UK New supply to granny annex

Tommyp100 · May 10, 2024

Hi, need more advise as getting two different views from electricians. Ty

Detached double garage 10m from house, split equally into garden room and garage on 32a supply from cu in house. Looking to convert into 30m2 granny annex with small kitchen (induction hob, single oven), shower room, heating, sockets, lighting etc. No gas so everything electric and as not much space prefer to avoid hot water tank / immersion heaters. So thinking electric underfloor heating, electric shower and electric instant hot water heater for sinks.

House on single phase 100A supply. House hot water via ground source heat pump but no hot water supply to annex.

One electrician said fine to increase 32A supply to 60A to connect to new cu in annex which would be enough to happily service annex and house

Other electrician says to contact the DNO to get the annex on its own dedicated supply. Existing power pole to property is next to annex so don't see massive task but understand would then have two meters etc.

Happy to provide any more info to help guide me.

Thank you.

buzzlightyear · May 10, 2024

Tommyp100 said:
One electrician said fine to increase 32A supply to 60A to connect to new cu in annex which would be enough to happily service annex and house

Other electrician says to contact the DNO to get the annex on its own dedicated supply. Existing power pole to property is next to annex so don't see massive task but understand would then have two meters etc.

What we have here is a load factor.
top spark is wrong.
Second spark either could not work out if it can be done
Calulations is in order in annex.
Get a third spark in and work out your diversity for you and design it.

UNG · May 11, 2024

Tommyp100 said:
Hi, need more advise as getting two different views from electricians. Ty

Detached double garage 10m from house, split equally into garden room and garage on 32a supply from cu in house. Looking to convert into 30m2 granny annex with small kitchen (induction hob, single oven), shower room, heating, sockets, lighting etc. No gas so everything electric and as not much space prefer to avoid hot water tank / immersion heaters. So thinking electric underfloor heating, electric shower and electric instant hot water heater for sinks.

House on single phase 100A supply. House hot water via ground source heat pump but no hot water supply to annex.

One electrician said fine to increase 32A supply to 60A to connect to new cu in annex which would be enough to happily service annex and house

Other electrician says to contact the DNO to get the annex on its own dedicated supply. Existing power pole to property is next to annex so don't see massive task but understand would then have two meters etc.

Happy to provide any more info to help guide me.

Thank you.

So there is a very distinct lack of information in your post

It ia all very well highlighting the electrical requirements of the granny annex but what does the load profile of the existing house look like as option 2 may be the best route to take if the house also has a significant load requirement

Tommyp100 · May 11, 2024

UNG said:
.. But what does the load profile of the existing house look like as option 2 may be the best route to take if the house also has a significant load requirement

Does this help? Looked back over the last year for the day with largest useage (18th Jan) (55.7kWh). Definitely don't consider us huge electric users but not sure that's 100% as little to compare with (for 2023 we used 6665kWh).

(and to compare recent useage - 9th May was 7kWh)

MDJ · May 21, 2024

Tommyp100 said:
Hi, need more advise as getting two different views from electricians. Ty

Detached double garage 10m from house, split equally into garden room and garage on 32a supply from cu in house. Looking to convert into 30m2 granny annex with small kitchen (induction hob, single oven), shower room, heating, sockets, lighting etc. No gas so everything electric and as not much space prefer to avoid hot water tank / immersion heaters. So thinking electric underfloor heating, electric shower and electric instant hot water heater for sinks.

House on single phase 100A supply. House hot water via ground source heat pump but no hot water supply to annex.

One electrician said fine to increase 32A supply to 60A to connect to new cu in annex which would be enough to happily service annex and house

Other electrician says to contact the DNO to get the annex on its own dedicated supply. Existing power pole to property is next to annex so don't see massive task but understand would then have two meters etc.

Happy to provide any more info to help guide me.

Thank you.

even allowing for diversity with the hob and oven, to have an electric shower and a instant hot water boiler for water and then to add the original house to the load a 100 amp single phase supply won't be enough IMHO.

What you need to do is work out the total load, allowing for diversity in the house and annex and go from there, I would suggest the second spark may be closer to being right than the first sparky.

I think an average house will use around 60-65 amps max at peak times (christmas etc) so 100 amps more than enough. But adding another premises to the load may be a bridge too far, get a proper calculation and go from there.

davesparks · May 21, 2024

Both options appear to be viable to me, both will work. Obviously the second option allows for a greater load than the first.

We will all have different opinion based on our own experience and understanding and would do the job differently.

Tommyp100 · May 22, 2024

davesparks said:
Both options appear to be viable to me, both will work. Obviously the second option allows for a greater load than the first.

We will all have different opinion based on our own experience and understanding and would do the job differently.

Thank you, I was hoping that in the background there would be some type of 'sparky bible' that gave a consistent answer to load demands but can see that things are too complex for that and need a case by case calculation.

We've now had a rethink and looking to install an LPG boiler and gas cylinders for the annex's heating, hot water and hob. Will leave just the a single oven, sockets and lighting on an electrical supply. Shared this with the electrician who previously recommended a new supply and he's now going to help us with a loading design.

Tommyp100 · May 22, 2024

MDJ said:
... an average house will use around 60-65 amps max at peak times (christmas etc)..

So interested in how this max demand works out to what we've actually used. 65A = 15.6kW but looking at the above useage profile on our peak day we maxed at 2.93kwh at 17:00 and 2.4kwh at 17:30. Total average over the peak hour on the peak day of 5.33kW. Is it therefore OK to say that we maxed out our demand at 22A?

I've looked through day to day useage and we did have a larger peak on another day (14 Jan) of 6.37kW but overall demand on the day was less.. That would be 26A at peak.

EalingBadger · May 22, 2024

Tommyp100 said:
Thank you, I was hoping that in the background there would be some type of 'sparky bible' that gave a consistent answer to load demands but can see that things are too complex for that and need a case by case calculation. We've now had a rethink and looking to install an LPG boiler and gas cylinders for the annex's heating, hot water and hob. Will leave just the a single oven, sockets and lighting on an electrical supply. Shared this with the electrician who previously recommended a new supply and he's now going to help us with a loading design.

Gas from cylinders is PHENOMENALLY expensive.

I would want to be very certain of the numbers / costs over the medium to long term before committing to this.

Avo Mk8 · May 22, 2024

Tommyp100 said:
I've looked through day to day useage and we did have a larger peak on another day (14 Jan) of 6.37kW but overall demand on the day was less.. That would be 26A at peak.

Your kWh figures are giving you consumption averaged over each hour period, but don't give an indication of total instantaneous current.
To try and illustrate the point I wanted to make (I know you may not have these particular loads), if you had a 7kW induction hob, and say a 9kW shower, and happened to run both at the same time, but for not very long, you might see only 2 or 3 kWh total consumption on your graph, but current might be in the order of 20A for the hob and simultaneously 37A for the shower resulting in getting on for 60A.
It would surely be more accurate to list the kW ratings (or current if easier) of your appliances, e.g cooker, hob, vacuum cleaner, kettle, toaster, heat-pump, standby immersion heater if any etc etc. and then apply diversity in the prescribed way.
Your numbers seem low at the moment, but it could be they are right - I feel it's worth doing the sums to confirm,