Single Phase v 3 Phase

[CathalX]

Generally speaking, what is the KVA limit for a single phase supply??

Can you have, for example, a 30 KVA single phase supply??

 

[plugwash]

i think single phase supplies fused at 125A are fairly common no idea what the declared max demand on them is though.

i think you can get bigger single phase supplies if you really need them but i belive you are likely to have to transform down from 11KV yourself for such a supply.

anything in particular you wan't such a supply for? (possiblly some type of welding?)

 

[pdcelec]

The REC's usually supply a 30, 60, 80 or 100A supply to dwellings in houses. The power factor of domestic situations is usually good so the KVA virtually equals the KW ratings. The meters measure the kWh used by the consumer as compared to some industrial and commercial meters which will measure the maximum demand kVA used

 

[CathalX]

4000 SQ Foot House - Underfloor Heating with 17 elements each drawing 10A - 120 Spots - 3 Ring mains etc.

The customer ordered a 16 KVA supply for 2000 Euro thinking it could run a factory!

I need to come up with a solution. The expected load would be 70A for power and light and 170A for heating if all elements were on.

I have installed quite a large single phase board and have two choices: increase the single phase supply to 70 KVA (impossible I presume) or install a three phase board to look after the heating and get the customer to order a 3 phase supply. (Big Cost) + (Upset Customer)

You are probably wondering how I ever thought I could power the house with a single phase supply - I thought the heating would only take 40A.

MISTAKE!!!

 

[andy]

looks like your gonna have to get a 3 phase supply. and 170A of heating is a bit OTT innit?

 

[plugwash]

you'll have to go with what the rec will give you. my guess is for that amount of heating a 100A 3 phase supply is in order.

why so much heating equipment anyway?

 

[ban-all-sheds]

40kW of heating for a 4000 sq ft house - doesn't sound OTT to me - 10w/sq ft.

If it was wet heating with a 40kW/136KBTU boiler, to serve a house which is what? - 3-4 times the size of the average semi?, would you think that was oversized?

 

[Agile]

You dont say where this house is situated or confirm that its a new build.

If its underground supply from the local sub station then there may already be a load limit problem either distribution or transformer rating. They have a habit of telling you that you have to pay to uprate the local transformer.

On the other hand if its fed with an o/h 11 kV line then they use two 11kv phases and a pole mounted transformer sized to suit but with about 130 kVa about the max on a pole.

Generally the costs start to seriously escalate if you ask for something out of the ordinary.

I do have to say that the design planning for this job seems to be rather odd. Such a large house would much more economically be heated by oil and perhaps still slightly cheaper even with LPG. You have not mentioned if its set up for an off peak tariff which would be needed if the running costs are not to be astronomic.

Tony Glazier

 

[CathalX]

The house is built on a remote site. The supply authority have erected a pole, mounted a transformer and given him his 16 KVA supply. You are right the design is not sensible at all. It is built with 12 inch blocks with a foot of stone added on. Without any power in the house, it is already very warm.

There is an off peak tariff.

I think that a three phase supply is now inevitable.

In answer to the question "why so much heating", the house was designed in a very 'money no object manner'. He wanted under floor heating in each room downstairs but because the rooms are so large, three to five elements were needed in each. He didn't really consider the supply implications or running costs.

Thanks for your help

 

[plugwash]

if its a dedicated polemount there is another option

SPLIT PHASE

the advantage of this is they don't have to run a third 11KV line to the pole.

the disadvantages are you only get two live conductors not 3 (which will mean either two seperate CUs or some form of adaptation of a 3 phase board and will mean you will need a higher current rating for the same total power than with 3 phase) and you can't run 3 phase equipment.

i'd say ask them for quotes for
250A single phase
125A split phase
100A three phase
and go with whichever one is cheapest.

 

[Stoday]

Think yourself lucky you're in Ireland. If you were in the UK you'd have to pay the full cost of the connection. ESB's connection policy is to charge half the cost (unless it's changed in the past 2 years).

 

[ban-all-sheds]

In answer to the question "why so much heating", the house was designed in a very 'money no object manner'. He wanted under floor heating in each room downstairs but because the rooms are so large, three to five elements were needed in each. He didn't really consider the supply implications or running costs.

Did he design it himself, in a position of ignorance, or did he employ an architect? If the latter I think he's been the victim of negligence...

 

[Big_Spark]

Cathal, I think your also labouring under a false premise, have you forgotten DIVERSITY??

Yes you have

170A Potential Heating load
6kW Potential Lighting Load
3 Rings with a potential loading of 96A MAXIMUM

In reality, due to occupancy and the way people use a home, you will have likely loadings of

60A Heating
9A Lighting
40A maximum across all three rings

Giving a grand total of 109A which would require a 30kVa supply to give an upper cieling.

The heating elements will be Thermostatically and Timer controlled, so it is highly unlikely that all will be on at the same time, if the house is as well insulated as you imply then they are likely to be on only intermittently.

I would simply recommend that the supply be uprated from 16kVa to 30kVa.

 

[andy]

Cathal, I think your also labouring under a false premise, have you forgotten DIVERSITY??

Yes you have

170A Potential Heating load
6kW Potential Lighting Load
3 Rings with a potential loading of 96A MAXIMUM

In reality, due to occupancy and the way people use a home, you will have likely loadings of

60A Heating
9A Lighting
40A maximum across all three rings

Giving a grand total of 109A which would require a 30kVa supply to give an upper cieling.

The heating elements will be Thermostatically and Timer controlled, so it is highly unlikely that all will be on at the same time, if the house is as well insulated as you imply then they are likely to be on only intermittently.

I would simply recommend that the supply be uprated from 16kVa to 30kVa.

but you cant have diversity on floor heaters according to the OSG. and what happens when he gets back from a weekend away and all heaters go on at the same time

 

[plugwash]

whilst the diversity guidelines are just guidelines the point you make is right. its quite feasible for everything to be running for a while when you turn the system on from cold (less so if controls are seperate but its still possible).