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I don't normally get involved with domestic work but this job is for a family member so I cant Simpley say no.
the house is a large property with a 3 phase 100A supply.
I think the earthing is TT but not 100% sure of that yet.
customer has bought a 12.5Kva generator to use as a backup supply when the grid fails.
I have suggested an auto transfer switch that is in an enclosure etc. and will be suitable for the task in hand.
first question is, will this be notifiable work?
I suspect it is but cant actually figure out why, it is a new supply, to the existing installation.
I have not got a problem either way, I can either use the inform LBC method or employ a domestic spark to run and sign off on the job. i know a few that I normally just pass the work onto without getting involved.
but as I say, this is family and they WANT ME to do it.
now, here is the technical issue.
loads are no where near balanced across the phases and in reality they never will be.
the house is split across multiple DB's and depending on what rooms or activities are active, the loads can change considerably from phase to phase.
e.g if they had a 12.5Kva 3 phase gen set, max current per phase would be 18A
if the load is not balanced, a kettle and a hair dryer together would be enough to overload a phase if they were both on the same one.
this is not acceptable for them.
in reality, when on backup power, the total load is unlikely to go above 10Kva, however there is no way of knowing if one of the phases will draw more than 4Kva which is the limit of supply for a 3 phase generator.
they have decided to go for a single phase generator that will provide them with around 50A
the idea being that when on backup power, the 3 phase installation will be provided with 3 x L1 by joining the phases together.
Grid side, there is not a problem because there is no interconnection between on site generation and grid.
I am wondering and cant seem to find any info.
is this solution acceptable?
personally i think it is a good solution, it means that the house can be provided with backup power to all areas and all circuits.
the obvious solution would be to buy a generator that could provide the max demand required on any phase at any time, and this would be how you would do it if it was a hospital or other critical infrastructure.
however to do this for this property would mean a generator of around 35Kva.
Ideally with a generator, you should be loading it to at least 75% for 1 hour in every 24 hours to prevent damage to the engine.
the loading would be to small for the gen set and would have issues with glazed cylinders and other issues with an under loaded engine.
also the fuel consumption would be in the region of 3 x what is required.
I.e the 12.5Kva set uses around 3L per hour a 35Kva set would use more like 10 to 15L per hour.
Finally I must add, the house is all single phase equipment, except for the following.
a 3 phase motor for a pool cover
a 3 phase motor on a lathe in the workshop.
obviously the 3 phase equipment would not work when under backup power, and due to the fact that all phases would be at the same potential, there would be no current flow through them if you tried to use them.
So there we have it, a proper problem that requires some real thought and consideration.
I have put this in commercial, because although it is domestic, it is well out of the remit of MOST but not all domestic Sparks.
the house is a large property with a 3 phase 100A supply.
I think the earthing is TT but not 100% sure of that yet.
customer has bought a 12.5Kva generator to use as a backup supply when the grid fails.
I have suggested an auto transfer switch that is in an enclosure etc. and will be suitable for the task in hand.
first question is, will this be notifiable work?
I suspect it is but cant actually figure out why, it is a new supply, to the existing installation.
I have not got a problem either way, I can either use the inform LBC method or employ a domestic spark to run and sign off on the job. i know a few that I normally just pass the work onto without getting involved.
but as I say, this is family and they WANT ME to do it.
now, here is the technical issue.
loads are no where near balanced across the phases and in reality they never will be.
the house is split across multiple DB's and depending on what rooms or activities are active, the loads can change considerably from phase to phase.
e.g if they had a 12.5Kva 3 phase gen set, max current per phase would be 18A
if the load is not balanced, a kettle and a hair dryer together would be enough to overload a phase if they were both on the same one.
this is not acceptable for them.
in reality, when on backup power, the total load is unlikely to go above 10Kva, however there is no way of knowing if one of the phases will draw more than 4Kva which is the limit of supply for a 3 phase generator.
they have decided to go for a single phase generator that will provide them with around 50A
the idea being that when on backup power, the 3 phase installation will be provided with 3 x L1 by joining the phases together.
Grid side, there is not a problem because there is no interconnection between on site generation and grid.
I am wondering and cant seem to find any info.
is this solution acceptable?
personally i think it is a good solution, it means that the house can be provided with backup power to all areas and all circuits.
the obvious solution would be to buy a generator that could provide the max demand required on any phase at any time, and this would be how you would do it if it was a hospital or other critical infrastructure.
however to do this for this property would mean a generator of around 35Kva.
Ideally with a generator, you should be loading it to at least 75% for 1 hour in every 24 hours to prevent damage to the engine.
the loading would be to small for the gen set and would have issues with glazed cylinders and other issues with an under loaded engine.
also the fuel consumption would be in the region of 3 x what is required.
I.e the 12.5Kva set uses around 3L per hour a 35Kva set would use more like 10 to 15L per hour.
Finally I must add, the house is all single phase equipment, except for the following.
a 3 phase motor for a pool cover
a 3 phase motor on a lathe in the workshop.
obviously the 3 phase equipment would not work when under backup power, and due to the fact that all phases would be at the same potential, there would be no current flow through them if you tried to use them.
So there we have it, a proper problem that requires some real thought and consideration.
I have put this in commercial, because although it is domestic, it is well out of the remit of MOST but not all domestic Sparks.
- TL;DR
- Generators, 3 phase to single phase
