Two story extension

[djelectric]

I'm wondering if someone could help me out.

I have been asked to give a quote to wire up a proposed double extension on a house for a client who plans to have her sister live in it as if it were a separate dwelling.

The extension will consist of a lounge, kitchen, shared garden room, shared porch, bedroom, and bathroom.

The existing property has a 100A TN-S supply. I have seen the current CU and have calculated max demand (after diversity calcs) at 76.8A.

The client hasn't specified whether or not she wants a separate consumer unit in the extension, but she definitely doesn't want another meter fitted.

I clearly don't have enough left on the supply to fit another separate CU.. I can make some alterations and bring the existing CU demand down a little but still not enough room to add another cooker, 2 rings, 2 lighting..

The house itself isn't huge, approx. 44m2 and will be around 80m2 after the extension. Average amount of sockets/lights.

Would it be acceptable to extend the existing circuits into the extension.. Bearing in mind she will have her own front door with her own key.. Only way to access the CU would be to have a key for the main house..

Would it be a legal requirement in this instance to have direct/easy access to a CU?

The only other option I can think of is to have the DNO upgrade the supply and have a three phase board fitted (expensive), or move the existing CU into the porch where both parties can access it easily..

Not sure which way to go about it, any advice?

 

[jackhammerJIM]

do they have gas ?

 

[djelectric]

do they have gas ?

Yes they do..

 

[jackhammerJIM]

use the gas for heating and cooking . Cant see a problem

 

[jackhammerJIM]

Cant see how you have arrived at 76 amps for demand of existing house ?

 

[davesparks]

76Amps? Is it a 24 bedroom mansion?

 

[djelectric]

Sorry, I have just had a look at Appendix A in the OSG and I've been calculating it wrong. I was using number 9 in Table A2 for the whole CU. I was adding all breakers together apart from highest rated, then multiplying by 0.4 (40%), then adding highest breaker back. I was taught at college to use this method for everything, but have just realised it is only for final circuits! Not good..

 

[davesparks]

Instead of using the imagined nonsense of the osg actually look at what loads are present.
The best way to assess demand is to actually measure the average and max demand over a couple of weeks using a data logging ammeter or simillar, but this isnt always an option for smaller jobs.
Things like boilers and burgular alarms will often have their own 16A mcb yet use less than an amp, a lighting circuit will never draw more than the number of lights connected to it, a 32A ring serving bedrooms will rarely see more than a hairdryer and a few phone chargers.

 

[Guest77]

I'm wondering if someone could help me out.

I have been asked to give a quote to wire up a proposed double extension on a house for a client who plans to have her sister live in it as if it were a separate dwelling.

The extension will consist of a lounge, kitchen, shared garden room, shared porch, bedroom, and bathroom.

The existing property has a 100A TN-S supply. I have seen the current CU and have calculated max demand (after diversity calcs) at 76.8A.

The client hasn't specified whether or not she wants a separate consumer unit in the extension, but she definitely doesn't want another meter fitted.

I clearly don't have enough left on the supply to fit another separate CU.. I can make some alterations and bring the existing CU demand down a little but still not enough room to add another cooker, 2 rings, 2 lighting..

The house itself isn't huge, approx. 44m2 and will be around 80m2 after the extension. Average amount of sockets/lights.

Would it be acceptable to extend the existing circuits into the extension.. Bearing in mind she will have her own front door with her own key.. Only way to access the CU would be to have a key for the main house..

Would it be a legal requirement in this instance to have direct/easy access to a CU?

The only other option I can think of is to have the DNO upgrade the supply and have a three phase board fitted (expensive), or move the existing CU into the porch where both parties can access it easily..

Not sure which way to go about it, any advice?

Hmm worrying, I am a little concerned you may be out of your depth in this case, how much experience have you got? do you know any sparks who could help you? if your struggling with this I think you may struggle with the wiring as well, sorry to sound rude, I don't intend to be so but it is a little worrying.

 

[malcolmsanford]

Sorry, I have just had a look at Appendix A in the OSG and I've been calculating it wrong. I was using number 9 in Table A2 for the whole CU. I was adding all breakers together apart from highest rated, then multiplying by 0.4 (40%), then adding highest breaker back. I was taught at college to use this method for everything, but have just realised it is only for final circuits! Not good..

And it is still a pretty good rule of thumb, and Office/light commercial is .65 and industrial .80.

But it is only a rule of thumb.

 

[djelectric]

Hmm worrying, I am a little concerned you may be out of your depth in this case, how much experience have you got? do you know any sparks who could help you? if your struggling with this I think you may struggle with the wiring as well, sorry to sound rude, I don't intend to be so but it is a little worrying.

No offence taken. I appreciate your concern but I am very confident with the rest of the wiring in the property. I admit I do have limited experience in this particular case of applying two consumer units to one property, but I am more than confident with the rest of the job.

I am a newly "qualified" electrician having just finished college, but have got around 6 years on site experience. Have just never came across this scenario before. I plan to get registered in January once I've got xmas out of the way and won't be doing the job until I'm registered, still need to wait for drawings to be finished, planning permission etc so plenty of time to get myself set up properly. I may even use this job to register with the NICEIC and have them assess it.

But anyway, thanks for your concern. Will start again with the calculations this afternoon as I'm not working far from the job.

 

[djelectric]

Instead of using the imagined nonsense of the osg actually look at what loads are present.
The best way to assess demand is to actually measure the average and max demand over a couple of weeks using a data logging ammeter or simillar, but this isnt always an option for smaller jobs.
Things like boilers and burgular alarms will often have their own 16A mcb yet use less than an amp, a lighting circuit will never draw more than the number of lights connected to it, a 32A ring serving bedrooms will rarely see more than a hairdryer and a few phone chargers.

Thanks, I inderstand this now having had a good look at the OSG. Will give it another go today, probably won't be using a meter but I can count the light fittings / Max ratings of them. Correct cooker calculation, final circuits etc.

Thanks for your help.

 

[Octopus]

And it is still a pretty good rule of thumb, and Office/light commercial is .65 and industrial .80.

But it is only a rule of thumb.

Problem is that with fuseboards having more circuits these days, the maximum demand calculations come up with figures are far too high. I add them up and multiply by 0.25

As for the extension circuits, best to run in separate ones IMHO

 

[Midwest]

If the owners can afford, why don't you opt for one CU populated with RCBO's. Unless the two occupants don't want to cross paths! 2 x electric showers might cause a problem for your max load calculations.

 

[telectrix]

without seeing the job. my first thought would be split the tails, 16mm SWA to 2nd CU, then treat extension as a separate set of finals. run the shower from the C/H boiler, then that's 40A that you can forget. fit a 80A KMF for the 2nd CU, ans a 100A isolatoer before your henleys, so there's single point isolation for the whole property.

 

[amlu]

without seeing the job. my first thought would be split the tails, 16mm SWA to 2nd CU, then treat extension as a separate set of finals. run the shower from the C/H boiler, then that's 40A that you can forget. fit a 80A KMF for the 2nd CU, ans a 100A isolatoer before your henleys, so there's single point isolation for the whole property.

^^^that, its under £200 in materials if you shop around. not a massive sum and will make your job easier. plus people living in the extension wont have to knock on others door if something trips etc...

 

[djelectric]

without seeing the job. my first thought would be split the tails, 16mm SWA to 2nd CU, then treat extension as a separate set of finals. run the shower from the C/H boiler, then that's 40A that you can forget. fit a 80A KMF for the 2nd CU, ans a 100A isolatoer before your henleys, so there's single point isolation for the whole property.

This is what I initially planned to do, but was worried about diversity.. Using common sense and looking around her house I am very confident that she never uses anything near the calculated sums I've come to.. (Re-calculated again today using the OSG, got around 50A which is fine because the extension will have the exact same circuits excluding the immersion.).

The lady lives on her own, has central heating, immersion is never used, there aren't any major power consuming items in the house other than the typical oven and immersion. Existing installation is up to scratch and was tested 2 years ago. No paperwork present though just going by the sticker on the CU.

So Murdoch, you reckon it's ok to just add all CBs and multiply by .25? Even though the OSG states otherwise.. I suppose it is only a guide.. And does leave me with a more realistic number..

I didn't get round to visiting today, have been calculating using pictures of the place.

 

[telectrix]

stick a clamp meter on one of the tails. switch on everything you can. a quid gets you 10 if it's over 40A ( shower excluded)

 

[Octopus]

This is what I initially planned to do, but was worried about diversity.. Using common sense and looking around her house I am very confident that she never uses anything near the calculated sums I've come to.. (Re-calculated again today using the OSG, got around 50A which is fine because the extension will have the exact same circuits excluding the immersion.).

The lady lives on her own, has central heating, immersion is never used, there aren't any major power consuming items in the house other than the typical oven and immersion. Existing installation is up to scratch and was tested 2 years ago. No paperwork present though just going by the sticker on the CU.

So Murdoch, you reckon it's ok to just add all CBs and multiply by .25? Even though the OSG states otherwise.. I suppose it is only a guide.. And does leave me with a more realistic number..

I didn't get round to visiting today, have been calculating using pictures of the place.

I think the OSG thinks that all houses have a maximum of 6 way CU's! - so the guidance is way out of date IMHO.

 

[djelectric]

stick a clamp meter on one of the tails. switch on everything you can. a quid gets you 10 if it's over 40A ( shower excluded)

Sounds like a plan, will give it a go. Thanks