Is this acceptable??

[Shpark]

So I understand that this is not ideal before people jump in, but I am stuck with a 2.5mm feed to the garage.

2.5mm feeding garage 27m
1.5mm in garage for lights 30m
4mm in garage feeding sockets 28m

16A mcb in house CCU feeing garage
16A mcb in garage feeding sockets
3A mcb in garage feeding lights

Volt drop to garage = 3.38%
Total volt drop for lights = 3.93%
Total volt drop for sockets = 5.52%

This seem acceptable?

 

[Dustydazzler]

27mtr of 2.5 in 'real world' application is a relatively short run, i rarely bother with calculating VD on such short runs.
Again at just 27mtr you should acceptable Zs as well.

What is your concern ?

 

[Shpark]

27mtr of 2.5 in 'real world' application is a relatively short run, i rarely bother with calculating VD on such short runs.
Again at just 27mtr you should acceptable Zs as well.

What is your concern ?

When would you calculate VD?

I mean I think just that’s my idea of doing it regarding length, current carrying capacity and VD and if that’s a decent suggestion? Or if there’s a better way?

 

[Dustydazzler]

When would you calculate VD?

I mean I think just that’s my idea of doing it regarding length, current carrying capacity and VD and if that’s a decent suggestion? Or if there’s a better way?

Technically you should 'always' do your cable calcs , apply your correction factors and do your volt drop etc etc

But in the real world unless you are doing very long runs or high amp loads I very rarely do all the calculations ( don't tell my scam assessor that thou )

 

[westward10]

It doesn't comply. Your volt drop is calculated from source not individual parts of the circuit so from your calculations, I haven't done it your power circuit has a drop of 5.52% which is above 5% for a power circuit.

 

[SJD]

If you want to have the socket VD comply, consider either a larger cable for the socket circuit in the garage (6mm2) (or an RFC in the 4mm2).

However, if you have two B16 MCBs in series, it is going to be pot luck which one trips first on overload. It might be better to have a design e.g. with a B20 for the distribution circuit from the house, and a B16 in the garage for the sockets. Assuming of course that the cable installation method allows this.

 

[Lister1987]

Are we just going to ignore that thry have 2.5mm FEEDING the garage but 4.0mm feeding tht SOCKETS?

 

[Dustydazzler]

Are we just going to ignore that thry have 2.5mm FEEDING the garage but 4.0mm feeding tht SOCKETS?

Probably just using up what ever cable they had at the time ?

 

[Lister1987]

Probably just using up what ever cable they had at the time ?

So use the 28m of 4mm and not the 29m of 2.5mm? Your calculations would fail from the start for breaching 433.1.1, not meeting coordination

 

[Electro-tech]

I'm with SJD What about the discrimination of protective devices. If a socket trips a 16 A mcb it will more likely be the one in the Mainboard. What are you planning on load-wise in the garage.?

 

[Pretty Mouth]

Your calculations would fail from the start for breaching 433.1.1, not meeting coordination

Why so?

 

[timhoward]

However, if you have two B16 MCBs in series, it is going to be pot luck which one trips first on overload. It might be better to have a design e.g. with a B20 for the distribution circuit from the house, and a B16 in the garage for the sockets. Assuming of course that the cable installation method allows this.

I completely agree with your point, but very few manufacturers state selectivity between MCBs, even Hager won't confirm that a B16 would trip before a B20.

Are we just going to ignore that thry have 2.5mm FEEDING the garage but 4.0mm feeding tht SOCKETS?

The total R1+R2 of distribution and sockets using entirely 2.5mm is about 1.2 ohms. So there (probably) isn't a need for 4.0mm for Zs purposes.
As above there's a selectivity issue between OCPDs, but that is at worst an irritation, it doesn't make it unsafe.
There's probably a "division of installation" issue regarding RCD protection too if we are talking ideals.

I assumed the use of 4mm was to attempt to reduce the reign in volt drop a little. I haven't done the maths.

Your calculations would fail from the start for breaching 433.1.1, not meeting coordination

I can't agree with you there, I can't see any reason that the sub-clauses of 433.1.1 are not met.

 

[pc1966]

Why not just run the 2.5mm / 4mm to sockets, and use a 3A FCU off that for the lights?

It is bizarre to have a smaller sub-main than final, but I presume the sub-main is already in and hard to replace?

 

[timhoward]

Why not just run the 2.5mm / 4mm to sockets, and use a 3A FCU off that for the lights?

It is bizarre to have a smaller sub-main than final, but I presume the sub-main is already in and hard to replace?

We're actually talking about the sockets, we haven't even started on the lighting yet! (Though these days LED lighting makes it a non issue in most cases)

 

[westward10]

I could be wrong but I am assuming this is an existing set-up.

 

[Pretty Mouth]

Why not just run the 2.5mm / 4mm to sockets, and use a 3A FCU off that for the lights?

It is bizarre to have a smaller sub-main than final, but I presume the sub-main is already in and hard to replace?

Any reason why he couldn't omit the FCU, and have everything off the 16A MCB?

 

[timhoward]

I've just run the maths on a few scenario's (my wife and daughters are shopping and I'm sitting in the car waiting for them!)

Sticking a 13A FCU inline for all sockets still leaves it just over 5% so doesn't help.
Going up to 6mm for the sockets circuit leaves it at 4.8%
Changing to a B10 breaker in the garage leaves it at 4.7%, but obviously could affect intended usage.
If it's left alone but 8m of cable can be lost and sockets moved accordingly then it becomes 4.9%.

 

[cliffed]

Yea basically VD is calculated from the origin to the circuit load using the appropriate formula

 

[Pretty Mouth]

I've put the numbers into my VD spreadsheet. I'm getting 1.3V drop for lights, 4.9V for sockets, and 7.6V for distribution. Assuming method C, those can be corrected to 1.1V, 4.4V, and 6.9V respectively, allowing for them never reaching the full 70 deg operating temp.

Just about meets VD for sockets, but not for lighting. I would still install it though, real world it aint going to be a problem.

 

[pc1966]

When would you calculate VD?

The usual answer is if what you have is not one of the "standard circuits" in the on-site guide.

As above, mostly you get an idea from that book of typical length/cable/load combinations and it is only when you get outside of that, like here with a mix of cables, would you look closely.

The regs have 3% for lighting still, though really that is a hang-over from the days of filament lamps that are very sensitive to voltage. With LED lights I personally would not care about applying 5% to both categories, even though it is still not the official thing.