Diversity calcs with economy 7

[pcboffinuk]

Hi,

Can anyone advise on how they would work out total demand including diversity in a situation with economy 7 storage heaters?

Working out the maximum demand on the main circuits gives me about 63A, which works out ok (the main house fuse is a 60A - the main CU has 100A isolation switch)

However, there are 3 storage heaters and a 3kw immersion heater on a separate economy 7 CU, all on 20A rewireable fuses.

The On site guide doesn't suggest what to do in this situation. Taking it directly would give maybe another 40A, but since these are only running overnight, when most of the other loads are off, is it reasonable to take the view that they can be ignored, or at least partially discounted?

Any thoughts on what is best practise in this situation would be appreciated.

Regards

Gavin

 

[breezer]

What rating (KW) are the storage rads?

 

[pcboffinuk]

Another early bird I see...

I was hoping noone would ask

They are rather ancient but I will be looking on Sunday. Although they are each on 20A fuses I doubt they are any more than 2kw.

Gavin

 

[breezer]

so as you said

there are 3 storage heaters and a 3kw immersion heater on a separate economy 7 CU, all on 20A rewireable fuses.

and

. Although they are each on 20A fuses I doubt they are any more than 2kw.

That is 3 x 2 =6 + 3 = 9kw about 39A

which makes

Working out the maximum demand on the main circuits gives me about 63A,

way out.

no idea how you arrived at 63A

 

[pcboffinuk]

no idea how you arrived at 63A

Sorry probably not being too clear at this time of the morning - the 63A is the maximum demand calculated for the main CU with the sockets, lights etc on.

The storage heaters are on a totally different CU, split using henley blocks.

For heating, the on site guide advises the first 10A plus 50% of the remaining A, but not sure if that is the fused rating or the actual rating for the appliance.

And besides, since the storage heaters are drawing load overnight, when the socket/light load is at its lowest, I just wonder if it would be considered good practise to take that into account and therefore reduce the anticipated maximum load.

To be honest, I only need it for the certificate, but since this job will be one of the ones used for a NICEIC assessment I would like to know what the accepted practise is.

Gavin

 

[Lectrician]

Whats your calcs for the 63amps??

Doubtful you will ever take this current for sustained periods.

Are you going to be showering and cooking between 00:00 and 07:00

20amp fuses is not really right unless the storage rads / immersions are fed via FCU's? The immersion should NOT be on an FCU and should have a DP switch instead and a 15amp fuse.

 

[Taylortwocities]

The diversity thing is a GUIDE in the On-Site GUIDE. Its up to you to determine if the GUIDE fits your application.
Per the above, if you have a split Henley block into the meter you need to think about the max peak load depends on what else is running at the time.
Those heaters will all come on at max load at the same time so I would not allow any diversity on the storage heaters. The load will reduce as the stats start clicking in and out but at when the time clock goes on its 100% load.

TTC

 

[Lectrician]

The diversity thing is a GUIDE in the On-Site GUIDE. Its up to you to determine if the GUIDE fits your application.
Per the above, if you have a split Henley block into the meter you need to think about the max peak load depends on what else is running at the time.
Those heaters will all come on at max load at the same time so I would not allow any diversity on the storage heaters. The load will reduce as the stats start clicking in and out but at when the time clock goes on its 100% load.

TTC

STORAGE rads can be allowed NO diversity - The rest of the install can.

 

[Taylortwocities]

Think thats what I said......
So in the case of the Henley Block as the supply is split you must add the full load of the off-peak (got any off peak immersions too?) and add this to the diversity calc of the main installation. Check your meter tails, earthing and the REC's fuse is big enough for the total load...

TTC

 

[plugwash]

as has already been said the diversity guidelines in the regs are just that, guidelines.

if people really followed those guidelines on domestic installations most houses would need a 3 phase intake!

yes the E7 will draw a lot of current initially as all the heaters are cold, but it will generally do it at a time when other load is likely to be low so overloads are likely to be rare, short-lived and small if indeed they happen at all. Neither cables nor fuses suddenly blow at small overloads it just shortens thier lifetime a bit.

if the service fuse blows call the DNO and tell them, when they come out to replace it they will almost certainly upgrade it to 80A or 100A. If not then i really wouldnt worry about it.