# Garage CU

If he's worked it out, he's worked it out wrong!
32 x 0.0073 x 29.5 = ..................?

6.89 V = 2.99%

That's all very well - for someone elses circuit

Ours is:

Design current - 22A

Volt Drop (3-core SWA) - 0.0064

Allowable volt drop - 5% = 11.5V

So,

22 x 0.0064 x ........ = 11.5

You still need to use the 2 core value unless you are talking about 3 phase, even if the cable is 3 core. The resistance of each conductor will be relatively the same in a 2 core or 3 core cable when used in single phase applications.
Allowable volt drop for a bog standard lighting circuit is now 3%, given we don't know the actual design current though.
I have had to drop 6mm submains down to 25A before owing to volt drop.

You still need to use the 2 core value unless you are talking about 3 phase, even if the cable is 3 core. The resistance of each conductor will be relatively the same in a 2 core or 3 core cable when used in single phase applications.
Allowable volt drop for a bog standard lighting circuit is now 3%, given we don't know the actual design current though.
I have had to drop 6mm submains down to 25A before owing to volt drop.

I stand corrected, Spark123, you're right, volt drop is 0.0073 v/a/m for this cable.
But the design current is 22A max, so the volt drop calc is wrong - it has to be done with the design current.

I should thank you, though, because what you pointed out strengthens my argument for the current carrying capacity.
This increases to 46A for a 6mm SWA cable Ref method D.
Even if you now halve this for running through thermal insulation, the cable is still fine.

Bearing in mind the overload protection of the circuit is provided by the MCBs in the garage CU, effectively restricting the current drawn to 22A, this is well below the current carrying capacity of our cable.

Short circuit protection for the cable is provided by the 32A MCB, and Earth fault protection by the RCD.

So there was never anything dangerous about what I suggested, even if it was run through someones loft

I should thank you, though, because what you pointed out strengthens my argument for the current carrying capacity.
This increases to 46A for a 6mm SWA cable Ref method D.
Even if you now halve this for running through thermal insulation, the cable is still fine.
er..no it's not as it's protected by a 32A breaker.. so needs to have a CCC of at least 32A... which is what I was saying earlier...

Bearing in mind the overload protection of the circuit is provided by the MCBs in the garage CU, effectively restricting the current drawn to 22A, this is well below the current carrying capacity of our cable.

still has to satisfy

Iz>In>Ib

But the design current is 22A max, so the volt drop calc is wrong - it has to be done with the design current.
you present me with a genuine scenario where the current carrying capacity of our 6mm conductor, from house to garage, drops below 32A.
It has to be done with 32A, because that's the figure you gave as being the upper limit.

It has to be done with 32A, because that's the figure you gave as being the upper limit.

It's done with the design current (6A lighting and 16A socket radial) - there's no argument around that but, in this situation, your argument is still irrelevant.
Changing the 16A MCB to a 32A MCB hasn't altered the volt drop of the circuits.

If the garage feed and installation were designed and installed taking volt drop into account, then nothing has changed to alter that - it will still comply.
The load hasn't changed, the length of the SWA feed hasn't changed and the design current hasn't changed - so the volt drop hasn't changed.

And Coljack, the principle is the same as with a spur off a ring.
Your 2.5mm spur is protected by a 32A MCB, oh dear, it doesn't comply with Iz>In>Ib - that's because overload protection is provided by limiting what you are able to plug into 2 socket outlets, 26A - then it complies

that is because you can have ONE spur, not a spur from a spur, there is nothing to stop someone changing the 16A breaker in the garage CU with a 20 Amp or a 32 Amp

that is because you can have ONE spur, not a spur from a spur, there is nothing to stop someone changing the 16A breaker in the garage CU with a 20 Amp or a 32 Amp

Then they will be altering the load and will have to take volt drop etc into account.

We can't install today for what someone might do next year - that's ridiculous.

So if I install a cooker circuit to feed a single oven with gas hob, I have to fit a larger cable coz someone might fit a double range cooker with electric hob in the furure - I don't think so

well YOU may well advise them to just up the breaker capacity, like you have done here, without considering things if a 6mm cable is installed then it would make sense and be good practice for it to be able to safely work with a 32 Amp load, there should also be provision for future expansion on a sub-main circuit like this.

Would you wire a socket circuit in a bedroom in 1mm cable on a 6A breaker due to the fact that there may only be a laptop, tv, lamp and phone charger used in there?

well YOU may well advise them to just up the breaker capacity, like you have done here, without considering things

I did consider things - I didn't just suggest it out the blue - 6mm cable can be protected by a 32 A MCB - WITHOUT A PROBLEM
It's current carrying capacity is well higher than this!
If you put a 20A breaker on 2.5mm and a 32A breaker on 4mm, why can't 6mm be protected by a 32A breaker??

Would you wire a socket circuit in a bedroom in 1mm cable on a 6A breaker due to the fact that there may only be a laptop, tv, lamp and phone charger used in there?

Technically,there's nothing wrong with this so bad example - what regulation does it not comply with?
If they plug anything larger in, they will blow the fuse - not a problem.
You wouldn't do it coz it's not convenient to be restricted like this.

If we followed your line of thinking, why don't we just wire everything in 10mm cable, just in case someone adds something in the future

You can't go blindly changing MCBs for larger ones. The MCB may be of that rating for a reason. Not just cable calcs.

The MCB could be supplying another cable of a smaller size somewhere. Or the cable has been joined somewhere with a smaller cable. Or the electrician only had a 16 amp MCB. Who can say? The possibilities are endless. Never assume anything without checking.

And Coljack, the principle is the same as with a spur off a ring.
Your 2.5mm spur is protected by a 32A MCB, oh dear, it doesn't comply with Iz>In>Ib - that's because overload protection is provided by limiting what you are able to plug into 2 socket outlets, 26A - then it complies

there's a special exception made for rings and spurs from rings..
there isn't however a special exception made for garage feeds..

It's done with the design current (6A lighting and 16A socket radial) - there's no argument around that but, in this situation, your argument is still irrelevant.

Changing the 16A MCB to a 32A MCB hasn't altered the volt drop of the circuits.
Indeed not, but that isn't what you asked.

You asked "you present me with a genuine scenario where the current carrying capacity of our 6mm conductor, from house to garage, drops below 32A."

When the circuit length goes above 29.5m the capacity of the cable drops below 32A.

If the garage feed and installation were designed and installed taking volt drop into account, then nothing has changed to alter that - it will still comply.
But that isn't what you asked.

You asked "you present me with a genuine scenario where the current carrying capacity of our 6mm conductor, from house to garage, drops below 32A."

When the circuit length goes above 29.5m the capacity of the cable drops below 32A.

The load hasn't changed, the length of the SWA feed hasn't changed and the design current hasn't changed - so the volt drop hasn't changed.
No, but that isn't what you asked.

You asked "you present me with a genuine scenario where the current carrying capacity of our 6mm conductor, from house to garage, drops below 32A."

When the circuit length goes above 29.5m the capacity of the cable drops below 32A.

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.

You appear to be getting confused between the design current and the rating of the OPD. Volts drop is based on the design current, NOT the In of the OPD - In must be lower than the rating of the cable and higher than the design current, that is all - In has NO BEARING ON VOLTS DROP WHATSOEVER.

Please don't make such basic errors when sanctimoniously insulting others.

Three times he said it and three times he got it wrong, but I bet we still don't get an apology.
Volt drop and cable length has no bearing on the current carrying capacity of the cable.

The genuine scenario I asked for, ban-all-sheds, is in relation to why I was wrong to recommend upgrading the MCB to 32A for a 3-core 6mm SWA cable.
No-one has come up with a valid reason, backed up by regulation or calculation, as to why it was wrong/dangerous for me to do so.

And Coljack, the principle is the same as with a spur off a ring.
Your 2.5mm spur is protected by a 32A MCB, oh dear, it doesn't comply with Iz>In>Ib - that's because overload protection is provided by limiting what you are able to plug into 2 socket outlets, 26A - then it complies

there's a special exception made for rings and spurs from rings..
there isn't however a special exception made for garage feeds
..

No there isn't, it's just how it works.

If you run a 10 meter long, 2.5mm spur off a 4mm radial(32A mcb) and terminate it in a 13A FCU, this is fine.

Now, is the 2.5mm cable protected by the 32A MCB, or is it protected by the fact that only a max of 13A can be drawn on the cable due to the FCU?

#### DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Replies
29
Views
8K
Replies
18
Views
2K
Replies
8
Views
3K
Replies
11
Views
1K
Replies
3
Views
1K