is this too much for 100a single phase!

[unclebubble]

I'm working on this guy's house in London and we are coming to the end of the job and my foreman thinks we have too many circuits for a 100amp supply!
We have a CU containing fifteen- 6 amp MCB's(all lights with no more than 10 on circuit)
The above is on one consumer unit the rest here is on another-
five-32 amp MCB's(all ring Mains)
three-20 amp MCB's(2 air con units,1 immersion)
one 16 amp MCB(1 socket)

The Foreman isn't completely clued up on Diversity and we have designed this installation ourselves so we want it to be right for building control!
Any help would really be appreciated to work this out!

 

[ricicle]

It's not the no. of circuits you need to worry about so much, it's the actual load!

Is this a normal domestic dwelling?
Why was it deemed necessary for 15 lighting circuits/what is the total load of the lighting?
Is the immersion the primary source for hot water?
Any electric cooking appliances?
Any electric showers?
What is the actual loading of the A/C units?

 

[Spark123]

Have you tried working out the loading using the charts in the On Site Guide? I suspect it may end up being over the 100A, using the diverstiy guide in the OSG gives 83A on Ring final circuits alone!

 

[Paul Barker]

I had a similar situation with diversity and after literally half a day on the phone trying to get the distributor to put a 100 amp fuse in place of a 60 amp one I came up with a minor consession that they might put an 80 amp one in.

So I rang my certifying body and they pointed out that in bs7671 (not the onsite guide) it does state in small print in the section on diversity that the "electrician" can decide to exceed the diversity. He said as long as it is a normal dwelling and doesn't have 15 showers, don't bother trying to get sense out of the distributor.

Incidentally I was pushed from pillar to post all afternoon being given yet another premium rate number to phone with the clainm this time I would definately get to talk to the right person, but always ended up at a numpty who said I "you have come through to the (useless moron)department". I kept saying I want to talk to an engineer not a telephone worker to which they would retort "are you one of our engineers?"

*******s to them! If their fuse blows it's their lookout!

 

[Stoday]

Look at it from the DNO's point of view.

At the Grid Supply Point the After Diversity MD of a typical house is 1 kW. Lower down the network at the local distribution substation, the ADMD is 2 kW. Now to the service of a house - 5 kW.

We next come to the difference between the electrician's measure of MD and the DNO's. The network MD is defined as ‘twice the number of kWh in a half hour’, i.e. the demand is averaged over half an hour. That’s fine for the network because it relates to the thermal time constants of the network components. But it’s no good for the cutout use which will react to a much shorter overload.

So you would want to allow at for least 15kW at the CU whatever the size of the dwelling.

 

[unclebubble]

To give more detail on what I wrote-
Four of the 6 amp circuits are -one is an alarm spur(normal house alarm supply)Two are for velux window supplies can't remember the load not a lot though!The other is a supply for a digital reading on a shower!
I had to wire a lot of these back to the CU direct as these were extras after everything had been decorated and it was the easiest route back!There are almost 90 downlts(mains voltage)(50w)and there is probably around 30 low voltage (10w) pelmet lts so that is why we put in so many 6 amp circuits!

I forgot one of the 32 amps is for a cooker(6mm cable)
then the other four are for normal rings on each floor and kitchen separate!
The immersion for the heating is only a back up(central heating)
The two air con units don't even say what KW rate they are any where all we were told was to give them a 4mm supply each!
This is a domestic dwelling - a town house its very big though and this guy has just wanted so much doing to it.
I'm guessing I could probably double up some circuits in the board ie velux supplies alarm etc.
Still not totally sure how to work on sit guide diversity as not sure if I'm calculating right(I've never been explained it)
It does seem excessive to put a 3 phase supply in to this guys house(expensive to say the least)as it would only use a fraction of another phase???
I appreciate you all commenting on my post any more insight is so helpful!Cheers!!!

 

[Spark123]

If you read the on site guide it is pretty self explanatory. The On Site Guide however is only a basic way of calculating the max demand, if you wish to calculate it another way it is up to you.
The KW of the air con needs to be made available to you as the designer.

 

[gazza2]

With regard to the lights, what happened to part L and the use of low energy bulbs ( especially to replace loads of 50W halogens) you can get 2W gu10 replacements and that means that 90 come in at < 200W

 

[sparkybird]

I agree with Paul B - the DNO aren't that bothered - I had a similar situation when client wanted new 45A shower on 60A company head (in 3 storey house with the usual circuits). DNO turned up, grunted and said - just get on with it.

Actually when I looked at the fuse curves as the back of the Regs book, I realised that it would take over an hour of 60A draw to blow the main fuse, so unless the guy has really long showers, it'll be OK.

SB

 

[Softus]

A whole lot words of grammar with appalling.

Did you go on a course to learn how to write so confusingly?

 

[JohnD]

You've been taking those Curmedgeon pills again, haven't you?

 

[Softus]

Nope - I just noticed that Stoday had been let out of the asylum, and I've lost the number for the White Coated Men.

 

[JohnD]

We next come to the difference between the electrician's measure of MD and the DNO's. The network MD is defined as ‘twice the number of kWh in a half hour’, i.e. the demand is averaged over half an hour. That’s fine for the network because it relates to the thermal time constants of the network components. But it’s no good for the cutout use which will react to a much shorter overload.


etc

Seems sane enough to me.

 

[ricicle]

We next come to the difference between the electrician's measure of MD and the DNO's. The network MD is defined as ‘twice the number of kWh in a half hour’, i.e. the demand is averaged over half an hour. That’s fine for the network because it relates to the thermal time constants of the network components. But it’s no good for the cutout use which will react to a much shorter overload.


etc

Seems sane enough to me.

No, I didn't see too much wrong with it either (Maybe a tad too deep for a DIY forum though )