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3 Phase

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  • 20-06-2020 10:46am
    #1
    margaretdublin
    Registered Users Posts: 54 ✭✭


    Hi

    I’ve just received a quote for 3 phase from the ESB.

    We need to run 2 to 3 pieces of machinery at 7kw each at the same time

    The quote we received was for 18KVA. Is this sufficient to run at least two items at once.

    Thanks in advance


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    #2
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    margaretdublin wrote: »
    Hi

    I’ve just received a quote for 3 phase from the ESB.

    We need to run 2 to 3 pieces of machinery at 7kw each at the same time

    The quote we received was for 18KVA. Is this sufficient to run at least two items at once.

    Thanks in advance

    Let's assume the machinery is 3 phase and each unit has a power factor of 0.9 (conservative value) then each unit would be 7/0.9 = 7.78 kVA.

    So 15.56 kVA when both are on.

    That leaves you with 18-15.56 = 2.44 kVA for everything else (not even enough for a kettle).

    I have no idea what else you have but it sounds like a larger supply would be best but 2.44 kVA is very unlikely to be sufficient. Another consideration maybe
    the high starting currents for these machines, depending on what they are.


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    #3
    margaretdublin
    Registered Users Posts: 54 ✭✭margaretdublin


    Thanks very much for your reply.

    We’ve also had a separate quote for single phase 18KVA

    Could you let me know what the difference between 18KVA single supply and 18KVA 3 phase supply.

    The machinery can be wired to single phase or 3 phase


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    #4
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    margaretdublin wrote: »
    We’ve also had a separate quote for single phase 18KVA

    3 phase is far better, always. Also that is most likely as large as you can go in terms of a single phase supply, you would be in trouble if you needed to go larger.
    Could you let me know what the difference between 18KVA single supply and 18KVA 3 phase supply.

    Than question deserves a very long answer to cover it fully!

    Some of the important things to know is that in general 3 phase equipment is:
    • Cheaper to buy
    • More robust (less down time)
    • More economical to run
    • Causes less issues within the installation (such as volt drop during starting)

    Given a choice always go 3 phase.
    The machinery can be wired to single phase or 3 phase

    Wire it in 3 phase.


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    #5
    John.G
    Registered Users Posts: 3,514 ✭✭✭John.G


    I always get a bit mixed up re 3 phase.... if the motors are 3 phase then the kva/motor will be 7/0.9/1.732 or 4.5 kva or 9 kva for both motors so I'm inclined to think that there is 9 kva/phase "to spare" but obviously not.


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    #6
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    John.G wrote: »
    I always get a bit mixed up re 3 phase.... if the motors are 3 phase then the kva/motor will be 7/0.9/1.732 or 4.5 kva or 9 kva for both motors so I'm inclined to think that there is 9 kva/phase "to spare" but obviously not.


    Not quite.
    The formula for power for any 3 phase load is:

    Power = root 3 x line voltage x line current x power factor
    ^^ The answer will be in watts. To convert to kW devide the answer by 1000.

    In Ireland 3 phase line voltage is generally considered to be 400V.

    The formula for VA is the same but without power factor. To convert to kVA devide the answer by 1000


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    #7
    John.G
    Registered Users Posts: 3,514 ✭✭✭John.G


    Quote "Let's assume the machinery is 3 phase and each unit has a power factor of 0.9 (conservative value) then each unit would be 7/0.9 = 7.78 kVA"

    If you assume that the units were single phase then is the KVA still the same ie 7.78?
    does root 3 come into your first calculation at all?.


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    #8
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    John.G wrote: »
    Quote "Let's assume the machinery is 3 phase and each unit has a power factor of 0.9 (conservative value) then each unit would be 7/0.9 = 7.78 kVA"

    If you assume that the units were single phase then is the KVA still the same ie 7.78?

    Yes.
    does root 3 come into your first calculation at all?.

    No, it was already applied to the values provided.


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    #9
    corkgsxr
    Registered Users Posts: 3,404 ✭✭✭corkgsxr


    Anything that says 3 phase or 1. Take it as 3 phase = running 100%power

    1 phase = 20% of rated power and uses a higher unit cost for that


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    #10
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    corkgsxr wrote: »
    1 phase = 20% of rated power and uses a higher unit cost for that

    What do you mean? This makes no sense to me.


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    #11
    margaretdublin
    Registered Users Posts: 54 ✭✭margaretdublin


    Hi me again asking a dumb question.

    Apparently ducting is a standard part of the installation of 3 phase. Does anybody know is this a big procedure how long it will take and how much should I expect to pay to get this done. I don’t know much ducting is required but has anybody any idea how much it could cost. Thanks


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    #12
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    That depends on a lot of factors such as how much ducting you need. You need to get a registered electrical contractor on board and they will organize a site visit from the ESB. Once this is done you will be in a position to get quotations.


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    #13
    margaretdublin
    Registered Users Posts: 54 ✭✭margaretdublin


    Hi

    Thanks we got the site visit and details required

    Our electrician has said he will try get a quote for the groundworks.

    Should we get quotes ourselves but from who? Small builders? We need approximately 15m for the ducting mostly soil. What should we expect to pay for this?

    Thanks for your help




    2011 wrote: »
    That depends on a lot of factors such as how much ducting you need. You need to get a registered electrical contractor on board and they will organize a site visit from the ESB. Once this is done you will be in a position to get quotations.


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