What size SWA cable for garden workshop?

[Jonny Vee]

I`m planning to build a summerhouse/workshop down the garden. Cable length would be around 45/50mt. Mostly power used 13A and maybe some mig welder 16A

Do I need to get a 3 core cable or 2 core would be fine and Earth can be sorted out locally? What would be the ideal size without spending a fortune?

Thanks

 

[Notch7]

a Play around with a cable size calculator will give you an indication.

for example a 6mm cable 2 core at 50m and 5kw will have an 8.6 volt drop

Im not suggesting that’s what you need, it’s just to give you an example

Im sure the pros will be along to give you proper advice….and they prob will ask for more info: like what is the max total load requirement, what Earth does your house have, what is the garden building made of etc.

do you know what the acceptable Voltage drop is for the welder

Voltage Drop Calculator

www.tlc-direct.co.uk

 

[Jonny Vee]

Guess welder is on 15A fuse. Not planning to have crazy stuff but just some standard workshop tools. 13A heater and a kettle on the summer house.

 

[JohnW2]

.... for example a 6mm cable 2 core at 50m and 5kw will have an 8.6 volt drop ... do you know what the acceptable Voltage drop is for the welder

Given the wide variation in permissible supply voltage, the important questions are not just "what the acceptable Voltage drop is for the welder" but, also, what the OP's 'usual supply voltage' is.

If, as is the case for many of us, his normal supply voltage is 245V, then even a 20V 'voltage drop' would not take the voltage getting to the welder to below the supply voltage which his installation theoretically could have.

Jonny: do you have any idea what voltage the supply to your house usually is?

Kind Regards, John

 

[Jonny Vee]

Hi, I`m not sure what the voltage supply is but on memory is around 240V.

 

[oldbutnotdead]

Go big with the cable- it's amazing how much better a cheap MIG works when it's getting full power.
The rest of it is down to your electrician. Best way to connect at the house end is via Henley blocks (and an OPD of some sort) before the consumer unit. Then RCD & garage cu in the shed for light/power segregation.
You don't have to duct SWA but you might want to chuck a duct in the trench (for data, alarm cables). Trench needs to be quite deep and backfilled to protect the cable (sand round and above the cable then marker tape then hard layer then more tape then soil or whatever).
The cost of the cable will fade into background noise in the whole scheme.

 

[JohnW2]

Hi, I`m not sure what the voltage supply is but on memory is around 240V.

Fair enough. Despite the "230V nominal", I think that a high proportion of people get at least 240V.

There is obviously a theoretical possibility that one's supply voltage could, suddenly and unexpectedly, drop to not much over the 'permitted minimum' (216.2V), but that is extremely unlikely, and probably a bridge to be crossed if one ever meets it. I was just making the point that the 'maximum recommended voltage drop' in a circuit (5% - i.e. 11.5V) (which some people might start talking about!!) is probably a bit 'over-cautious' if one has a usual supply voltage of 240V or above.

Indeed, that 'recommended' 5% (11.5V) 'maximum voltage drop in a circuit' still applies if the supply voltage is right at the lower limit at 216.2V, in which case supplied equipment would only be getting 204.7V - which would require a voltage drop of around 35V (about 15%) from a starting voltage, like yours, of around 240V.

Having said all that, I agree with oldbutnotdead that the most sensible course is to 'go big' for the cable - particularly given that a larger cable does not add appreciably to cost,

Kind Regards, John

 

[scousespark]

The design voltage is 230v and has been for a while.

 

[Jonny Vee]

When you say go big. What size would you recommend?

 

[oldbutnotdead]

That sort of distance I'd go 10mm as a minimum & consider 16mm

 

[Jonny Vee]

That sort of distance I'd go 10mm as a minimum & consider 16mm

2 core or 3 core?

 

[oldbutnotdead]

I'm not an expert on whether to export the earth- if you are exporting earth I'd go 3 core- no problems with corrosion of the steel sheath increasing impedance.

 

[JohnW2]

The design voltage is 230v and has been for a while.

It is, but I don't think that alters the generality and 'common sense' of what I said, since the generality of what I said remains the case if one thinks in terms of the nominal ('design') voltage, rather than the voltage that most people probably get..

As I said, the recommended (but not 'required') maximum VD in a final circuit of 5% (i.e. as I said, 11.5 V on the basis of the nominal/'design' supply voltage) applies whatever the supply voltage which, theoretically could be as low as 216.2V. As I said, a circuit complying with that guideline would, if supply voltage were 216.2V, result in the connected equipment receiving only 204.7 V. If that is deemed OK, then, on the basis of the 'design voltage' of 230V, a VD in the final circuit of up to 25.3V (11% of 'design voltage') ought to be acceptable, oughtn't it?

Kind Regards, John

 

[TimboTwo]

Max volt drop for circuits containing lighting is 3%, 6.9v.

Voltage drop is calculated on design current, length of cable and cable size.

 

[JohnW2]

Max volt drop for circuits containing lighting is 3%, 6.9v.

The maximum 'recommended' voltage drop for lighting is 3%

The only requirement of BS7671 for voltage drop in any circuit is that "the voltage at the terminals [of connected equipment] shall be such as not to impair the safe functioning of that equipment." - and I seriously struggle to think of any way in which any degree of voltage drop could 'impair the safe functioning' of a light (or almost anything else)!

As I often write, even 'recommendations' regarding the degree of voltage drop in a final circuit seem to be really a bit silly, given the wide variation in permitted supply voltage.

Although I do not doubt that some people would be able to cite some very 'voltage-sensitive'(in terms of 'satisfactory functioning', not 'safe functioning') items of equipment (see ** below), I don't think that, in general, voltage drop in a final circuit will hardly ever be an issue. Provided that the impedance of the circuit satisfies the "maximum Zs" criterion (which IS 'a requirement, VD is extremely unlikely to ever be an issue.

[ ** if an item of equipment really is 'very voltage sensitive' then, on the basis of the 'recommendations', it would, if not lighting, have to be able to function satisfactorily down to a voltage of 204.7 volts ]

Kind Regards, John