Garage CU

[RF Lighting]

We don't know what reference method it is installed to. That's the point. It might run though 5m of insulation. Would a 32A CB still be ok then?

What is the volts drop for that run of cable? Can it cope with a 32A load, or has it been designed to just supply 16A like it does now?

 

[Steve]

You're doing it again!

What makes you think that you can just swap the 16A CB for a 32A CB, no questions asked?

6mm Armoured cable - Ref method D (direct in ground or in ducting in ground, in and around buildings) - three or four core cable - current carrying capacity 38 Amps

Unless you think he's doing something worse with it

Yeah, maybe its going 100 metres. He hasnt mentioned the distance.

 

[Steve]

ffs rf!

 

[Electrifying]

We don't know what reference method it is installed to. That's the point. It might run though 5m of insulation. Would a 32A CB still be ok then?

What is the volts drop for that run of cable? Can it cope with a 32A load, or has it been designed to just supply 16A like it does now?

Tell you what, instead of arguing backwards and forwards - me saying one thing then you contradicting me and so on - let's do it this way.

If you think that I am wrong in suggesting a 32A MCB for a 6mm SWA three core cable, then you present me with a genuine scenario where the current carrying capacity of our 6mm conductor, from house to garage, drops below 32A.

Could you also provide figures and references to support your argument.

I'm saying it's fine - you prove me wrong

 

[Steve]

volt drop becomes an issue over a certain length.

heat buildup becomes an issue when cables are bunched together.

Both the above would cause you to have to derate cable.

 

[Electrifying]

volt drop becomes an issue over a certain length.

Volt drop does become an issue over length - doesn't affect the current carrying capacity of the cable, though

heat buildup becomes an issue when cables are bunched together.

This is taken into account by the figure in the table - eg. operating temp of cable and reference method

Both the above would cause you to have to derate cable.Not true

This is hardly what I've asked you for.

 

[Steve]

volt drop becomes an issue over a certain length.

Volt drop does become an issue over length - doesn't affect the current carrying capacity of the cable, though

heat buildup becomes an issue when cables are bunched together.

This is taken into account by the figure in the table - eg. operating temp of cable and reference method

Both the above would cause you to have to derate cable.Not true

This is hardly what I've asked you for.

Does it matter what YOU asked me for? The fact is, there may be something the OP hasnt told us about HIS installation, which is what this thread is supposed to be about. We dont KNOW that his garage isnt 100 metres from his house, we dont KNOW that he doesnt have 10 of them with all the cables tie-wrapped together.

Stop assuming and wait for the OP to respond. If he dares now

 

[Electrifying]

Does it matter what YOU asked me for? The fact is, there may be something the OP hasnt told us about HIS installation, which is what this thread is supposed to be about. We dont KNOW that his garage isnt 100 metres from his house ](irrelevant), we dont KNOW that he doesnt have 10 of them with all the cables tie-wrapped together.(ah, the genuine scenario I asked for)

Stop assuming and wait for the OP to respond. If he dares now

Do you know what, you can't give a genuine scenario that will reduce a 6mm cables current carrying capacity to less than 32A - that's why you are taking this attitude.

Nothing else can derate that cable now, unless it was run over the whole distance from house to garage, wrapped in thermal insulation - that would bring it down to about 21A capacity - I suppose you think that's likely?

More likely that it doesn't come into contact with any thermal insulation, so you're knackered - 32A MCB it is.

And please stop going on about the distance of his garage from the house.

He could build his garage in the next town, and run a cable to it, and it wouldn't affect the current carrying capacity of the cable

 

[Steve]

How about instead of debating this, we wait for the OP to come back with HIS REAL SCENARIO so that we can advise him.

Whilst the CCC of the cable remains the same over long distance, it MUST be derated for the distance doe to VOLT DROP. Stop picking holes.

 

[WOH64Dek]

Thanks for all your replys

The 3 core wires of the swa are connected to the rcd protected side of the cu, but theres no earth wire connected to the banjo on the gland fitting. The cable is burried and I would say the cable is no more than 20ft in lenght. Both the house and garage cu's are virtually in line with each other.
Thanks again for all the posts, hope I've not caused to much trouble.

 

[ban-all-sheds]

volt drop becomes an issue over a certain length.

Volt drop does become an issue over length - doesn't affect the current carrying capacity of the cable, though

heat buildup becomes an issue when cables are bunched together.

This is taken into account by the figure in the table - eg. operating temp of cable and reference method

Both the above would cause you to have to derate cable.Not true

This is hardly what I've asked you for.

I don't suppose he asked you to insert your words into a "quote" of his.

Do you know what, you can't give a genuine scenario that will reduce a 6mm cables current carrying capacity to less than 32A - that's why you are taking this attitude.

There is a genuine scenario - that of the cable being more than 29.5m.

And please stop going on about the distance of his garage from the house.

He could build his garage in the next town, and run a cable to it, and it wouldn't affect the current carrying capacity of the cable

On the reasonable premise that you'd want it to comply with the wiring regulations, try reading 525.

 

[ColJack]

Do you know what, you can't give a genuine scenario that will reduce a 6mm cables current carrying capacity to less than 32A - that's why you are taking this attitude.

he lives in a bulgalow and will run the cable up to the loft from the high level CU, across the top of the insulation and out through the soffit, clipped down the wall behind a drain pipe and underground from there to the garage..
he then decides to add more insulation to the loft (since he noticed while up there runing the cable that it was a bit thin.. ), adding another 200mm laid crossways to the 100mm he already has between the rafters

the cable is now surrounded by insulation for lets say at least 500mm, which derates it to 0.5x it's capacity, bringing it down to 26.5A..
there you go..


or if you prefer a more likely scenario, cable run the same way but not covered in the loft, on top of the recomended 270mm of insulation..
this means it has to pass through 270mm of insulation to enter the loft.. so that derates it to at least 0.63 of it's capacity.
the figure given is for 200mm of insulation, so probably closer to 0.57 so that's 30.2A

 

[Electrifying]

There is a genuine scenario - that of the cable being more than 29.5m.

He could build his garage in the next town, and run a cable to it, and it wouldn't affect the current carrying capacity of the cable

On the reasonable premise that you'd want it to comply with the wiring regulations, try reading 525.

Both of these comments relate to volt drop and so are irrelevant to the discussion about current carrying capacity.

If you had to replace the cable with a 10mm because of the volt drop, that's a different issue, nothing to do with the MCB.

The design current of the circuit, governed by the MCBs in the garage CU, hasn't changed, so neither has the volt drop - so it's not an issue.
The only issue is cable routing - and this is a bit of a stretch, come on!:-

he lives in a bulgalow and will run the cable up to the loft from the high level CU, across the top of the insulation and out through the soffit, clipped down the wall behind a drain pipe and underground from there to the garage..
he then decides to add more insulation to the loft (since he noticed while up there runing the cable that it was a bit thin.. ), adding another 200mm laid crossways to the 100mm he already has between the rafters

the cable is now surrounded by insulation for lets say at least 500mm, which derates it to 0.5x it's capacity, bringing it down to 26.5A..
there you go..


or if you prefer a more likely scenario, cable run the same way but not covered in the loft, on top of the recomended 270mm of insulation..
this means it has to pass through 270mm of insulation to enter the loft.. so that derates it to at least 0.63 of it's capacity.
the figure given is for 200mm of insulation, so probably closer to 0.57 so that's 30.2A

You're running a cable from the ground floor, up into the loft to pass through insulation, then back down again - I said 'genuine'.

As we're quoting regulations, read the first line of 523.7

 

[ColJack]

like I said.. in a bungalow.. I never said a 2 storey dwelling.. and if it happens to have a concrete floor and the CU at a high level then that's the option to run it..
it is also the way I would run mine, living as I do in a first floor flat with a garden, the only options are straight through the wall ( which would leave a loop outside ) or the method I stated, up into the loft, along to the corner where the drainpipe is, cliped to the wall next to / behind the drainpipe and then under the paving slabs to the shed..

 

[ban-all-sheds]

Both of these comments relate to volt drop and so are irrelevant to the discussion about current carrying capacity.

No they are not - you may not put 32A down a 6mm² cable more than 29.5m long.

If you had to replace the cable with a 10mm because of the volt drop, that's a different issue, nothing to do with the MCB.

No, but it is to do with the capacity of the cable.