# Help with workshop rewire please



## Westwood (16 Mar 2020)

I am planning to rewire my small detached workshop, 6 metres by 3.3 , former garage, as the wiring is some 60 years old and has recently been shorting out the house. Project all discussed with electrician and I will do the first fix work and he will do all the wiring and certifying .
Installation consists of 5 overhead LED tube lamps; surface mounted 20mm plastic conduit with ten metal clad double sockets at low, middle and high levels; two external lights - two way switched from the house; all run from a new 6 way Hager consumer unit. Five active circuits and one spare, which I might use for two low level heaters in winter.. Possible future addition might be a dust extraction unit- SIP or similar.
I'm strictly an amateur woodworker with very limited power tools but do have a SIP ten inch table saw with induction motor. 
Two or three questions please:
How do I work out what size of breakers to fit in the consumer unit, and should they be RCD or MCB units ?
It seems sensible to plan a low level 16amp socket for the table saw, and possible future bandsaw. Should this be on its own separate circuit ?
Power will reach the garage via a sub main cable run from the main house consumer unit to this workshop CU. House supply is 100 amp three phase. Most of the submain cable will run below a concrete balcony slab and doesn't need to be buried. Can I use a suitable cable in conduit or does it need to be a SWA. ? The cable run is about 25 metres


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## MikeG. (16 Mar 2020)

You shouldn't be involving yourself with a consumer unit. Leave that to the electrician. You can do everything after the CU, but that box of tricks is his business, and his alone. They study for quite a long time to get that thing right. Same with sizing the supply cable.


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## Lons (16 Mar 2020)

What Mike said!

Have you given the electrician your list of requirements? In which case he will advise what size wiring for you to lay. He will also calculate the required MCBs to suit maximum load.

I have a SIP 01332 table saw and did run it from a standard 13 amp socket for a while until I had a dedicated 16 amp socket fitted and would definitely advise that to future proof your workshop as it will be much cheaper to have it done now.


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## flying haggis (16 Mar 2020)

as others have said, let your sparky do all the tricky calcs re cable sizing etc. it will after all, be his signature on the documentation. one bit of advice though, have you considered running square trunking round the top of the shop and having conduit drops to the sockets, switches etc. easy to add/remove if you want to change anything. as you have three phase at the house you could also run three phase to the shop.


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## sunnybob (17 Mar 2020)

Definitely 3 phase to the shop, the extra cost at this stage is minimal compared to the future usefullness.

And, like all of the above, its not your place to determine wire sizes or breakers. you can run all the conduits where he says, and chase out any walls to save him time and you money, and even run the wiring around waiting for him to connect the ends, but electrical design is in the hands of the man who signs off on it.
If push comes to shove, it will be HIM in the dock explaining to the coroner. Thats why he's paid the big bucks.


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## Steliz (17 Mar 2020)

If you do run the 3 phase to your workshop your electrician will have to make sure that the potential loads on each phase are balanced.
If there is only one cable from the house CU then your workshop will be all on one circuit with the satellite CU providing local protection.


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## Westwood (17 Mar 2020)

Thanks for all the helpful advice. I'll leave the consumer unit and submain to the sparky but do the meccano bits myself, i.e. the conduit runs and socket positioning ready for him to do his bit. 
For the TS 16 amp socket, is that a blue female unit ready to accept a blue male "plug" ? Curious as to what I order at the wholesalers....


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## Lons (17 Mar 2020)

I fitted one of these, got it from Screwfix they're only £8.50 so cheap enough and it's reasonable quality. Mine has been in a number of years in regular use and I also use it as a feed to keep my motorhome batteries charged.https://www.screwfix.com/search?search= ... e_start=60


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## BigMonka (17 Mar 2020)

Westwood":3sy7p9cv said:


> For the TS 16 amp socket, is that a blue female unit ready to accept a blue male "plug" ? Curious as to what I order at the wholesalers....


Yes, that's right - all sockets have the live side as a female, otherwise you'd have live pins sticking out that someone might touch. Blue is the standard for a 230V single-phase connection, red is 400V three-phase (and yellow is 110V) so that part depends on what you want to connect to the socket.


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## flying haggis (17 Mar 2020)

just checked and 5 core swa 10mm is round about £6/m so definetely worth considering running three phase to the shop. you could even run the cable and wire it for single phase to begin with


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## Westwood (17 Mar 2020)

"If there is only one cable from the house CU then your workshop will be all on one circuit with the satellite CU providing local protection."
I'd still like to have several different circuits as stated in my original post. 16amp for the TS; lights, exterior lights and general sockets on a ring. That way if I get a break in say, the power , I don't lose the lights.
Its potentially available but not sure why I would need three phase ? In 20 years or more of using the garage , I've never come across the need to upgrade.
And still interested to know how I would rate or size the CU breakers , if only so that I can be informed about this when the sparky makes his choices


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## flying haggis (17 Mar 2020)

treat the shop like a house sockets on a ring 32A breaker, big machines on a 20A radial to dedicated sockets (poss blue 16A) and light on 6A breakers. you could put radial ccts in for sockets on a 20A breaker if it saves cable. your original post stated light (on the shop?) switched from the house?? akward but possible. might be easier with remote switches ie like a car key fob. again get your tame sparky involved


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## Sideways (17 Mar 2020)

Westwood":3j7bzlo8 said:


> I'd still like to have several different circuits as stated in my original post. 16amp for the TS; lights, exterior lights and general sockets on a ring. That way if I get a break in say, the power , I don't lose the lights.
> And still interested to know how I would rate or size the CU breakers , if only so that I can be informed about this when the sparky makes his choices


Your electrician is required to do the design and distribute the loads sensibly across a set of circuits so that - as you say - you don't lose the lights if a machine overload trips a shared breaker (potentially leaving whatever other machinery still running and you in the dark...)
Simply, the design process is something like this :
Understand the size of the various loads and how they are used so they can be grouped sensibly. A ring for small machines, power tools, extractors and heaters plus some dedicated circuits for bigger motors is very sensible. Lights on their own circuit.
Ring circuits are standardised. 30 or 32A breaker, 2.5 square mm conductors, as many 13A outlets as you want in effect. Simplistically the floor area served is used to decide when to install more than one ring.
Radial circuits are sized according to the load you intend to put on them. e.g. take the full load currents of the machines' motors, scale them UP according to the power factor of the motor(s) and add them together. Pick the nearest standard circuit breaker with a rating EQUAL to or ABOVE the current you need (6A, 16A, 20A, 30A, 40A, 60A etc)
Pick the circuit breaker TYPE according to the nature of the load - common breakers are a type B, you may need a type C or D for heavily inductive loads like fluorescent lighting and motors so that the startup surge doesn't trip the breaker.
You will need 3omA RCD protection on many, possibly all circuits - unless you hard wire your machines in and run your cables in steel conduit. This can be a separate device or built into each breaker (devices called RCBO's).
Your circuit breaker is there to protect the fixed wiring, your tablesaw should typically be operated and protected by a starter with a thermal overload device.
Your cables sizes are calculated based on the size and type of the circuit breakers (not the loads) and also to ensure that the limits for voltage drop are not breached (one example, not more than 3% of the supply voltage can be lost between the consumer unit main switch in the house and any point in your workshop lighting circuit. this lost energy goes into heating the cables and components along the circuit. Longer runs and certain types of installation may require the use of thicker cables). 
Cable sizes must be thick enough that if a short circuit occurs, the wires are big enough to carry a large fault current. The bigger the fault current, the faster the circuit breaker will trip. Circuit breakers must trip within specified time limits, so "time current" graphs for each size and type of circuit breaker are used to work out the minimum fault current needed and then another set of calculations are done to find which standard size cable will deliver this.
And then there's a whole bunch of calculations to take into account the way the cables are installed because this affects how much they warm up and how much current they can carry before the insulation melts. These (usually) derating calcs may force the choice of thicker cables.
And the rules that decide the size of the conduit according to the size of the wires inside.
And the rules about maximum spacing between conduit fixing brackets, and the need for fire resistant clips
And the rules about earths and outbuildings
etc etc
The wiring regs run to over 400 pages. Unless you buy a copy and study it, you'll never anticipate everything. The best way to work is to decide where you want your various machines, sockets and lights to go. Explain this to your spark and let him tell you what hardware to buy and how to route and install it.


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## Westwood (18 Mar 2020)

Thanks for that, Sideways really comprehensive and helpful. My shop is very basic, only larger machines are the table saw and a possible future bandsaw, plus may be a dust extraction unit. Others include a bench top pillar drill, small bandsaw , scroll saw - occasional use only. Then there are more portable power tools, drill, router half inch and a quarter inch one, electric planer and hopefully a Tormek sharpening station. I guess I also plug in regularly a radio, stereo receiver, drill charging point, phone charger and a Henry hoover ! Occasional other plug in things include a reciprocating saw often at the end of my longest extension lead
Not sure of the difference between a ring circuit and a radial one ??


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## Lons (18 Mar 2020)

Westwood":2cscu7ua said:


> Not sure of the difference between a ring circuit and a radial one ??



If you google it you'll find plenty of straight forward, easy to understand explanations and drawings on the internet.

As far as your workshop is concerned, you can never have enough sockets though you seem to have allowed lots but if you can try to future proof your workshop as much as possible as it's cheap to put in the wiring at this stage.


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## Phil Pascoe (18 Mar 2020)

My old garage/workshop was about 6m x 5m, and had twenty five double sockets (inc. one in the pit  ). These were cheap and nasty, but easy to install during the build. I never needed to replace one. It guaranteed no trailing leads anywhere, and that no matter what was stacked against the walls I could reach one. You'd also be surprised how many sockets end up with things plugged into them permanently.


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## Lons (18 Mar 2020)

Phil Pascoe":18ub0gfn said:


> My old garage/workshop was about 6m x 5m, and had twenty five double sockets (inc. one in the pit  ). These were cheap and nasty, but easy to install during the build. I never needed to replace one. It guaranteed no trailing leads anywhere, and that no matter what was stacked against the walls I could reach one. You'd also be surprised how many sockets end up with things plugged into them permanently.


I have the equivalent of 16 doubles plus a 16 amp socket in my double garage size workshop and often think I need twice as many.


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## Sideways (20 Mar 2020)

The good thing in a one man shop is you can only run so much stuff at once however many sockets you have and tools you choose to leave plugged in. The power hogs will be the 3kW fan heater in winter plus a shop vac or dust extractor, plus a kettle, plus one of a welder / tablesaw / planer thicknesser / router.
So about 10kW max. That's about 40 amps to bring down from the house. Make your tea in the kitchen and you might well get away with 30A 
Cheers


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## Lons (20 Mar 2020)

My first workshop had only 1 double socket but what happens is that you use a number of extension cables as batteries have to be charged, maybe a radio, a vac and and there is a real danger of trip hazards especially if they get covered in sawdust.


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## flying haggis (25 Mar 2020)

somewhere in the back of my cluttered brain i seem to recall that you are supposed to notify your elec supplier if you want to use a welder(might be only if it is only of a certain power though)


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