Supply cable size

[Matthewben]

Hi I am not electrically qualified but would like some advice please. I am currently having my house renovated. The supply from western power goes into a main switch that is in an annexe that we have near to the main property. The builders have installed a new armoured 3 core 25mm cable that goes from the main switch to the new consumer unit, approx 25m in length. The guy that is doing the electrics is a mate of the builder and although we have asked to see qualifications, he has yet to produce them. The missus overheard a conversation between them which suggested that the cable that they have installed was some that they had lying around and she thinks that it may not be adequate for the job and so she wants me to check. The house has 2 ring circuits at present, plus 1 for the kitchen. 2 lighting circuits, a double oven, 2 boilers, 3 showers and we are having an external circuit fitted. Sorry for the long post but the missus wants me to make sure all ok and as there are builders on site every day I can't get someone in to take a look. Please let me know if you require any more info.
Thanks

 

[GMES]

Hi Matt and welcome to the forum, I have placed your thread in the general section until a member of Admin can move it to diy.

 

[davesparks]

Why does builders being on site stop you getting someone in to look at it? If it's your house and you are employing the builders then you call the shots, not them.

If it is indeed 25mm 3 core steel wire armoured cable then at 25mtrs long it should be more than adequate for a normal house.

What does need to be checked is how the cable has been installed. Is it buried in a duct or buried loose in the ground? Are there any signs of damage to the outside sheath of the cable?
If you can post some pictures of the cable we might be able to give more guidance


I think, for once, this one doesn't need to be in the DIY section!

 

[Leesparkykent]

25mm should be adequate for the job.

 

[GMES]

Davesparks raises a good point why can't you have another electrician come and look or price the job, remember you are in charge not him and also if you are worried about the builder kicking off just get a sparky to call round after 3pm most builders have downed tools by then.
@ Dave I have left a message for Admin to leave it in the general forum and not move it to diy.

 

[Octopus]

Maybe you should ask the builder/ sparky for the EIC and confirmation that your part p notice has been dealt with before paying for the electrics?

 

[Matthewben]

Thanks, I'll ask and see what they say. With not knowing all the ins and outs then I didn't want to sound like I was telling the guys how to do there job. I had calculated all the values of current for each circuit from calcs I found on the internet and looked up the current rating of 25mm cable and it didn't go. How's that work then.

 

[Marvo]

I'm guessing you overlooked diversity in your load calculations, without seeing them that would be my best guess. Diversity

 

[Octopus]

I'm guessing you overlooked diversity in your load calculations, without seeing them that would be my best guess. Diversity

Which is what 99.9% of DIYers do

 

[davesparks]

Thanks, I'll ask and see what they say. With not knowing all the ins and outs then I didn't want to sound like I was telling the guys how to do there job. I had calculated all the values of current for each circuit from calcs I found on the internet and looked up the current rating of 25mm cable and it didn't go. How's that work then.

The average family house will normally present a load of around 20Amps at most with occasional peaks around 40/50Amps if there's an electric shower.

The incoming supply from the electricity board will be a maximum of 100Amps. The electrician will probably choose to protect this new 25mm cable with an 80A fuse.

Assuming the cable is in the ground for most of its run and not surrounded by insulating material then 25mm should be quite happy.

 

[Matthewben]

So how does diversity work then, I know I = p/v so for my 3 showers, if all were going at once isn't that around 120 amps. Plus we have all the other circuits as well.

 

[telectrix]

So how does diversity work then, I know I = p/v so for my 3 showers, if all were going at once isn't that around 120 amps. Plus we have all the other circuits as well.

rare to see all 3 showers on at once. if they ever are, it's for short periods. cable ratings are for continuous use, so they and their fuse/MCB will safely withstand up to a 50% overload for short periods.

 

[loz]

So how does diversity work then, I know I = p/v so for my 3 showers, if all were going at once isn't that around 120 amps. Plus we have all the other circuits as well.

It doesn't quite work like that, most houses only have a max of 100amp supply, so if we looked at it as there may be 3 showers on at once everybody would have 3 phase supplies and bigger electricity bills, diversity is taken on assumptions that not all 3 showers will be running at once while the cooker and hob are all on with 6 2kw heaters running and a kettle boiling.

Regardless of this your supply will be a max of 100a which a 25mm cable can take if not buried in insulation, so I wouldn't worry if I was you

 

[Matthewben]

Thanks for that, so the cable is fine, I'll check there is an 80amp fuse at the start of the cable. Is there anything I should be looking for at the consumer unit end or just the armoured cable straight into the box. So I assume you guys do the diversity thing across all the circuits in the box to bring the current value right down, right.

 

[Andy78]

Thanks for that, so the cable is fine, I'll check there is an 80amp fuse at the start of the cable. Is there anything I should be looking for at the consumer unit end or just the armoured cable straight into the box. So I assume you guys do the diversity thing across all the circuits in the box to bring the current value right down, right.

Diversity can be assessed in many different ways and each installation needs to be individually assessed by the electrician designing the job. A lot of it is based on experience and hard to impart to someone asking for advice on a forum.
you really should be asking the electrician doing the work to explain all this to you. If he is a professional he would have no bones about explaining things in as much or little detail as you require. If you don't have any confidence in him, then as previously mentioned it may be worth paying another electrician for an hour of his time for a second opinion before the first guy carries on.