Am I Limited to a 20A Ring In Shed Build?

[ILikeDIYS]

Need to order the consumer unit for my garden shed and thinking about the MCBs to use...

It will be run off the house consumer unit... currently have a 32A MCB spare, in 6mm cable all the way to the exterior SWA terminal - from then on it is buried 4mm SWA (10m length) up to the shed.

For the MCBs in the shed, it looks like I'm limited to 20A (ring), 6A (lighting) and 6A (alarm).

Would this be ideal or is there any way I can push the ring to be higher than 20A? The next MCB up is 32A... which would mean a 40A MCB in the house... then connecting the alarms to the lighting MCB (6A) totalling 38A... applying diversity and all of that this should be OK on 4mm SWA right? Just wanna minimise the chances of music equipment tripping out from overload.

Thanks.

 

[EFLImpudence]

It will be run off the house consumer unit... currently have a 32A MCB spare, in 6mm cable all the way to the exterior SWA terminal - from then on it is buried 4mm SWA (10m length) up to the shed.

Ok.

For the MCBs in the shed, it looks like I'm limited to 20A (ring), 6A (lighting) and 6A (alarm).

Why?

Would this be ideal or is there any way I can push the ring to be higher than 20A?

You do not need any MCB in the shed for the sockets. It is protected by the 32A MCB in the house.
So. either 2.5mm² ring or 4mm² radial.

The next MCB up is 32A... which would mean a 40A MCB in the house...

40A would be too high for the 4mm² SWA (although there are other considerations) and it would not discriminate with the 32A MCB.

then connecting the alarms to the lighting MCB (6A) totalling 38A... applying diversity and all of that this should be OK on 4mm SWA right?

Yes, the total of MCBs is irrelevant if the load of the lights and alarm are negligible.
It will all be protected by the 32A in the house.

Just wanna minimise the chances of music equipment tripping out from overload.

Then you will have to design it the other way round and install circuits to cope rather than start with the circuits.


Outside power is not really a DIY job. There are other things to consider.

 

[ILikeDIYS]

You do not need any MCB in the shed for the sockets. It is protected by the 32A MCB in the house.
So. either 2.5mm² ring or 4mm² radial.

Yeah I was just wanting to separate the 3 circuits as the most likely candidate for overload is the ring... so at least I would have fire alarms and lights working still after a trip out.

40A would be too high for the 4mm² SWA (although there are other considerations) and it would not discriminate with the 32A MCB.

Hmm, maybe I could have a 32A in house AND a 32A for the shed ring + 6A for lights/alarms given the negligibility of those and at worse would only cause the house MCB to trip as well. Alarms have battery backup so would continue working in event of a full trip out. Think I just answered my own question.

Then you will have to design it the other way round and install circuits to cope rather than start with the circuits.

I cheeped out on the SWA and bought 4mm instead of 6mm and kinda regret that, but too late to change.

 

[JohnW2]

You do not need any MCB in the shed for the sockets. It is protected by the 32A MCB in the house. So. either 2.5mm² ring or 4mm² radial.

If there were no local MCB, then, strictly speaking (I know you like that ) "either 2.5mm² lollipop-circuit or 4mm² radial"

For what it's worth, I agree with everything else you say.

Kind Regards, John

 

[EFLImpudence]

Yeah I was just wanting to separate the 3 circuits as the most likely candidate for overload is the ring... so at least I would have fire alarms and lights working still after a trip out.

Fair enough, would 20A not be enough?

Hmm, maybe I could have a 32A in house AND a 32A for the shed ring + 6A for lights/alarms given the negligibility of those and at worse would only cause the house MCB to trip as well. Alarms have battery backup so would continue working in event of a full trip out. Think I just answered my own question.

... and the same for the light - an emergency light with battery.

I cheeped out on the SWA and bought 4mm instead of 6mm and kinda regret that, but too late to change.

That was only relating to putting a 40A MCB in the house which would be of little advantage anyway.

As I said there are other considerations.
With a 32A MCB in the shed (and negligible lighting and alarm) you could have 63A MCB in the house as long as certain earth fault conditions are met but you would need an electrician to verify that.

 

[ILikeDIYS]

Fair enough, would 20A not be enough?

Shed power is on a 13A plug at the moment and I blew a fuse running a heater, dehumidifier, heat gun and some other equip already when I was doing the decorating. To be honest I might as well do 20A for the ring and if it trips only then consider switching it to 32A.

With a 32A MCB in the shed (and negligible lighting and alarm) you could have 63A MCB in the house as long as certain earth fault conditions are met but you would need an electrician to verify that.

I was also gonna ask about earth electrodes. I'm rushing to patch the last piece of plasterboard which covers the electrical cables to the consumer unit... and am thinking whether I should dangle an earth cable down there for a future earth rod?

 

[EFLImpudence]

I was also gonna ask about earth electrodes. I'm rushing to patch the last piece of plasterboard which covers the electrical cables to the consumer unit... and am thinking whether I should dangle an earth cable down there for a future earth rod?

It's complicated. That's why I said outside power is not a DIY job.

It depends if you have metal extraneous-conductive-parts (metal bits coming out of the ground) in the shed.

If you have and don't want an electrode (which would be at the shed) you could need a 10mm² bonding cable from e-c-ps in shed to house CU.

If there are no e-c-ps then you don't need an electrode nor anything else anyway.

 

[ILikeDIYS]

It depends if you have metal extraneous-conductive-parts (metal bits coming out of the ground) in the shed.

If you have and don't want an electrode (which would be at the shed) you could need a 10mm² bonding cable from e-c-ps in shed to house CU.

If there are no e-c-ps then you don't need an electrode nor anything else anyway.

The water cold feed is an MDPE pipe and the only copper is inside the building (just a small run from MDPE to the tap/sink) so I guess the SWA earth from the house CU should do then. Will test everything in the end and see. Cheers.

 

[AndyPRK]

Sometimes you can get 25A mcbs.

You could put lights and alarm on same 6A Mcb

Who cares about discrimination if it’s unlikely to trip in normal use ? It’s only a shed after all. I’d go for 32A both ends

Ok you could end up in the dark.
But if you only use in day time and it has windows.

 

[JohnW2]

Who cares about discrimination if it’s unlikely to trip in normal use ? It’s only a shed after all.

Well said. I think there is a tendency for people to 'knee-jerk' about things like 'lack of discrimination', without giving consideration as to whether it really 'matters'. However, 'lack of discrimination' very often (probably nearly always, unless one is looking for 'redundancy') means that one of the devices is 'unnecessary' ....

I’d go for 32A both ends

As above, I'm not sure that the one at the shed end would serve any useful purpose, would it? If there were a 32A MCB at the shed (as well as in the house), the OP would have to have either a 4mm² radial or 2.5mm² ring in the shed - and that would remain the case (other than that we would then call a 2.5mm² ring a 'lollipop' circuit) even without any MCB in the shed, wouldn't it.

Lights etc. in the shed could obviously e fed via an FCU.

Kind Regards, John

 

[securespark]

Lack of discrimination is contrary to the regs.

No self-respecting spark would suggest such an arrangement.

There is absolutely no need for a 32A device at the shed.
Take a 4 milli radial from the house via a 32A device, then fuse down for lighting.

 

[JohnW2]

Lack of discrimination is contrary to the regs.

Indeed - but that might be taken to indicate that the regulations are one of those "people" who I suggested "'knee-jerk' about things like 'lack of discrimination', without giving consideration as to whether it really 'matters' "!

Deliberate 'lack of discrimation' can be used to enhance safety by providing redundancy. At least some people sometimes do that 'deliberately' with RCDs, but it's always surprised me a little that they never seem to want to do the same with OPDs, even though (unlike RCDs) they are effective not testable.

No self-respecting spark would suggest such an arrangement.

Do they never 'obey MIs' and install 3A fuses in series with 6A MCBs for extractor fans etc.? I also suspect that, despite what we both say (below) a good few of them have installed 'mini-CUs' in garages and outhouses which are fed from a 20A or 32A MCB in the house's CU.

There is absolutely no need for a 32A device at the shed. Take a 4 milli radial from the house via a 32A device, then fuse down for lighting.

Yes - I've just said all that

Kind Regards, John

 

[EFLImpudence]

Deliberate 'lack of discrimation' can be used to enhance safety by providing redundancy. At least some people sometimes do that 'deliberately' with RCDs, but it's always surprised me a little that they never seem to want to do the same with OPDs, even though (unlike RCDs) they are effective not testable.

Maybe but the logical outcome of that belief is two (or more) of every such device - clearly not practical.

Perhaps MCBs should contain two OPDs just in case..

 

[bernardgreen]

There is absolutely no need for a 32A device at the shed.
Take a 4 milli radial from the house via a 32A device, then fuse down for lighting.

I would consider running the sockets from a 20 Amp MCB in the shed. This could then avoid the situation where a overload on the socket circuit trips the 32 Amp MCB at the house leaving the shed with out lights.

I accept that a sudden hard overload from a fault is likely to take out both a 20 Amp MCB at the shed and the 32 Amp MCB in the house but a soft overload ( too many items switched on ) would take out the 20 Amp MCB before the 32 Amp MCB in the house.

Depending on what activities were planned for in the shed I might go further and split the sockets in the shed between two 16 Amp MCBs in the shed.

 

[EFLImpudence]

Yes, there are so many options, it is difficult to advise the OP which is why I keep saying that outside power is not a DIY job.

The circuit(s) should be designed in the normal sequence and manner.