what size mains cable do we need?

[batmanbrooks]

Hello we have recently moved into a farm house and apparantly the main swa cable feeding the main consumer unit is too small. It comes from the main suppliers 100A fuse and meter which is located in the top field then runs underground in the field for about 65m then into the house. I think at the moment it is only about 10mm swa. What size should it be??? The consumer unit has 5 lighting cuircuits, 4 socket circuits, then a circuit which feeds another board which has 2 socket circuits and 2 lighting cuircuits. Thank you

 

[Andy™]

400mm

 

[ProDave]

The only thing for certain, is that cable is way too small. I'll bet the lights dim when you turn the kettle on.

As well as an electrician, you will need a digger and driver to dig a trench for the new cable.

In the mean time, don't even think of adding extra circuits to the installation.

Perhaps a PIR (periodic inspection report) may not be a bad idea, I'm willing to bet with an installation like that (obviously old) that may be only 1 of many faults it may have.

Unless there is any need for the supply and meter to be so far away, I would investigate the possibility of getting the DNO to re route the supply directly into the house. They will come and give you a quote for that if you ask them. (same usually applies, they will expect you to dig the trench)

 

[Doc Hudson]

Hello we have recently moved into a farm house and apparently the main swa cable feeding the main consumer unit is too small. It comes from the main suppliers 100A fuse and meter which is located in the top field then runs underground in the field for about 65m then into the house. I think at the moment it is only about 10mm swa. What size should it be??? The consumer unit has 5 lighting cuircuits, 4 socket circuits, then a circuit which feeds another board which has 2 socket circuits and 2 lighting cuircuits. Thank you

Welcome to the forum, batmanbrooks, I assume you are not an electrician judging by your question. The cable should be big enough for the expected load, which at this point we have not been told. Who told you the cable was way to small, didn't they suggest what size is needed. Without actually seeing the site and doing a proper calculation of maximum demand I doubt any practical advice can be got from the internet. Saying you have five circuits etc does not give any actual connected load ratings. You are best employing a qualified electrician to come and do a full site survey and discuss with you exactly what you would like to have connected.

Doc H.

 

[NozSpark]

Welcome to the forum, batmanbrooks, I assume you are not an electrician judging by your question. The cable should be big enough for the expected load, which at this point we have not been told. Who told you the cable was way to small, didn't they suggest what size is needed. Without actually seeing the site and doing a proper calculation of maximum demand I doubt any practical advice can be got from the internet. Saying you have five circuits etc does not give any actual connected load ratings. You are best employing a qualified electrician to come and do a full site survey and discuss with you exactly what you would like to have connected. Doc H.

Maybe if you put your location, or have a search through the "find a contractor" part of the site one of the members might be able to help you!

 

[batmanbrooks]

400mm? isnt that huge?? that wont even fit in the main switch terminals on the consumer unit will it. The person that pointed it out to me said that its normally 25mm cable that comes into the consumer unit. He sugested 25mm 3 core armoured cable. Also i have the machinery to dig so that isnt a problem. Ill do all the digging my self before the electrician comes to save me money. Thank you

 

[Sidewinder]

Hello batmanbrooks,

Welcome to the forum.

If you are intent on doing this yourself and not complying with building regulations, then start with a 25mm sq or 35mm sq SWA use 3 core, and then calculate toe volt drop out for the length based on the expected load, don't forget to allow for the volt drop in the final circuits so that your maximum volt drop at connected accessories lies within the requirements of BS7671 to see if these are large enough, if not you will need to keep upping the size of your sub-main until you reach the required levels of volt drop.

Once you have done that you need to verify the earth fault loop impedance of the sub main is suitable for the protective device at the origin of the circuit, this is again by calculation.

Once installed the earth fault loop impedance must be verified by live measurement with a suitable test instrument to confirm your calculations and for entry onto the electrical installation certificate that must be completed to comply with BS7671.

Please note well also the other responses to your question.

 

[batty]

10mm armoured its a farm what else would be expected.

 

[Andy™]

10mm armoured its a farm what else would be expected.

10mm T&E, washing line style?

 

[Phoenix]

bare overheads hanging low enough that they have to be lifted up with a wooden stick to let tractor through?

 

[batmanbrooks]

why would it be 10mm just cause its a farm? shouldnt i have something like 35mm armoured

 

[Manator]

why would it be 10mm just cause its a farm? shouldnt i have something like 35mm armoured

To be honest its a common problem, the cable in question could have been laid years ago, when the maximum demand was around 40 amps or at most 60 amps.

Any good electrician would provide you with some calculations to show why the cable is good, or bad. For instance this is one I did for a job recently.

Cable ID / REF number : DB3 Supply Cable

Supply voltage = 230 Volts

Ib - design current = 60 Amps

Protective device type = BS1361

In - protective device rating = 60 Amps

Length of run of cable = 26 metres

Power factor = 1

Cable Type : Twin & Earth

Installation Method : Single-core or multicore cable direct in masonry having a thermal resistivity not greater than 2 K.m/W - without added mechanical protection.

method C

Ambient temp = 30

 

[batty]

why would it be 10mm just cause its a farm? shouldnt i have something like 35mm armoured

Twas a joke i have done work on farms and generally find farm electrics are the poorest to be found. I think as been said you need a realiable electrician to calculate cable size needed and how it needs to be installed by yourself.

 

[Andy™]

why would it be 10mm just cause its a farm? shouldnt i have something like 35mm armoured

may even need to be more than that. for a start, if existing circuits in the property are already at or near max volt drop, then a larger cable will be needed. same with earthing

your not going to get far here with the almost nom-existant information you have given

 

[batmanbrooks]

At a rough guess what size could i put in. Anything has to be better than what is in at the moment.

 

[Andy™]

At a rough guess what size could i put in. Anything has to be better than what is in at the moment.

like i said before, 400mm

 

[SLIPSHOD & SLAPDASH]

Give us some basic information

Is any heating electric

Is any water heating electric (inc electric showers)

How many bedrooms

Are there any pumps for either water, sewerage etc

Are any outbuildings, or farm machinery connected to the house supply

Are there any outside floodlights connected to the house supply

Are there any cables going to other parts of the farm from the meter position

 

[Doc Hudson]

At a rough guess what size could i put in. Anything has to be better than what is in at the moment.

Cable size is dependent upon load and the length of the supply cable. Without knowing this no one can give any realistic ideas. Asking for a rough guess is honestly as sensible as asking us to guess how much paint and wallpaper you need to decorate your house. If you cannot supply additional information about the prospective maximum demand of the installation then there is very little anyone can do to help you. There is no standard off-the-shelf wiring guide for farms or any other property. Every installation needs individual design considerations.

Doc H.

 

[Badger]

[quote name='Andy

 

[Andy™]

nah id go 500mm2 that will give you a much better earth path on the pme supply :run :slap just dont tell steps :slap

what about an un-insulated earth running along side the SWA?