Domestic 3 phase advice

[Aok1]

I would appreciate some advice on the following as i am not a full time spark and most of my electrical work is normally limited to minor works.

I was at a clients house in the week and while talking she mentioned that she needed to get a 3 phase supply installed at a cost of £6.5K + for her under floor heating which apparently draws 80A. The spark who done the re-wire in January thinks the total load is going to be around 135A which includes everything in the house.

He has said all the db's need changing to 3 phase.

There house is large but there are no electric showers, cooker is gas so the only items are really pull any current are on the ring and kitchen appliances.

I had a look quick look at the current system which has a 60A fuse on the incoming and then 2 x 12 way boards, 1 of which is for the underfloor 4 x 20a mcbs - Hall, living room, kitchen and small bathroom.

I think the figure of 135A is way out and no diversity has been allowed and there would be no need to change the db's either.

Would i be correct in thinking that no diversity is allowed for the underfloor heating but more common sense in that she has said that there would only ever be a maximum off 2 x areas on and never all 4.

I am due back on Friday to finish some AV and security stuff so i was thinking of advising possibly getting the 60A upgraded to 80A or 100A and also clamp testing the supply with the 2 x areas of the underfloor on and some other items to get some sort of idea on the actual load.

Thanks

 

[davesparks]

So is this an existing installation with all of the loads currently connected and working?

If it is then I'd suggest that a 60A supply is enough based in it currently working fine on an 60A supply.

What is the actual load of the underfloor heating? Just because it has a 20A mcb does not mean it will use 20A when operating!
If no data is available just measure the resistance of the underfloor mats/cables and calculate the power.

 

[Plonker 3]

As Dave said

 

[Aok1]

Sorry forgot to put that the underfloor MCBS have been taped off and the underfloor has never been used due to the above.

No data available for the underfloor load as the job has been a bit of nightmare for the client.

 

[davesparks]

Forget the 3 phase nonsense, a house with no electric cooking or showers won't ever achieve a constant current of 20A, and then 4x underfloor mats will achieve a degree of diversity whatever the damn OSG says!

 

[davesparks]

Sorry forgot to put that the underfloor MCBS have been taped off and the underfloor has never been used due to the above.

No data available for the underfloor load as the job has been a bit of nightmare for the client.

Just measure the resistance of the underfloor heating and calculate the power!

I'd be happy to bet that a house without electric cooking or showers will never achieve a steady current of 20A.

 

[Aok1]

Thanks chaps.

 

[telectrix]

think the spark that did the rewire needs to go back to the fast track training centre. do week 6. 135A my arse!

 

[snowhead]

Sorry forgot to put that the underfloor MCBS have been taped off and the underfloor has never been used due to the above.

No data available for the underfloor load as the job has been a bit of nightmare for the client.

If the U-Floor heating is already installed, try powering up 1 zone at a time and see what the load is.

I'll bet there's a fault on one zone and the last Spark thought it was overloading in normal use and switched it all off.

 

[telectrix]

more likely the last spark did not think at all.

 

[Octopus]

If the mats really are 20a each do you know if there are contactors involved?

 

[snowhead]

If the mats really are 20a each do you know if there are contractors involved?

Maybe the Contractors fitted some Contactors??

 

[7029 dave]

I would, clamp it,whack all on see the kind of load you are pulling.

 

[davesparks]

I would, clamp it,whack all on see the kind of load you are pulling.

I'd be wary of doing that, if it is a hefty load of underfloor then turning it all on at once might be enough to tip an old 60A fuse over the edge.
I'd switch the underfloor heating on gradually (a good few hours between each) to give diversity a chance to come in to play.

Also there is no point in just measuring the max connected load, as it does not represent the maximum realistic demand. You may still end up thinking you need a bigger supply when you don't.

Clamp meters only give a snapshot of the current demand at a particular point in time, you need to record current over a period of time, a week for example, to really know what is going on.

 

[7029 dave]

But it would be a good identifier. yes no.