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]

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?