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Discuss Max demand & diversity advice needed in the Electrical Forum area at ElectriciansForums.net

stuarth

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Arms
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I have been asked to do some work on a large 25 room mini mansion spread over 3 old properties made in to 1 with 6 consumer units on a single phase 100amp supply. The total loading of the breakers is 452 amps if I apply diversity at 40% I arrive at 180.8amps demand but this dose not take in to account fixed loads like 2 x water heaters & under floor heating etc, when calculating as per OSG the demand comes to 355amps.
When I mentioned the large potential demand the customer got very upset that a additional 100amp phase may be a wise idea and said the classic its "been"like this for years with no problems" my question is am I being over cautious?
 
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GBDamo

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Supporter
Sorry don't have an answer to the "I've never had a problem with it" situation other than to assess how they use their supply and how future occupants will use it.

A retired couple in some large pad may never challenge an overloaded supply however a large family with teenage kids may do so.

So for me if it's there and could reasonably expected to be used in such a manner manner as to overload the supply you have to raise it and look to lower it or upgrade the supply.

I managed to convince one customer to ditch the electric showers, three of them, in favour of bar mixers as they already had a ample gas heated hot water. This was part of a major renovation so bathrooms were being changed anyway.

Likewise with the twin electric hobs, the old dear didn't take much convincing to get a shiny new gas range.

On this occasion we managed to reduce the theoretical demand by a few hundred Amps so the theoretical overload was in the tens of Amps rather than hundreds.
 

Midwest

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Arms
Esteemed
Is this just some minor alterations your being asked to do, or is it something like a complete refurbishment, requiring major alterations of the electrical installation?
 

Lucien Nunes

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Mentor
Arms
Esteemed
Unless there is electric space heating or filament lighting, to a great extent it's the people who create the load, not the building, e.g. four occupants will use the same number of hairdryers in a large house as in a small one. Hence a particular family might have a relatively modest energy usage if they don't entertain much etc, and the 100A supply will be ample.

Applying electrical design rules for a permanent installation that might have to serve a more proportionate use of the building would result in a calculated maximum demand much higher, as it will allow for dozens of people using dozens of hairdryers and the dishwasher and washing machine running all day every day. Even so, the 100A supply will do more than many people expect.
 

Strima

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Arms
Esteemed
I personally don't think the diversity guidance within the OSG is very accurate for modern installations. Technology has moved on significantly and gone are the days of three bar heaters and 100w filament lamps.

You need to look at the demographic of the people intending to live within the property and go from there.

What is the intended use of the property/properties?

Pensioners tend to use less than a teenager, I've lost count of the times I've told the kids to turn the lights off if they've left the room.
 

stuarth

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Arms
  • Thread Starter Thread Starter
  • #7
I got asked to fix tripping RCD (underfloor heating in conservatory) then we started to basically do a EICR and discovered the installation is a mess, I have found a supply to a garage on a sub main with only a main switch no protection for its entire length of 60 meters in SWA, starts in 10mm then its split into 1 x 6mm to garage cu then 6mm again around the house to a conservatory cu (installed 5 years ago) approx 45 meters, no selectivity between consumer units, no RCD protection on circuits though bathrooms etc. original rewire approx 25 years ago by a BT engineer, no oil bonding on brand new boiler, the 50+ garden lights connected in chocbox connector boxes wrapped in tape the list just keep going.
 

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Wilko

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Arms
Esteemed
Hi - my 20p worth is as per Others, that the domestic load stems from the people, not the installed circuits. If they’ve gas heating, cooker and hot water then a single 100A supply may be ok. But if it’s really 3 houses without gas then it may not be.
 

Midwest

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Arms
Esteemed
I got asked to fix tripping RCD (underfloor heating in conservatory) then we started to basically do a EICR and discovered the installation is a mess, I have found a supply to a garage on a sub main with only a main switch no protection for its entire length of 60 meters in SWA, starts in 10mm then its split into 1 x 6mm to garage cu then 6mm again around the house to a conservatory cu (installed 5 years ago) approx 45 meters, no selectivity between consumer units, no RCD protection on circuits though bathrooms etc. original rewire approx 25 years ago by a BT engineer, no oil bonding on brand new boiler, the 50+ garden lights connected in chocbox connector boxes wrapped in tape the list just keep going.
I've no experience in specifying max demand etc. As the others suggested, who will be using this property, existing owners, new owners or change of use?
 

stuarth

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Arms
  • Thread Starter Thread Starter
  • #10
I've no experience in specifying max demand etc. As the others suggested, who will be using this property, existing owners, new owners or change of use?
Existing owners for a few years then planing on selling.
 

Midwest

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Arms
Esteemed
Existing owners for a few years then planing on selling.
I haven't the experience of others, but I think a new 3 phase supply might be appropriate here. EV chargers etc might be installed by new owners. But then I suppose the existing owners might not what to pay for that extra considerable expense, for somebody else. But I guess these sorts sales might attract some inspection by surveyors, by the buyer. Bit late when its all done.

As you might be held responsible (for poor advice) if you become the contractor that refurbishes the property, I would seek for them to get some advice from a consulting firm with experience in this field, or not least have it in writing your concerns.

I did some work subbing on a large barn conversion, which had a new 3 phase supply. It did have a swimming pool & gym, but it was just for a young family. Also did some work in an existing largish manor house (again with pool), this had a 3 phase supply. Just a couple of family members, and a summer let.

I think you are right to have concerns.
 
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SJD

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Mentor
Arms
Esteemed
The original post mentioned 3 properties converted to one large one.

While perhaps not ideal (compared to a new 3-phase supply), is there actually more than one single phase 100A supply coming into the property (that is still live, even if not in use)?
 

sparks1234

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Arms
It doesn't matter what the total load of the breakers adds up to, thats a load of old rubbish, look at what loading to have, you might only be drawing 50 amps , who knows, only you can have a guess at what they need
 

TJ Anderson

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Mentor
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Esteemed
At the end of the day .......the DNO's BS88 will have the final say lol. That's real world diversity. For the record, I've never seen one blow from overload and I've seen commercial places that you would not believe we're on a 80A BS88 supply fuse.

Install a TPN DB with single phase kit. Then you are covered for an upgrade if ever required.

Put a current monitoring relay on it switching a mains Klaxon.....then they will know if the are getting close hahaha

It may be of interest......the DNO's base their calcs based on loads of just 2 kW per house and it works just fine.
 
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marconi

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Mentor
Arms
Esteemed
In a similar way to a gas powered hot water and central heating system which switches between one and t'other but never both at the same time, could you have some priority switching between high current loads such as the underfloor heating, hot water and any electric showers using this sort of device - where a temporary interruption in power to a load is of little consequence?:

https://www.meteorelectrical.com/distribution-control/consumer-units-accessories-1/garo-priority-shower-board-choose-priority.html

or

https://media.automation24.com/datasheet/en/900CPR-1-1-BL-230V-CE.pdf
 

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