Thicknesser 1650W
• Router Table 1500W
• Chop saw 1200W
• Dust collector 1100W
• Extraction 1000W
• Pillar drill 550W

Thanks for taking the time folks, it is much appreciated. My safe and cosy machines and fine woodworking will thank you!

 

[poni]

we would need to know a lot more , like what is the Ze at the MET, PSC, and how much cable and type, also what containment was used for each circuit, then we may be able to tell you if the MCB's are ok

 

[Shocker1]

Okey doke, I’ll do my best.

Everything in the garage is surface mounted by the way.

- SWA is 6mm 3 core spanning 55m
- Metalclad boxes with standard 2.5mm
- Metalclad switch with standard 1.5mm

I’ve never tested Zs at MET or impedance myself on a system so I can’t tell you those right now, sorry

 

[Shocker1]

Okey doke, I’ll do my best.

Everything in the garage is surface mounted by the way.

- SWA is 6mm 3 core spanning 55m
- Metalclad boxes with standard 2.5mm
- Metalclad switch with standard 1.5mm

I’ve never tested Zs at MET or impedance myself on a system so I can’t tell you those right now, sorry

I have a meter, nothing fancy but I guess can carry out these tests. Could you briefly outline where I connect to get the values?

 

[NozSpark]

Personally i think that your submain is too small… with the volt drop you’ll be able to have a max of 3.6kW before you run into problems

 

[Shocker1]

How do you know the size of the sub-main? And how are you calculating that value?

I’m also concerned that I haven’t seen the electrician run a single test so far, and didn’t even check my socket wiring for cross polarity had the plates off ready for inspection and everything! He spec’d 4mm SWA too which didn’t seem like enough given the distance and draw?

 

[NozSpark]

Well you have said that you have installed 60m of 6.0mm SWA cable (in your opening paragraph),,,, that’ll allow for a load of upto 3.6kW before the voltage drop becomes non-compliant…. It just takes a quick calculation using an app or using the appropriate sections of the regs/onsite guide to work it out

 

[Shocker1]

Interesting, as that’s exactly the type of calculator I used to calculate what I would need, one on TLC direct. It’s actually about 55m

 

[poni]

is this the one you used? it shows 10mm2 2 core SWA, and I have attached the type of tester you should use

 

[Shocker1]

is this the one you used? it shows 10mm2 2 core SWA

That’s 2 core, I’m using 3 core.

I’m showing these results with the same data but 3 core:

Required Cable Size	6mm
Voltage Drop	6.83 Volts
Percentage Drop	3.0%
Current Load	18.3 Amps
Max Cable Load	46.2 Amps

 

[poni]

why use 3 core?

 

[poni]

the number of cores don't matter its the number you use

 

[Shocker1]

why use 3 core?

Because everything I read and researched led me to believe that it is what I required for an electrical supply to a workshop outbuilding 55m away running machines.

SPD requires earth right? I’ve fitted an earth rod and spec’d in the SPD too, so that requires the 3rd core.

Please tell me otherwise, as I’m interested to learn. Not that I can change it now, it’s buried!

 

[poni]

did you see this bit at the bottom

 

[poni]

this is getting silly now, as far as I know its frowned upon to export the earth outside the equipotential zone ,

where's the earth rod and have you tested it ?

 

[Shocker1]

I did indeed. I used a couple of other calculators too, and it seemed like 6mm 3 core was within the bounds of what I need.

I refer to:
Except where installed in a conduit or duct which provides equivalent protection against mechanical damage, a cable buried in the ground shall incorporate an earthed armour or metal sheath, or both, suitable for use as a protective conductor, or be of insulated concentric construction.

As I am a DIYer, I felt it best to just use 3 core rather than trying to use the steel wire as my ground, I am more comfortable with the extra copper core. Even if I am at the top end of what I will be running, I don’t think I’m concerned about the 6mm being the issue?

As I said, please inform me if you have helpful and relevant information!

 

[poni]

I do like your tag name "Shocker", what's your day job?

 

[Shocker1]

I specialise in timber framing, mostly shipwright work, but a cabinet maker historically and plenty restoration, which covers general building. What’s yours!?

I would really love some more detailed information if you have some to offer, rather than such vague short answers? I truly wish to learn more, but I’m not really getting much from you other than perhaps you think the cable should be 10mm 2 core and not 6mm 3 core?

 

[poni]

50 years an electrician

 

[poni]

I can't get my 50 years knowledge across in a few short post's
you need more experience to do a job like this, this is not a DIYer job?