Realistic cable size for voltage drop

[HappyHippyDad]

My customer currently has a shed at the bottom of his garden. The shed is approx 70m from the house as it is a huge garden.

The shed has electrics going to it. They are from another shed 20m away on a 1.5mm cable. This shed is from a spur off a socket in the house on a 13A FCU. It is 2.5mm.

So basically we have a ring in the house. Spur off to a FCU. 2.5mm for 50m to shed 1 and then 1.5mm 20m to shed 2.

It has been like this for many years and they have plugged all sorts into shed 2 without any problems with voltage drop. The house is a TT and Ze is 15ohms at the house (don't know at the shed)

I have suggested it is wired properly. This would mean 10mm cable purely to meet the VD requirements (for lights), assuming max demand 16A for sockets and 1A for lights.

This just seems extravagant. I realise we cant use the argument 'well it's worked fine for years', but it has. I am saying this purely with regards the VD, not any of the other poor design which requires improvement.

What I'm getting at is...

1. Do you feel the 6.9V VD for lighting is too small?
2. Has anyone ever come across a situation in a domestic premise where VD has been a problem due to an undersized cable being used?

 

[Silly Sausage]

What was the voltage at the house?
Where the shed lights a bit dim?
Does the kettle seem to take ages to boil in the shed?

 

[Wilko]

Regulations aside, the voltage drop is important if it impacts on the loads used, as per Archy. With LED lamps I’d hazard they won’t change their output near as much as incandescent lamps used to. I haven’t tested it, but I reckon it’s likely they’ll work well below Cmin.

I’m just wondering if the light Vd could be a deviation noted on the EIC - no safety impact - works for agreed loading etc.

 

[Bobster]

Regulations aside, the voltage drop is important if it impacts on the loads used, as per Archy. With LED lamps I’d hazard they won’t change their output near as much as incandescent lamps used to. I haven’t tested it, but I reckon it’s likely they’ll work well below Cmin.

I’m just wondering if the light Vd could be a deviation noted on the EIC - no safety impact - works for agreed loading etc.

Most LED drivers will be a small switch mode power supply. I'd hazard a guess that they will accept 110 - 240V AC. So they meet the US market (among others) without any design change.

I'd imagine they aren't using the shed outlets for anything substantial, or certainly not for any length of time.

 

[Andrew Hunter]

Down grade the fuse in the FCU to 3 amps problem solved.
6.09 volts dropped.

 

[Andrew Hunter]

So just to clarify my previous comment' that result was from calculating the full run at 1.5mm2. However when calculating as seperate runs it actually drops to 4.44 volts dropped. Unfortunatly doing same for a 5 amp fuse falls just outside the requirement at 7.4 volts dropped. so if the voltage at the source is a little higher (The norm) then on volt drop alone the installation is ok to a 5 amp supply plenty to run lights and small hand tolls just dont go having tea parties and wrap up in layers in winter (No heating)

 

[Lucien Nunes]

Yes, with the demise of tungsten lighting I think the 3% requirement is now past its use-by date. Voltage drop has to be kept under control for all types of load, a) to ensure the load works correctly and b) to prevent energy wastage by excessive heating in the circuit cables. But with tungsten there was an important 3rd factor, which is that not only does the quality (colour) of light degrade with low volts, but the light output falls off catastrophically even with modest voltage drop, so to make up the correct light output you need much more lamp power. It's a triple whammy - more power loss in cables, lower quality light, and most of all, much lower efficiency in the lamps leading to yet more cable losses when the lamp wattage is increased to compensate.

With electronic LED drivers, as mentioned above there is usually a huge latitude for voltage variation, up to 50% without loss of performance OR significant change in efficiency in the driver itself. Obviously it would be ridiculous to reduce the efficiency of a lighting installation as a whole (counting cable losses) to 50% just to save on copper, especially as we've come so far recently in reducing lighting energy consumption and cables can be smaller than ever before. But 3% VD for what is now a very forgiving load, is not necessary IMO.

 

[static zap]

.....the demise of tungsten lighting...the 3% ... past its use-by date.....
.... with tungsten there was an important 3rd facto.... the light output falls off catastrophically....

May have been a marketing ploy ,to get lamps to run "allotted" Hours .
As consistently under run tungsten lamps do last longer.
(Unsure-about modern Halogen/quartz)
Many street lamp bulbs were actually 260v rated.
(If my memory / eyesight is still accurate)

 

[Risteard]

I think you'll find that the 3%/5% is not actually a requirement, but forms part of an Informative Appendix (and therefore might be seen as best practice). The actual requirements for voltage drop don't specify any specific permissible drop for BS7671 installations.