Max demand fun

[timhoward]

This isn't sitting right with me.
So if I rock up at a house with a 60A suppliers fuse, it has a dear old Wylex with
1 RFC 30A (no diversity)
1 Cooker 30A (no data to hand, plate has socket, assuming 21.6A for now)
1 Shower 30A (7.5Kw - full rating)
2 Lighting 5A ( 21 points total * 100w * 0.6 = 6A ish)

Using OSG method its fairly clearly stuffed by RFC and Shower before I start isn't it?!
30+21.6+30+6=87A

The thing is there is only ONE person living there, there isn't a tumble drier or a dishwasher, it's mostly LED lights, and they can hardly be using all four hobs with a roast in the oven while having a shower and using a hair dryer at the same moment. In this particular case the calculation seems way over the top.

I'm erring on C3 for this noting current use is mitigating factor.
Would this situation bother anyone enough to C2 it for max demand, as there are no signs at all of thermal damage or overload issues. I also can't help thinking the OSG method doesn't take account of increasingly efficient appliances, LED lighting etc.

The regs say in section 3 that max demand shall be determined, and the suitability of the supply for the requirements of the installation, including max demand shall be determined by measurement, enquiry or inspection. Does anyone ever use a method other than the OSG way? For example it looks as though measuring demand for a week and looking at peaks and average would be acceptable to BS7671. Can smart meters help here?!
Thanks for thoughts - I do realise that if the house is sold the situation could dramatically change.

 

[Strima]

How often will everything be turned on at the same time?

LED lighting so that would be a couple of amps max if all lights are on.

No significant white goods so in reality the RFC would be 5 amps average.

Once cooker is up to temperature it would cycle through.

Does the occupant spend all day in the shower?

Single occupant.

I would say 38 amps would be more than ample.

The OSG can't give you all the variables especially with more efficient equipment being used, you need to assess each installation on it's own merit and use.

 

[timhoward]

How often will everything be turned on at the same time?

LED lighting so that would be a couple of amps max if all lights are on.

No significant white goods so in reality the RFC would be 5 amps average.

Once cooker is up to temperature it would cycle through.

Does the occupant spend all day in the shower?

Single occupant.

I would say 38 amps would be more than ample.

The OSG can't give you all the variables especially with more efficient equipment being used, you need to assess each installation on it's own merit and use.

Thanks I value someone else sanity checking it and that is exactly what I was thinking.

 

[telectrix]

my dad's house ran on a rewireable 30A main fuse for 60 years. (1926 - 1986). only blew when we added an electric hob and a shower.

 

[UNG]

The regs say in section 3 that max demand shall be determined, and the suitability of the supply for the requirements of the installation, including max demand shall be determined by measurement, enquiry or inspection. Does anyone ever use a method other than the OSG way? For example it looks as though measuring demand for a week and looking at peaks and average would be acceptable to BS7671. Can smart meters help here?!
Thanks for thoughts - I do realise that if the house is sold the situation could dramatically change.

Can you look at the bills for the energy usage over 6 months or so for a daily average to get some indication. Smart meters would not necessarily capture a peak load incident , although I believe the energy supplier can get meter readings every 15 minutes or possibly less if they choose to.
The only real way to get more accurate information would be to install a power logger which would give more detail while identifying the maximum peak load.

 

[GBDamo]

For me you have to calculate your demand for the likely usage of the property, not who is in it.

So a 3 bed, 3 bath all with electric showers, immediately 120+Amps, wouldn't be OK just because old Doris is rattling round in it on her own and can't use them all at once.

 

[Dartlec]

I can't really see many situations where a C2 seems right for maximum demand on a 60A main fuse (individual circuits perhaps).

The length of time in an average house's life where it would draw anything like that are going to be minimal.

The only situation where it might raise any alarm would be with multiple electric showers - where there might be people using them simultaneously (before/after work)...

But then the issues are more likely to be with overloading an RCD causing failure, or the possible damage of lower quality switchgear, than any likely damage to the likely 16mm tails....

Has anyone ever come across meter tails that have actually been damaged by overload, not by loose connections or external factors?

 

[loz2754]

Has anyone ever come across meter tails that have actually been damaged by overload, not by loose connections or external factors?

Nope

 

[GBDamo]

I can't really see many situations where a C2 seems right for maximum demand on a 60A main fuse (individual circuits perhaps).

The length of time in an average house's life where it would draw anything like that are going to be minimal.

The only situation where it might raise any alarm would be with multiple electric showers - where there might be people using them simultaneously (before/after work)...

But then the issues are more likely to be with overloading an RCD causing failure, or the possible damage of lower quality switchgear, than any likely damage to the likely 16mm tails....

Has anyone ever come across meter tails that have actually been damaged by overload, not by loose connections or external factors?

Never seen heat damaged tails other than due to poor termination.

What I have seen a few times is bitumen oozing out of older heads which is indicative of sustained high load, and associated high temperature.

 

[UNG]

Never seen heat damaged tails other than due to poor termination.

What I have seen a few times is bitumen oozing out of older heads which is indicative of sustained high load, and associated high temperature.

So it couldn't possibly be a loose connection in the service head then

 

[GBDamo]

So it couldn't possibly be a loose connection in the service head then

There's a clue in the use of words like 'indicative'

An even bigger clue is the use of the term 'poor termination'

I love the internet ?

 

[timhoward]

I love the internet ?

Pre-coffee posts on a Sunday are always risky....

 

[cliffed]

Max demand is noted not coded…maybe observation