New Workshop Electrics

[aesmith]

Hi,
I'm planning a new workshop, think "big shed" rather than industrial unit, but I will still want a decent electrical system.

First thing is how to get the supply in. There's a nearby garage which is served by 25sq.mm SWA from the house, split off with henly blocks before the house CU. I was thinking of taking supply from there using an MCB (maybe 40A) and feed the new shed with SWA (maybe 10sq.mm). Shed will have a main switch board with RCBOs, probably two for sockets, one lights and maybe two for future dedicated 16A supplies.

Question.. do people think there's anything inherently wrong with this?

Any other info needed for comment? House earth is TNS, I don't think the shed will have extraneous metalwork, unless the concrete floor counts.

Next question, Hager or Fusebox or anything else? I like the idea of switched neutral, and it looks like Hager C curve RCBOs are ludicrously expensive.

Thanksin advance. Schematic shows existing in blue, proposed in red.

 

[addjunkie]

If the shed not built yet, Id put a conduit in the ground and pull your cable through it. Or at least dig your SWA in first. Cable size should be calculated on load and distance.

 

[Dave-sparks]

Avoid using an MCB to feed a distribution circuit if at all possible, they offer no chance of any selectivity under fault conditions.

 

[aesmith]

Thanks. The shed's not yet built but it's going on an existing concrete base. I'm planning to carry the external part of the SWA inside a steel pipe, crossing between the buildings well above head height.

Cable length house->garage is about 45m in 25sq.mm. Garage DB to new shed DB cable will be about 10m.

I don't really have a clue how to estimate likely load. There'll just be me so you could guess the worst case might be running two things at once making it 6kW. Plus lighting and odds and sods like battery chargers adding a few hundred watts.

My other point of reference is that in a former house all the outbuildings were supplied via a 4sq.mm cable on a B40 MCB (I know). I can't think of much I'll be doing now that I don’t do there.

 

[Dave-sparks]

Next question, Hager or Fusebox or anything else? I like the idea of switched neutral, and it looks like Hager C curve RCBOs are ludicrously expensive.

Any particular reason you like the idea of switched neutrals?
I'm interested as there seems to be a growing obsession with switched neutrals within the industry but, whilst there is very good reason for it in some circumstances, many people don't seem to know when it is advantageous and when it can actually be a problem.

I don't know where you've looked at prices for Hager C curve switched neutral RCBOs but they shouldn't be ludicrously expensive, Edwardes brothers list them for as little as £22!

 

[addjunkie]

My biggest loads are lathe, or welder…..but cant use both at once. But got caught out in the last house as Id left the compressor on. It cut in while I was mid weld.

 

[aesmith]

Avoid using an MCB to feed a distribution circuit if at all possible, they offer no chance of any selectivity under fault conditions.

Thanks. So a fuse would be better, if that's possible to fit into a Hager board?

I did wonder about using a C curve MCB at the origin, but I guess that would only help with discrimination during overload, not high current fault.

 

[Dozzer]

When rewiring my little workshop was advised to look at the switching curve for the breakers, as some motors & welders will trip a normal breaker.

Tripping Curves of Circuit Breaker. B, C, D, K & Z Trip Curve

Types of Circuit Breaker Based on its Tripping Curves. What is a Trip Curve & How to Read it? Why We Need Different Tripping Curves for MCB?

www.electricaltechnology.org

 

[knighty]

I'd bump up to 25mm cable for the new bit too

when fuse it higher - to future proof it

quick online calculator says you're fine at 80 amps for 70m of 25mm csa

not a massive cost difference when it's only a 10m run

 

[aesmith]

Any particular reason you like the idea of switched neutrals?
...
I don't know where you've looked at prices for Hager C curve switched neutral RCBOs

Switched N, maybe no rational reason but I like the idea the if switched off (or tripped) the circuit is completely isolated.

I'm clearly looking at the wrong place for Hager. My favourite electrical place doesn't do Hager 1PSN RCBOs at all, and C curve only 10kVA rated which we don't need.

 

[Misterg]

Next question, Hager or Fusebox or anything else?

Sample of 1, but I liked the Fusebox board I put that appeared in my garage:

It's slightly irritating that the test button for the type C RCBO isn't in the same position as the type Bs.

My worst-case load was welder (or plasma) + compressor + dust extractor + halogen floodlight that came to ~30A that was just doable with 10mm^2 for a ~40m run.

Avoid using an MCB to feed a distribution circuit if at all possible, they offer no chance of any selectivity under fault conditions.

Did you mean to type MCB? The second part of the comment makes more sense if referring to an RCD.

 

[Dave-sparks]

Did you mean to type MCB? The second part of the comment makes more sense if referring to an RCD.

Yes I did, MCBs should be avoided for distribution circuits as they offer little to no selectivity in a fault.

You don't get any meaningful selectivity within the standard range of 6A - 63A MCBs

 

[Misterg]

Yes I did, MCBs should be avoided for distribution circuits as they offer little to no selectivity in a fault.

You don't get any meaningful selectivity within the standard range of 6A - 63A MCBs

My apologies - What should you use instead? A full sized breaker?

 

[aesmith]

Any particular reason you like the idea of switched neutrals?
I'm interested as there seems to be a growing obsession with switched neutrals within the industry but, whilst there is very good reason for it in some circumstances, many people don't seem to know when it is advantageous and when it can actually be a problem.

Missed this earlier. When is switched neutral a problem?

 

[Dave-sparks]

My apologies - What should you use instead? A full sized breaker?

Good old fashioned fuses are actually one of the best options.

If by full sized breaker you mean an MCCB then yes they are good for the job, but not worth the price for a domestic or other small installation.

 

[Dave-sparks]

Missed this earlier. When is switched neutral a problem?

It's a complex subject but generally more applicable to 3 phase installations than single phase.

In a single phase installation a switched neutral is generally no problem and it makes no difference.

In a three phase installation a switched neutral can become a serious problem if the neutral pole opens before the lives.
The risk is very small as most switches will operate as designed for all of their lives but if the neutral pole does fail then it can lead to unwanted voltage rises destroying a lot of expensive equipment. So it was always the case that the neutral was always kept solid unless there was a very good reason to switch it

 

[aesmith]

I'd bump up to 25mm cable for the new bit too

That's worth considering. I wasn't keen because of handling, routing, terminating that size of cable. But if I did could I simply branch off at the garage, with no fuse or breaker needed, all protected by the 80A fuse at the house?

 

[Dave-sparks]

Yes you can extend the existing distribution circuit to feed both buildings, there's nothing in the regulations to say you can't.

But you will find a lot of electricians out there who will tell you that it's impossible, can't be done, the world will end if you do it etc.

 

[aesmith]

Still musing on this. I really don't think I need 25mm2 to the new shed, the house->garage cable was over sized mainly for voltage drop given the 45m length and the fact that other buildings would be fed onwards from it. Also because of excavation and an awkward route inside the house it definitely needed to be future proof.

So I'm looking at 10 or maybe 16mm2. But I'd like to verify voltage drop taking into account the 45m to the garage as well as the proposed garage to shed.

Does anyone know an online calculator that will give voltage drop for different cable types or sizes? The very easy TLC one only gives figures for the minimum cable size, ie if you say 9kW over 45m it gives only gives figures for 10mm2. And looking at the proposed new run garage->shed it gives 4mm2.

Voltage Drop Calculator

www.tlc-direct.co.uk

 

[knighty]

I think you'll make things much easier using 25mm and tapping into the supply of the garage instead of coming out of it's consumer unit

I don't know the regs... sure someone else will be along soon who does... but if you put it in a duct you don;t need to use armoured cable, which means you could get something much easier to run

also... I'll have to measure but I think I have a length of 5corn 25mmho7z cable which is more than 10m long... if it's long enough you're welcome to it
(just use 3 cores and ignore 2 of them)