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  • Jan 19, 2007, 07:08 PM
    totallywiredinc
    Electrical sizing help
    I am currently building a new building. We as the owners have decided to install a diesel back-up generator unit with auto transfer. The building uses well water and therefore requires a water storage tank for the fire suppression sprinkler system. It also will be using a 100 hp 480 volt 3 phase fire pump. The building as designed currently has 800 amp 277/480 3 phase service. The fire pump requires a separate service and meter completely independent of the 800 amp main service. How do I install the auto transfer switch to supply power to both the fire pump and main service? There is a electrical utility pad and transformer within 5 feet of the main switchgear inside the building. The genset is a 500 kw unit 277/480 3 phase. How much amperage will I get from it? Can the transfer switch be rated at 800 amps or do I need it rated for the combined main service and fire pump amperages? The building is a trucking terminal and 800 amps is probably overkill but I have 74 dock doors with a 20 amp 120 volt outlet for each door to plug in block heaters (typically 1000-1500 watts each) for the trucks. If it matters, I am in unincorporated cook county in Illinois... thankx for the help... mike
    p.s. the indoor switch gear will use current transformers with outside mounted meters.
  • Jan 19, 2007, 10:48 PM
    tkrussell
    This installation, with a Fire Pump, two services, and a genset, is tricky, and will need engineering and coordination with the local electrical inspector, and the local Fire Marshall.

    I will point out some important facts and issues to be concerned with, but by no means use this information for sizing or engineering.

    A 500 KW diesel genset will be approximately 625 KVA, and capable of approximately 750 peak amps.

    Yes a Fire Pump needs a separate service, tapped ahead of the Main Service, directly from the transformer. But if this is done, then two transfer switches are needed, one to handle the utility service #1 for the main plant, and another to handle service #2 for the fire pump.

    And the fire pump service equipment cannot be grouped with the main building service equipment.

    The 100 HP 480 volt motor needs 124 amps running load. This motor can have a locked rotor amps of 725 amps. The genset needs to be able to handle that load, so the standby load may need to be shed if the fire pump needs to run. Or the genset needs to sized to handle the starting and running load of the Fire Pump and any other load connected to it.

    The starting KVA of motors can be determined by the code letter on the motor, and then refer to NEC Table 430.7(B) KVA per HP values as listed by Motor Code letters.



    The genset will then need two distribution breakers/circuits, one for each transfer switch.

    And with interlocking between the two ATS, the genset can be dedicated to Life Safety equipment, such as egress lighting, fire alarm, and fire pump, and locked out any normal loads on the main service transfer switch.


    How far along is this project? Sounds like the genset and 800 amp transfer switch is on site,and you already have two services from the utility transformer?

    If so, and only one transfer switch, then I don't know what your going to do, because this just won't work.
  • Jan 20, 2007, 09:38 AM
    totallywiredinc
    We have not yet broke ground for this building. We are trying to finalize all the permits and plans at this time. I am currently looking for the genset and transfer switch. So if I understand you correctly, I will need a distribution panel on the output of the genset to divide the power between the two transfer switches, one for the fire pump and associated emergency equipment and the other for the main building itself with the fire pump switch being set up on a priority system. If the utility power fails, the genset will start and pick up the main building load and the fire pump/em lighting circuit unless there is fire pump activation at which time the building will be shed to direct power to the fire pump. Does the genset have to have output sufficient to supply the locked rotor requirements of the fire pump or just a percentage?
  • Jan 20, 2007, 10:10 AM
    tkrussell
    OK whew, I thought you were farther along with this project.

    A generator, when installed for both purposes of standby power for normal uses, and for Life Safety , such as egress lighting and fire pumps, can be ordered with two breakers on the genset, one for the normal load distribution panel that you would like to keep running in the event of a power failure, and another for a Life Safety distribution panel.

    You will need two distribution panels and feeders, cannot just use one and divide it.

    The Fire Pump still needs it's own separate utility service, therefore again, two ATS and distribution panels.


    And once you leave the genset with feeders from two main breakers, then each needs it's own transfer switch.

    Here is another tricky issue, if the intention is too have the genset power the normal loads AND the Fire Pump, all at the same time, then the genset must be able to handle the total of the normal load AND the Locked Rotor Amps of the Fire Pump. This is not usually practical or needed, as if there is a fire, and the process that normally occurs in the building is not critical, such as a hospital or a data center, then the putting out the fire is usually much more important, the normal load transfer switch can be Locked Out using electrical interlock control wiring, and prevent the normal load ATS from switching , and allow ONLY the ATS for the Fire Pump to transfer and energize the fire pump to suppress the fire.

    This ATS will be called the Life Safety distribution, and will also need to energize all emergency lighting, exits signs, fire alarm panel, and any other devices or systems deemed necessary for Life Safety and safe egress of all occupants if a fire occurs.

    The 100 HP Fire Pump motor, if started will have it's full Locked Rotor Amps applied to the power source, so the genset must be able to handle this full LRA, and per the NEC Table 430.151(B) and depending on the Code Ltter, can either be 725 amps (typical code letter) or may be 937 amps,(rare but may be this code letter).

    So before ordering a genset set you must get all the specs of the Fire Pump, to be sure the genset is large enough to handle the full 100 % of the LRA of the motor.

    This system requires a great deal of planning and engineering before any equipment is purchased.

    Just the distribution of the utility feeder to the Fire Pump needs special consideration. Any disconnect switch needs to be sized to handle the LRA of the 100 HP motor. With a LRA of 725 amps, and Main Disconnect switch for the utility feed will need to be rated for the next size larger that is available, which should be 800 amp rated switch. The wiring can be sized for the running load of 124 amps.

    This will look a little weird with #2 copper wire connected to an 800 Amp switch, but this is how it is.. Fire Pumps are wired with the intention of delivering all the power it needs, continuously, to insure the fire is put out, even if the fire pump is destroyed during starting or running.
  • Jan 20, 2007, 11:20 AM
    totallywiredinc
    The genset I am looking at is a used unit (only 360 hours) and it only has a single breaker output rated full load of 800 amps. Can I bring this full rated load into the building then using switchgear, divide it into two circuits to supply both transfer switches or do I need to convert the genset to two independent outputs to supply the two transfer switches? The genset does not have a transfer switch included but the same company that is trying to sell me the genset has two identical, slightly used transfer switches each rated at 800 amp at 277/480 volt.
  • Jan 20, 2007, 12:09 PM
    tkrussell
    I believe, thou cannot find the specific code at the moment, that if a genset provides power for normal standby power, and emergency Life Safety power, the Life Safety feeder must be segregated from the normal load feeder, so that if there is a problem with the normal load distribution and it cause the beaker at the genset to trip, the Life Safety feeder will not be affected and can continue to serve power.

    I am not sure what will be acceptable to the local inspector to convert the genset from one feeder to two feeders each protected differently.

    I have not done this before, try to use one genset breaker for both feeders, as all gensets we order for standby and Life Safety power are specified to have two breakers.

    I realize you may have found a used one, almost new, and a good deal, but really need to check with your engineer and inspector to determine if you can get away with the used genset as configured before you commit to the purchase.

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