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Topic Title: Convention in description of a supply.
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Created On: 09 February 2019 08:08 PM
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 09 February 2019 08:08 PM
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AncientMariner

Posts: 904
Joined: 14 December 2004

If a customer requests a 3-phase supply from a DNO and the DNO replies offering 30 kVA, would you take that as 10 KVA per phase ie 30 kVA total, or 30 kVA per phase?

(This relates to my questions last year regarding a supply upgrade and the size of the DNO 11 kV/400 transformer.)

As a rider to this question above, when requesting a supply from a DNO, either initial or upgrade, what would be a typical requirement for a farm house with oil fired CH and cooking, with washing machine & dishwasher and two 10 kW showers? (Family of four - two daughters.)

Clive

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Clive S Carver GCGI IEng MIET MITP
 09 February 2019 08:27 PM
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alancapon

Posts: 7492
Joined: 27 December 2005

That would be 10KVA per phase, and I would expect the cutout to be fused at 45A per phase.

Regards,

Alan.
 09 February 2019 08:57 PM
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AncientMariner

Posts: 904
Joined: 14 December 2004

Thanks Alan.
The confusion I have is that the 3-phase 30 kVA applies to the farm site. A house, large workshop building and an underway barn conversion. holiday let.

The workshop is currently 1-phase with a 80 amp fuse in the cut-out (the meter was replaced yesterday and my friend asked what size it was.

The house with the showers etc is currently 1-phase with unknown size cut out fuse; but I am guessing at currently more than 45 amp.

The 3-phase supply will be going to the workshop and possibly the barn conversion holiday let.

The holiday let will have a couple of electric showers plus electric water heating and they are considering an air-sourced heat pump for underfloor heating. I would guess too a dishwasher + washing machine and maybe a tumble dryer.

So, would 3-phase be better for the heat pump, especially if a single phase 45 amp supply is being considered for this building? Similarly, I would have thought that 45 amp would be pushing their luck for the farm house.

Their electrician says that for the farmhouse, 4kVA would be sufficient - even though two electric showers and two daughters etc..

(Our soon to be rented property with no electric showers was found to have a by-passed blown 60 amp fuse in the cut out)

Thanks
Clive

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Clive S Carver GCGI IEng MIET MITP
 09 February 2019 08:59 PM
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chrispearson

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Joined: 15 February 2018

Originally posted by: AncientMariner

If a customer requests a 3-phase supply from a DNO and the DNO replies offering 30 kVA, would you take that as 10 KVA per phase ie 30 kVA total, or 30 kVA per phase?


30 kVA per phase would be > 100 A so it must be total.

As a rider to this question above, when requesting a supply from a DNO, either initial or upgrade, what would be a typical requirement for a farm house with oil fired CH and cooking, with washing machine & dishwasher and two 10 kW showers? (Family of four - two daughters.)


Not a huge amount. The person whom I know who spends the most time in a bathroom is a man who loves to soak in a bath. Diversity will take care of the showers even if there are other loads at the time.

Originally posted by: alancapon

That would be 10KVA per phase, and I would expect the cutout to be fused at 45A per phase.


I cannot comment upon normal practice, but I have been allowed 30 kVA after diversity with 100 A fuses.

If I get anywhere near that consumption for any prolonged period, I shall be bankrupt!
 09 February 2019 10:22 PM
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alancapon

Posts: 7492
Joined: 27 December 2005

Not sure about your part of the world, but our standard fuse is a 63A for a single phase supply.

Regards,

Alan.
 09 February 2019 10:46 PM
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AncientMariner

Posts: 904
Joined: 14 December 2004

Here in the old MANWEB area, our house, a 3 bedroom 1 bathroom detached house, has a 80 amp fuse in the cut out (advised to me during failed attempt to fit a smart meter - gas and electricity too far away to communicate with each other). Used to have storage heaters fitted by MANWEB. No electric shower.

Identical house next door, which I believe had storage heaters at one time, 60 amp. But was found blown and bypassed when smart meter fitted last year; meter man fitted a 63 amp replacement. No electric shower.

Clive

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Clive S Carver GCGI IEng MIET MITP
 09 February 2019 11:06 PM
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broadgage

Posts: 3170
Joined: 07 August 2007

For a farmhouse as described with two electric showers each of 10Kw, I would consider that a single phase 100 amp supply would be needed.
If both showers are used at the same time, then that is about 80 amps., and 20 amps for everything else. A short term overload of the supply seems likely to occur if both showers coincided with several other loads. Not ideal, but widely accepted in practice.
With an 80 amp supply I perceive a small but real risk of the cut out fuse operating.

A three phase supply at 45 amps per phase may prove marginal, despite being 135 amps in total.
 10 February 2019 06:08 AM
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mapj1

Posts: 12039
Joined: 22 July 2004

Well you'd want the showers on different phases for a start.
It is certainly true that a great many houses with a full family manage on a 60A single phase supply, and there are plenty of flats and 'park homes' for one on 32A supplies, and the fact that the breakers in the box add up to 3 times that is neither here nor there.

However, I'd check if they really saying they will fuse at 45A, as they may say 'the nominal supply is 45A /phase ' and then fuse at one size larger, allowing some occasional overshoot. A look at the electricity bill will tell you the long term average demand is far lower.
It is also worth staring hard at fuse curves
as they show that there is long enough for the kettle to boil or toaster to do its thing, without actually blowing an apparently fully loaded fuse.
(your 45A fuse can give more like 100A for 15 mins, which is almost the 1000 seconds line)
However, thinking of future proofing, in the coming era of the all electric car, 45 A per phase may be a bit light for what sounds like 3 buildings.

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regards Mike
 10 February 2019 07:13 AM
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normcall

Posts: 8550
Joined: 15 January 2005

Being realistic, If electric cars become popular, then massive investment in our supply network will be required!
We are all right as house built in road without gas and 100A 3 phase supply. All it needs now is electric cars, able to tow caravan for 200 miles otherwise a no go for us.

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Norman
 10 February 2019 09:15 AM
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Zoomup

Posts: 6117
Joined: 20 February 2014

I know of a bed and breakfast premises on a 100 Amp single phase supply. It has three electrical showers in guest rooms , several dishwashers and washing machines and cooking appliances. The heating is by gas boiler. The main fuse has never blown in many years if at all.

Z.
 10 February 2019 11:51 AM
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ebee

Posts: 6735
Joined: 02 December 2004

Luck (good or bad) conspires.

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
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