Garage power

[Electrapeace]

If I connect 4mm2 armoured from a 32A mcb to a garage 30metres away do I need to sub-fuse box in the garage or can I just use it as a radial circuit?
The garage will have two sockets and fused down lighting.

 

[telectrix]

is the 32A MCB RCD (and hence your new work) protected? you could feed to the sockets and then fit a 5A FCU for the lighting.the FCU can also be used as the light switch.

 

[Taylortwocities]

If I connect 4mm2 armoured from a 32A mcb to a garage

New circuit = notifiable to the local authority.
Otherwise as telectrix says.
cables between sockets and the feed side of the FCU also need to be in 4mm.

 

[Dustydazzler]

do you really need 32a at the garage ?

if you don’t then you Could save yourself some money and use 2.5 on a 20a

 

[Electrapeace]

is the 32A MCB RCD (and hence your new work) protected? you could feed to the sockets and then fit a 5A FCU for the lighting.the FCU can also be used as the light switch.

Yes it’s protected at the CU with an RCD. So then I can go straight to the sockets as you say. Thanks. Just out of curiosity, if I did put in a small garage fusebox I would end up with two RCDs in the same circuit. That seems unnecessary.
[automerge]1591001101[/automerge]

New circuit = notifiable to the local authority.
Otherwise as telectrix says.
cables between sockets and the feed side of the FCU also need to be in 4mm.

Ahh ok that’s good to know as I was going to use 2.5 between the sockets.
[automerge]1591001176[/automerge]

do you really need 32a at the garage ?

if you don’t then you Could save yourself some money and use 2.5 on a 20a

Yes the owner wants 32A I case if future needs.

 

[John-SJW]

Electrapeace, having 4mm between sockets and FCU, will be for voltage drop on such a length. If a garage CU is fitted, if the Main CU has an RCD, you wrote it did, then no RCD needed in the garage. Just have an incomer and mcbs for the sockets and lights. Keep them radials. Then from the CU to the sockets can be 2.5mm providing that the voltage coming into the garage is fine (has not dropped substantially from the main supply).

 

[pc1966]

If the owner has future plans then maybe as John-SJW says use a garage CU with just a switch incomer to split between lights and sockets. The sockets could then be on 2.5mm at 32A as a ring final, or radial with local 20A MCB used. If the CU has 3 MCB slots (eg. the BG one) then 6A lights / 20A sockets / spare space for a future circuit should the decide what they want.

Of course you won't get good selectivity with the upstream MCB but maybe acceptable. What might be a bigger concern in the case of a split RCD board in the house is the garage tripping half of it on a fault.

 

[John-SJW]

A prime aim must be to keep all fault and over-current tripping inside the garage while at the design stage. No need to compromise here.

 

[pc1966]

A prime aim must be to keep all fault and over-current tripping inside the garage while at the design stage. No need to compromise here.

In that case they can't feed it from the home CU. The MCB there (assuming 32A B-curve) will trip "instantly" at less than 160A so practically any sort of cable short fault will cause that.

The only way you will get tolerable selectivity is to split the meter tails and used a fused-switch, then you may be in the kA range. But you then need a local RCD, and you need to check the supply Ze and make fuse selection to be as high as practical for selectivity, while still meeting the limits for clearing a short on the SWA cable in under 5s. Better desing, but a major increase in work cost!

 

[davesparks]

A prime aim must be to keep all fault and over-current tripping inside the garage while at the design stage. No need to compromise here.

That is impossible as it will all be fed from a domestic supply and a 32A MCB.
[automerge]1591044385[/automerge]

Electrapeace, having 4mm between sockets and FCU, will be for voltage drop on such a length. If a garage CU is fitted, if the Main CU has an RCD, you wrote it did, then no RCD needed in the garage. Just have an incomer and mcbs for the sockets and lights. Keep them radials. Then from the CU to the sockets can be 2.5mm providing that the voltage coming into the garage is fine (has not dropped substantially from the main supply).

The CU is pointless as the MCB's in it will not discriminate with the upstream OCPD.

 

[Taylortwocities]

The CU is pointless as the MCB's in it will not discriminate with the upstream OCPD.

Agreed, discrimination between MCBs is quite difficult to achieve. Just gave a look at the tables and you will see.
as a rule of thumb the upstream MCB will heed to be at least twice the value of the downstream MCB for there to be any hope that the garage MCB will trip. Usually it’s the house MCB that trips because the design hasn’t been done properly.

 

[John-SJW]

Agreed, discrimination between MCBs is quite difficult to achieve. Just gave a look at the tables and you will see.
as a rule of thumb the upstream MCB will heed to be at least twice the value of the downstream MCB for there to be any hope that the garage MCB will trip. Usually it’s the house MCB that trips because the design hasn’t been done properly.

I assume you are referring to fault not over current. The CU will split the circuits into logical divisions, lights, sockets, maybe even into a small fixed heater & sink water heater. They are easily isolated for whatever reason. Having a small 2.5mm ring will keep voltage up as it will effectively be 5mm. OK, e
[automerge]1591047679[/automerge]ffectivceky you could have 4 FCUs in a line (skts, lights, Htr, water Htr), that will look amateurish, when a small compact CU enclosure does it nicer.

The CU is pointless as the MCB's in it will not discriminate with the upstream OCPD.

Depends on what mcbs are fitted in both. How to do it.

Electrical Calculation Tool

 

[davesparks]

I assume you are referring to fault not over current. The CU will split the circuits into logical divisions, lights, sockets, maybe even into a small fixed heater & sink water heater. They are easily isolated for whatever reason. Having a small 2.5mm ring will keep voltage up as it will effectively be 5mm. OK, e
fectivceky you could have 4 FCUs in a line (skts, lights, Htr, water Htr), that will look amateurish, when a small compact CU enclosure does it nicer.

Where have you got a water heater and heater from?
The OP clearly states 2 twin sockets and an SFCU to feed some lights.

It is a simple installation which doesn't require any over-complication.
[automerge]1591049347[/automerge]

Depends on what mcbs are fitted in both. How to do it.

Electrical Calculation Tool

60898's won't fully discriminate whatever you fit.

 

[John-SJW]

Where have you got a water heater and heater from?

I am generalising.

60898's won't fully discriminate whatever you fit.

You can make matters better.

 

[davesparks]

A prime aim must be to keep all fault and over-current tripping inside the garage while at the design stage. No need to compromise here.

Why 'must' this be a prime aim in a garage installation of two sockets and some lights?