Maximum Demand?

[PrenticeBoyofDerry]

I'd be interested know the opinions of you guys and the requirements related to Maximum demand in a Consumer Unit.
At what point do you consider that a CU with a main fuse rated at 100A, is overloaded with breakers and what are the alternatives?

 

[plugwash]

I belive on a domestic most electrcians worry about it if and when the service fuse starts blowing. This VERY rarely happens.

Typically the next step up supply wise would be a basic three-phase supply though larger single phase supplies are occasionally installed.

 

[ericmark]

As plugwash says we don't normally worry.
There are rules on diversity to in the real world we all know many items will not be run together.

There are of course exceptions and I remember where a very large house was fitted with economy 7 with 10 rooms all with a 3Kw heater which of course will all switch on together and then a three phase supply had to be used.

This is a problem as one wants to avoid two phases in a room so the whole house was rewired.

In a large house I wired I used a 150A three phase switched fuse to supply three consumer units with 80A fuses in each way. The consumer units were not all in the same place as one reason to split was volt drop. This arrangement allowed the supply to be up-graded to either 150A or three phase at a later date without any re-wiring.

As far as I know it ran without a problem with 100A fuse main incomer but because it was a converted mill it was a long building and I fitted consumer unit each end and a central one for kitchen.

But a family of 5 living in a two up two down except for heating use about the same as family of 5 in a mansion. Only when the house has multi-families as when converted to flats does the power usage rise. So normally I would not worry.

 

[PrenticeBoyofDerry]

So it would be possible to increase the main fuse rating and tails size to 150A single phase.
I ask this question as I have conflicting opinions where I work and the old grey matter has now become inquisitive.

 

[Spark123]

I wouldn't be happy with that setup unless the CU, main switch etc were also uprated.
As has been said if you look in your On Site Guide at diversity the figure you come out with for a domestic is usually fairly small anyway.
The next natural step from a 100A single phase service is a 100A 3 phase service.

 

[PrenticeBoyofDerry]

Thank guys, what would be considered a potential overload value as I have seen many boards carrying protective devices in excess of 240Amps and If a board carrying that was loaded up to 40% of it's demand it would be close to the rating of the main fuse. So what is acceptable?

 

[oharaf]

If you calculate Maximum demand (following the instructions in OSG) you very quickly get the value below the rating of the Main Fuse, even if it begins to approach this value, for example I calced MD at last job to be 72.8A on a 80A fuse.

The maximum demand details given in the OSG are very conservative and so the value is probably far lower!!!

I am going to look at a 4 storey re-wire on a large Elizabethan property tomorrow, expecting there to be ALOT of extra demand on the supply.

Unless the fuse is 60A or less then I am quite confident that the fuse will be upto the job.

Will let you know more tomorrow

 

[sparkyspike]

I recently wired up a large factory in Sheffield. The two main workshops, each 600msq, comprised twenty welding sets, three overhead cranes, two canteens, three general rings, 25 32A outlets, eight 63A outlets, four industrial extract fans, electric shutters, 18 hibay lights, 6 SONs, hand dryers, etc etc.

Using CT metering, each workshop runs at about 60A - rarely going over 80A. So I'm well under the 100A maximum you're talking about for domestic installations.

 

[plugwash]

So it would be possible to increase the main fuse rating and tails size to 150A single phase.

The trouble with that is:

1: normal CUs are only usually rated for IIRC 100A short term and 80A continuous. So you would need to use something else to distribute the power, e.g. switchfuses or a TP board with a single phase conversion kit.
2: The DNO would almost certainly need to replace the service head and service cable
3: Direct connect meters max out at 100A IIRC so CT metering would be needed.

Large single phase supplies do exist but normally the next step up from a normal domestic supply would be an 80A or 100A three phase supply.

 

[PrenticeBoyofDerry]

Again thanks guys for your valuable input, I have read up a little on the subject but the guidelines given don't really offer a good and full understanding or explanation, so I appreciate the first hand knowledge given by the fine people on this forum.