20a DP switch spur for 6 sockets

[G100917]

Hi everyone, I'm currently upgrading my workshop and before I get quotes for the electrics I'd like to understand it better myself. If I want 6 sockets from a ring mains circuit can I spur off an existing socket to a 20a DP switch and run the 6 sockets from that? It's unlikely the load will exceed 4kw, there will be an electric heater running, as well as a kettle, microwave and occasional use of electric saw etc. Or would I be better off running 3 sockets each from two separate 13a FCU switches spurred off two different sockets on the ring circuit . Any advice would be appreciated many thanks.

 

[telectrix]

no you can't spur off a ring without fusing down. for the load you anticipate, you need to extend the ring. i.e. split at convenient socket and loop round the workshop, making all of the sockets on the ring. should end up with something like this. 2 existing cables and 2 new cables .

 

[G100917]

no you can't spur off a ring without fusing down. for the load you anticipate, you need to extend the ring. i.e. split at convenient socket and loop round the workshop, making all of the sockets on the ring. should end up with something like this. 2 existing cables and 2 new cables .

I see many thanks for the reply telectrix.

 

[7029 dave]

Is the circuit RCD protected, because the additional one's you propose to fit will have to be.

 

[G100917]

Is the circuit RCD protected, because the additional one's you propose to fit will have to be.

Yes it is protected

 

[7029 dave]

Yes it is protected

Thats good, one less obstacle to address.

 

[pc1966]

As above, best option is to extend the existing ring final, after checking that:

• It has RCD protection, and
• The ring is OK (end-end continuity and insulation resistance good)

(OK, just read that you have an RCD, but leaving it for anyone else reading this in the future)

 

[pc1966]

One final though, presumably you don't actually need an isolation switch (or emergency stop) for any other workshop safety reason?

If it is just for hobby stuff without any seriously intimidating equipment then just go with the ring extension. Otherwise if you really need some means to isolate the whole area you would be better with a separate 4mm radial from the CU just to feed the 6 sockets via a local isolation switch.

 

[G100917]

One final though, presumably you don't actually need an isolation switch (or emergency stop) for any other workshop safety reason?

If it is just for hobby stuff without any seriously intimidating equipment then just go with the ring extension. Otherwise if you really need some means to isolate the whole area you would be better with a separate 4mm radial from the CU just to feed the 6 sockets.

My understanding was that you could only spur off to multiple sockets if you had a fused switch coming off the ring first. And because I planned to be able to run an electric heater, kettle and microwave at the same time for example I presumed that would blow a 13a fuse, hence why I presumed a 20a DP switch could just be used instead. I'll be getting a professional round at some point but would be handy to know the options beforehand. Also the CU is a fair distance from my workshop hence why I thought the spur would be easiest/cheapest option!

 

[telectrix]

a 20A D/P switch is no good. it's not fused. you could fit a 20A MCB in a small enclosure to feed a spur, but for the load you mention,better as a ring extension (you only need to access a local existing socket, no need to go back to CU) so you have a 32A capacity. the 32A MCB will allow small overloads for short time periods without problems.

 

[G100917]

a 20A D/P switch is no good. it's not fused. you could fit a 20A MCB in a small enclosure to feed a spur, but for the load you mention,better as a ring extension so you have a 32A capacity. the 32A MCB will allow small overloads for short time periods without problems.

Excellent, many thanks for your feedback telectrix much appreciated.

 

[Intoelectrics]

My understanding was that you could only spur off to multiple sockets if you had a fused switch coming off the ring first. And because I planned to be able to run an electric heater, kettle and microwave at the same time for example I presumed that would blow a 13a fuse, hence why I presumed a 20a DP switch could just be used instead. I'll be getting a professional round at some point but would be handy to know the options beforehand. Also the CU is a fair distance from my workshop hence why I thought the spur would be easiest/cheapest option!

Extending the ring maybe your cheapest and easiest option but if you intend to use the workshop on a regular basis with multiple equipment, then personally, if its an option, I would seriously consider a separate supply from the CU.

 

[Strima]

I would look at running a separate circuit for your workshop. Last thing you want is something tripping the power and taking out a circuit in the house as well.

 

[G100917]

Post deleted

 

[G100917]

I would look at running a separate circuit for your workshop. Last thing you want is something tripping the power and taking out a circuit in the house as well.

Many thanks for the reply, out of interest if I extended the ring circuit would that mean the max possible amp on the circuit would be 32A before it trips?

 

[telectrix]

Many thanks for the reply, out of interest if I extended the ring circuit would that mean the max possible amp on the circuit would be 32A before it trips?

basically yes. but MCBs will withstand small overloads for short periods. this also applies to the cable/e.g. a 32A MCB will trip in approx 17 minutes at a steady 50A current.

 

[pc1966]

Many thanks for the reply, out of interest if I extended the ring circuit would that mean the max possible amp on the circuit would be 32A before it trips?

Typical trip curves are given here:

https://storage.electrika.com/manu/man-0150/pdftech/psb06-b.pdf

Most commonly a ring-final is a 32A B-curve MCB (or RCBO), but often what trips the MCB is a short taking out a 13A fuse. The let-through of the fuse can often be high enough to hit the "instant" magnetic trip point of the MCB at 3-5 * 32A (in the B-curve case).

To some extent you have to judge how likely that is, and how significant losing the house sockets until you can reset the breaker. A bigger concern is if the same MCB also feeds the workshop lights, as loss of lighting along with a "bang!" cable damaging incident in a room full of potentially dangerous tools is not a good thing.

 

[G100917]

Typical trip curves are given here:

https://storage.electrika.com/manu/man-0150/pdftech/psb06-b.pdf

Most commonly a ring-final is a 32A B-curve MCB (or RCBO), but often what trips the MCB is a short taking out a 13A fuse. The let-through of the fuse can often be high enough to hit the "instant" magnetic trip point of the MCB at 3-5 * 32A (in the B-curve case).

To some extent you have to judge how likely that is, and how significant losing the house sockets until you can reset the breaker. A bigger concern is if the same MCB also feeds the workshop lights, as loss of lighting along with a "bang!" cable damaging incident in a room full of potentially dangerous tools is not a good thing.

Interesting good to know, thanks for you insight pc1966

 

[G100917]

Many thanks for your feedback everyone. I'll most likely look in to getting the ring circuit extended as the workshop will only be for light and occasional use, and there's a socket nearby that can be connected to.

Just out of interest, why do multiple sockets from 1 unfused spur overload and damage the cable, when multiple sockets on a ring circuit won't do the same? If you used several appliances at once on a ring circuit doesn't that draw a lot of amps through the circuit in the same way it would through sockets connected to a spur?

 

[loz2754]

With a ring circuit you've got 2 cables feeding each socket instead of 1 for a radial circuit.

 

[telectrix]

asabove^^^ on a ring the load is shred between both legs. equally shared at mid-point but nearer the CU, the shorter legg carries mor than the longer leg, that's why it's a 32A MCB and not a 40A or 45A.

 

[ipf]

Like you say in your first post, get quotes from electricians for the job. Just because you have decent information from this forum, it doesn't mean you know and understand all involved.
In general, a DIY installer wouldn't have the slightest chance of following or understanding 'trip curves' or methods of protection.....or getting their head around RFC concepts, for that matter.

 

[telectrix]

isn't littlespark up Glasgow way?

 

[G100917]

Ok I see! Many thanks. I will of course get a professional I just wanted to understand best I could before hand and in case it affected the workshop layout/which walls would need cables mounted etc

 

[pc1966]

isn't littlespark up Glasgow way?

I think @littlespark is borders, so a bit of a drive to Glasgow, but I think @JK-Electrical is Glasgow area, as is @MFS Electrical

 

[streamer]

I'm in the Southside of Glasgow so I could have a look and give you a quote if you would like. I'll send you a message.

 

[littlespark]

Yeh, @telectrix you got the Borderers wrong. @streamer was exiled to Glasgow for things we can’t really talk about here. ??

 

[telectrix]

i thought the border was a line drawn under glasgow across to edinbrug. there used to be a wall but the jocks nicked all the stones.

 

[streamer]

And here was me looking for anonymity in the big smoke

 

[telectrix]

anonymity? is she fit?

 

[streamer]

Yeh, @telectrix you got the Borderers wrong. @streamer was exiled to Glasgow for things we can’t really talk about here. ??

And here I was, looking for anonymity in the big smoke. Cover blown.

 

[streamer]

anonymity? is she fit?

Worth getting exiled for!

 

[littlespark]

i thought the border was a line drawn under glasgow across to edinbrug. there used to be a wall but the jocks nicked all the stones.

Well there’s the pot calling the kettle black....

I can’t say that, can I?

 

[telectrix]

you just did.