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Thread: Conductor sizing for new fire pump service.

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    Conductor sizing for new fire pump service.

    I’m in the middle of designing a service to a new fire pump building. I have attached a PDF for reference. Does this sizing seem ok or does anyone have any issues with this set up?

    Note 2 - will be sized as normal for the 60A panel.

    Note 7 - will indicate wire size for 125% of the FLC of the jockey pump. (480, 3p, 1.5 HP Pump = 3A FLC (per 430.250)*1.25 = 3.75A = #12G wire)

    Note 8 - will indicate wire size for 125% of the FLC of the fire pump. (480, 3p, 75HP Pump = 96A FLC (per 430.250)*1.25 = 120A = #1G wire)

    Note 1 (service conductors) –Here’s where I’m not so sure. I’m assuming I would just add the 60A+120A+3.75A and then size the wire for that which would equal 183.75A or 3/0 wire. Note – voltage drop on this size is under the required 15%. This seems like the common sense way to size things, but I can’t find anything that verifies my thought process and I know fire pumps can be tricky.

    If anyone could provide some insight I’d appreciate it.

    Thanks,
    - Drew
    Attached Images Attached Images

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    Shouldn't there be a service disconnect?

    The service entrance conductors should be sized according to the max available current from the service XFMR.

    For example:

    Let's say that the utility XFMR is 150kVA 480/277V.
    Secondary full load current is:
    150kVA/(480V * 1.732) = 180A
    Which would require you to use 3/0 AWG feeders.

    As you see, it makes sense to add up the loads (and you would have to to size the XFMR). But the proper way to size feeders is always to base it off of what amperage is available i.e. XFMR full-load or Overcurrent device rating.
    Last edited by jeremy.zinkofsky; 02-05-16 at 01:37 PM.

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    Quote Originally Posted by jeremy.zinkofsky View Post
    Shouldn't there be a service disconnect?

    The service entrance conductors should be sized according to the max available current from the service XFMR.

    For example:

    Let's say that the utility XFMR is 150kVA 480/277V.
    Secondary full load current is:
    150kVA/(480V * 1.732) = 180A
    Which would require you to use 3/0 AWG feeders.

    As you see, it makes sense to add up the loads (and you would have to to size the XFMR). But the proper way to size feeders is always to base it off of what amperage is available i.e. XFMR full-load or Overcurrent device rating.
    Sorry - I should have been more clear. Each control panel has it's own disconnect. I'm utilizing the 6 disconnect rule in this application.
    Also - Fire pumps are treated a bit differently. The wire is not sized to the rating of the OCPD. The wire is sized at 125% of the FLC, the OCPD however is sized based on the Locked rotor current.

    - Drew

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    Quote Originally Posted by Fitzdrew516 View Post
    Sorry - I should have been more clear. Each control panel has it's own disconnect. I'm utilizing the 6 disconnect rule in this application.
    Also - Fire pumps are treated a bit differently. The wire is not sized to the rating of the OCPD. The wire is sized at 125% of the FLC, the OCPD however is sized based on the Locked rotor current.

    - Drew
    Right, but we're talking about sizing the service cables and not the motor feeder cables. If you size the cable appropriately based on the XFMR secondary's full load amperage and the cable gets overloaded, the XFMR will get overloaded as well. In that scenario, the XFMR has not been sized appropriately and you have bigger problems.

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    How did you size the secondary cable of XFMR 'T3' in your drawing? The Utility XFMR will be sized using the same approach.

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    Quote Originally Posted by jeremy.zinkofsky View Post
    Right, but we're talking about sizing the service cables and not the motor feeder cables. If you size the cable appropriately based on the XFMR secondary's full load amperage and the cable gets overloaded, the XFMR will get overloaded as well. In that scenario, the XFMR has not been sized appropriately and you have bigger problems.
    In this application they're the secondary conductors of the utility transformer. The utility is not governed by the NEC. All you do is tell the utility what your approximate load is going to be, then they size the xfmr. If it were a house transformer I would agree with you as far as sizing for the secondary. This application is pretty different in many aspects so I'm trying to make sure I'm covering all bases - I just don't know 100% if I am.

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    Quote Originally Posted by Fitzdrew516 View Post
    In this application they're the secondary conductors of the utility transformer. The utility is not governed by the NEC. All you do is tell the utility what your approximate load is going to be, then they size the xfmr. If it were a house transformer I would agree with you as far as sizing for the secondary. This application is pretty different in many aspects so I'm trying to make sure I'm covering all bases - I just don't know 100% if I am.
    Then you still size the conductors to that approximate load. It doesn't matter who is sizing what or who is providing the XFMR. When you make the connection to the service, the cables you bring will have to be able to handle the same ampacity of the ones provided by the utility. So you will have to do the same calculation that the Utility will do.

    Just make sure that you are adhering to NEC 240.21

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    Quote Originally Posted by jeremy.zinkofsky View Post
    Then you still size the conductors to that approximate load. It doesn't matter who is sizing what or who is providing the XFMR. When you make the connection to the service, the cables you bring will have to be able to handle the same ampacity of the ones provided by the utility. So you will have to do the same calculation that the Utility will do.

    Just make sure that you are adhering to NEC 240.21
    Yeah. I agree. What I'm asking is do I size the load to 125% of the Fire/Jockey pumps or would that only be a consideration for the wire that feeds directly from the control panel to the motor itself?

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    Not sure if this is your answer.

    695.6

    (B) Conductor Size.
    (1) Fire Pump Motors and Other Equipment. Conductors
    supplying a fire pump motor(s), pressure maintenance
    pumps, and associated fire pump accessory equipment shall
    have a rating not less than 125 percent of the sum of the fire
    pump motor(s) and pressure maintenance motor(s) full-load
    current(s), and 100 percent of the associated fire pump accessory
    equipment.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    I guess a question regarding 240.21 would be - The OCPD for the control panels are sized for the locked rotor current current of the pumps - Now would you need to size the wire from the wireway to the controllers based on the OCPD of the controllers? I know that's how you would normally approach the tap rule, but feeding 600 amp wire off of some 3/0 service conductors seems asinine.

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