Is this acceptable??

Shpark · Jun 18, 2022

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?

[ElectriciansForums.net] Is this acceptable??

Pretty Mouth · Jun 18, 2022

pc1966 said:
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 · Jun 18, 2022

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 · Jun 18, 2022

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

Pretty Mouth · Jun 18, 2022

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 · Jun 18, 2022

Shpark said:
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.

pc1966 · Jun 18, 2022

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

Probably none, as the MCB provides fault protection and the loads are unlikely to be capable of overloading the cable. But I still like to have protection for lights that is below the rating of the cable and/or switches, even though it is probably unnecessary.

I guess it stems from seeing those now-long-banned adaptors in the 70s that would allow you to plug in a socket to a bayonet lamp fitting!

Shpark · Jun 18, 2022

So the way that I’ve look at is as such. I cannot change the 2.5mm feed to the garage, it’s set, buried. I was going to leave a 16A feeding the garage from the house so that it does not draw more than this. The lights are obviously set at drawing a max 1.47A, the sockets could technically draw 16A, but I doubt ever will. Thinking like using a vacuum for a car, trickle charger for battery and maybe the occasional grinder/drill.
I’ve calculated VD using 70C tables. 3.93% for lights. 5.52% for power, but only using a 4mm. A 2.5 for power would make total VD 6.85% which is obviously too high. But it seems that people this both power and lights are still too high, but why? It doesn’t exceed 3% and 5% respectively, just decimal places, is this not acceptable?

westward10 · Jun 18, 2022

Don't worry about it the volt drop is based on full load which is not likely to happen.

pc1966 · Jun 18, 2022

Realistically, what load(s) are going to be used?

If it is a ghetto blaster and occasionally a 2kW heater then check the VD for that 9A load!

But do check the end of circuit Zs is good enough for the 16B MCB, that is a bit more important for safety.

Electro-tech · Jun 19, 2022

pc1966 said:
Realistically, what load(s) are going to be used?

If it is a ghetto blaster and occasionally a 2kW heater then check the VD for that 9A load!

But do check the end of circuit Zs is good enough for the 16B MCB, that is a bit more important for safety.

I can see in winter something warming up without a Zs reading how will you know the 16 amp mcb will trip. I do hope you have the means to test.