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Thread: Need help on sizing a transformer

  1. #1
    Join Date
    May 2009
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    12

    Need help on sizing a transformer

    Hi all,
    I need some help on sizing a transformer for an upcoming job. It's a new restaurant in a shopping plaza. The service coming in the building has a 200 amp 3 phase 480v panel, what's the largest transformer can I install to get 3 phase 120/208? I'm not sure how much power would be needed since the customer does not have the spec for the equipments. I would like to install a 3 phase 120/208 400 amp panel just to be safe. Is a 150kva transformer good for 400amp? 208*400*1.732/1000=144kva, is that correct? Is the existing 200amp breaker ok for the primary size?

  2. #2
    Your supply has about 165KVA (408V, 3P, 200A) capacity. If you want to take out 144kva (208/120V,3P,400A) that should be fine, leaving about 10KVA for other use.

  3. #3
    Join Date
    Jul 2005
    Location
    Tennessee
    Posts
    11,867
    Welcome to the Forum...

    You're good on your calculations. Inrush might pose a problem on your 200 amp breaker. You might see if it has adjustable trip.
    alos make sure your HVAC isn't 480. Since it's an existing service, the HVAC may have already been spec'ed.

    If you have restaurant experience you don't need the next "not asked for "advice, but if it's a first....before you get in too deep, get an equipment schedule. Since you apparently don't have a print, it looks like it may not be a "designed" restaurant and those folks are notorious about "picking" equipment and you end up with multiple voltage/phases, etc.

    You could probably get a number of horror stories from Forum members.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  4. #4
    Join Date
    Feb 2003
    Location
    Massachusetts
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    47,012
    You may want to use two 75 KVAs that supply two 225 amp panels.

    Reason being is the inrush of the 150 Kva transformer may trip the 200 amp main.

  5. #5
    Join Date
    Jan 2010
    Posts
    10
    From doing numerous restaurants, (also no offense if you have the experience as well) first thing I always ALWAYS find out is if they have a gas service. If they have gas 200A at 480 is usually fine, but typically 400A at 480 if no gas is available to play it safe. I also agree that (2) 75kVA xfmrs is a much better way to go. I almost always end up with a double section panel anyway and you can save a bunch of money on the transformer primary breaker by avoiding adjustable trip settings.

    -Jeff
    Electrical Engineer
    Advanced Engineering, Inc

  6. #6
    Join Date
    Sep 2007
    Location
    Omaha, NE
    Posts
    104
    The capacity of the 200 amp, 277/480 volt, 3-phase service is 166.3 kVA (200 amps x 480 volts x 1.732). This equates to 461.5 amps at 120/208 volt, 3-phase.

    If you want to convert the entire 200 amp 277/480 volt, 3-phase service to 120/208 volt, 3-phase, you would need a minimum of a 225 kVA transformer. However, a 150 kVA transformer will likely suffice if the HVAC equipment is 480 volt. It is highly unlikely much of anything else in a restaurant will be rated at 480 volts.

    If using the 150 kVA transformer, it will deliver 416.7 amps at 120/208 volt, 3-phase. Per NEC Table 450.3(B), the overcurrent protection of the secondary would be limited to 600 amps ((150 kVA x 1000) / (208 volts x 1.732), rounded up to the next standard OCPD size). Therefore, having a 400 amp, 120/208 volt, 3-phase panel as you propose would be permitted.

    Also per NEC Table 450.3(B), the maximum rating of the primary OCPD for the 150 kVA transformer would be 250 amps ((150 kVA x 1000) / (480 volts x 1.732), rounded up to the next standard OCPD size).

    I agree with augie47 that you need a kitchen equipment schedule to determine the voltages of all of the kitchen equipment. Otherwise you could end up setting a 240 volt panel and transformer from the 480 volt service or end up buck/boosting 240 volt equipment from the 120/208 volt panel.

    Whether inrush at the 200 amp panel may or may not be a concern is a subjective issue that can only be determined by evaluating the loads served by the transformer and the likelihood of those loads starting simultaneously.
    Jason Rohe, P.E.

  7. #7
    Join Date
    Feb 2003
    Location
    Massachusetts
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    47,012
    Quote Originally Posted by jrohe View Post
    Whether inrush at the 200 amp panel may or may not be a concern is a subjective issue that can only be determined by evaluating the loads served by the transformer and the likelihood of those loads starting simultaneously.
    My statement had nothing to do with the loads, with the secondaries open the inrush alone may still trip the 200 breaker. I do agree it is not a certainty but it is something to consider.

    It is my understanding that this is the reason the NEC allows up to 250% on the primary side.

    Seeing as the full load current would be 180 the 200 amp breaker is only about 10% above seems like a gamble to me.

  8. #8
    Join Date
    May 2009
    Posts
    12
    Thank you very much for the advice. I've done a few restaurants in the past but none require to install a step down transformer. The last restaurant I did almost max out the 400amp 120/208 3 phase service, the dishwasher alone required 150amp 3 phase 208v. The equipment schedule is not available at this point since they have not decided on the equipments, but I'll make sure none of the equipment is at 480v. There will be at least 12 exhaust and make up fan for this restaurant, and it's about 5000 sq ft. The building owner has agreed to bring in another 200 amp if the current service is not enough. As for you suggesting, two 75kva vs one 150 kva transformer, how much more would that cost me?

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