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Thread: Generator Sizing for 240/120V 3-phase system

  1. #1
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    Generator Sizing for 240/120V 3-phase system

    Hi All,

    I've got a retrofit job for a commercial building where I need to install a standby genset. Service is open delta 3-phase (120/240V). From my research, it appears like I will have to provide a generator wired for high leg delta. Given that a good bit of the load is single phase loads (apart from the elevator and three AC units), one of the phases (with the neutral, lets call it A-B) will be quite a bit more loaded than the other two. So, how do you take that into consideration for sizing.

    Thinking out loud, I just need to size it to ensure that phase A-B is not overloaded, right? For example, if I have 50kW of single phase loads, and I have a 150kW 3-phase unit, would the real capacity for each phase be 43.5kW (87% of 50kW)?

    Forgive my ignorance, ALL of my experience has been in large industrial plants. Never looked at an open delta service until this week!

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    Quote Originally Posted by JMLewis View Post
    Hi All,

    I've got a retrofit job for a commercial building where I need to install a standby genset. Service is open delta 3-phase (120/240V). From my research, it appears like I will have to provide a generator wired for high leg delta. Given that a good bit of the load is single phase loads (apart from the elevator and three AC units), one of the phases (with the neutral, lets call it A-B) will be quite a bit more loaded than the other two. So, how do you take that into consideration for sizing.

    Thinking out loud, I just need to size it to ensure that phase A-B is not overloaded, right? For example, if I have 50kW of single phase loads, and I have a 150kW 3-phase unit, would the real capacity for each phase be 43.5kW (87% of 50kW)?

    Forgive my ignorance, ALL of my experience has been in large industrial plants. Never looked at an open delta service until this week!
    Been a while since I did one, but Cummins and the other major vendors used to make gensets with a generator end that allowed you to load the machine up to 100% on the 2 center tapped legs when connected in a 240/120 center tapped delta arrangement. You'll have to check with them. Without this you waste an awful lot of engine capacity.

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    Quote Originally Posted by JMLewis View Post
    Thinking out loud, I just need to size it to ensure that phase A-B is not overloaded, right? For example, if I have 50kW of single phase loads, and I have a 150kW 3-phase unit, would the real capacity for each phase be 43.5kW (87% of 50kW)?
    If this were an open-delta transformer bank, I would use this fomula for the maximum size; KVA = T + 1.5S, where T=three phase load, and S=single phase load.

    So, in your case, I would be asking the generator manufacturer.
    Just because you can, doesn't mean you should.

  4. #4
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    Yeah, I’ve got an SOS out to Cummins right now. Everybody is selling so many sets, I don’t know when I’ll hear back from them. Will post my finding here for anybody interested.


    Sent from my iPhone using Tapatalk

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    Wouldn't this fall under the 87% of two transformers or 57% of three transformers?

    Further clarification:
    Since both transformers have to supplied three phase loading, one transformer will be at 8.6 KVA (150 kVA/87%). The other transformer has to supply single phase and three phase loading thus the second transformer is rated at 58.6 KVA (8.6 KVA+50 KVA).

    Total KVA for transformer = 67.2 KVA (8.6 KVA +58.6 KVA)
    Last edited by Fishn sparky; 01-15-19 at 07:57 PM.

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