Garage MCB ratings on 6mm cable

[slider45]

Hi.
Just wondering what is a safe MCB rating for the setup I've currently got on my garage.

In house consumer unit is new & currently has a 40A MCB (non-RCD) protecting the 6mm supply to the garage.
In garage, there is a second mini consumer unit (RCD protected) supplying two circuits:
One powers the main ring (on a 32A MCB).
Other supplys a single radial to power my compressor or welder.
(Compressor motor is 17a but specifies being on a 30a circuit.)

At present, the cable between the two CUs is 6mm and approx 6 or 7m long.

My question is, what MCBs would be best in the two CUs?

I'd like a 40a type D in the house and
a 32a type D in the Garage for the radial and
a 32a tybe B in the Garage for the ring circuit

Does this sound feasible on a 6mm cable?

 

[ricicle]

You seem to have (had this) done backwards in the fact that you have ran everything in knowing the circuit requirements, then ask what protection to fit when there already appears to be some in place????

Bit more info/background required please.

 

[Softus]

Just wondering what is a safe MCB rating for the setup I've currently got on my garage.

In house consumer unit is new & currently has a 40A MCB (non-RCD) protecting the 6mm supply to the garage.

What cable, and how installed (i.e. under and/or within what)?

 

[slider45]

You seem to have (had this) done backwards in the fact that you have ran everything in knowing the circuit requirements, then ask what protection to fit when there already appears to be some in place????

Bit more info/background required please.

Yeah I wasn't expecting anything needing beyond a 13a supply really - but it was the size of the compressor I've just picked up which has prompted the question.
On purchase noticed the big sticker on it saying it invalidated the warranty being connected to a supply less than 30a. (having said that, the guy I bought it from had it on a 20a cartridge fuse withut any issues).

Big 3.5Hp motor > dedicated 30A radial > current rethink on whether the 6mm cable is enough.

Changing to a 10mm is not the end of the world, but its just a pain in the back side running it under the floor etc - sepecially as its only a matter of weeks since my spark came and did the installations & checks.

Cable run is pretty simple. Down into the floorspace(0.5m), across the kitchen floor(3m), Up the wall inside a cupboard (2m) & through the wall straight into the garage CU. Not amongst insulation or other cabling anywhere along its route.

Cheers

 

[Adam_151]

Not sure you'd get a D type to fit a domestic consumer unit, though I think hager commerial industrial breakers (Types C and D, 10ka breaking capacity) are stated as being compatable with their domestic consumer units

ELFI would be the bigger problem though, max 0.28 Zs for D40 from a fag packet calc (before 75% rule of thumb), so I'd suggest unless you happen to be on a TNCS and not too far away from the local 11kv transformer, then its a non starter. Type C would make it much easier to comply!

EDIT: I'd also consider a BS1361 fuse to feed the submain, probably best not to feed it through your CU anyway!, was thinking you could drop down to 5s disconnection, but as its outside the equipotential zone (unless your garage is internal) I think it still has to be 0.4 (would have to consult the brown book)

 

[slider45]

Not sure you'd get a D type to fit a domestic consumer unit, though I think hager commerial industrial breakers (Types C and D, 10ka breaking capacity) are stated as being compatable with their domestic consumer units

ELFI would be the bigger problem though, max 0.28 Zs for D40 from a fag packet calc (before 75% rule of thumb), so I'd suggest unless you happen to be on a TNCS and not too far away from the local 11kv transformer, then its a non starter. Type C would make it much easier to comply!

EDIT: I'd also consider a BS1361 fuse to feed the submain, probably best not to feed it through your CU anyway!, was thinking you could drop down to 5s disconnection, but as its outside the equipotential zone (unless your garage is internal) I think it still has to be 0.4 (would have to consult the brown book)

The Type D MCBs are straight out of the Wylex catalogue for domestic CUs.
And in answer to one of your other points - yes, the garage is an integral one within the house.
In reality, there would probably be no draw in excess of 20a - but I need to be sure my 6mm would be sufficiently protected or whether I should use a 32a type D instead.

 

[jamie.aston]

If i were you i would skip the 10mm and go straight to 16mm with the kind of kit you have you may need it in the future.

Jambo

 

[Adam_151]

I'd probably be looking at an appropiatly sized SWA submain fed from a 45A/60A 1361 to feed the garage board , as the garage is internal the cable length should be shortish and it shouldn't cost the earth

a D32 will protect a cable from overload exactly the same as a B32, the thermal element is the same, its protection against indirect contact and thermal withstand (the whole I²t not exceeding K²S² thing) in the event of an earth fault or short circuit that is the issue, and I have my doubts about that 2.5mm² cpc in an earth fault situation... oh it'll hold alright, but the cable might not be fit to use again afterwards , and if you manage to keep your Zs low enough for the D type, then chances are the fault current will be high enough to clear not only the D type, but also the company fuse as well!

 

[slider45]

Thanks for that Adam. Had to look a few things up, but think I follow that. All bar the last bit.

and if you manage to keep your Zs low enough for the D type, then chances are the fault current will be high enough to clear not only the D type, but also the company fuse as well!

Could you explain (in simpleton terms ) how low resistance would prevent an RCD, MCB and company fuse all failing to protect the installation? What sort of true life fault might casuse this kind of catastrophe?

cheers.