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Thread: Calculation for 5 mobile radiology imaging trucks

  1. #1
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    Calculation for 5 mobile radiology imaging trucks

    Currently the customer has a 480v 3ph 400 amp service to meter then splits to 2-200 amp fused disconnects with 200 amp fuses that feeds 2-480v 3 phase 200amp industrial receptacles.

    The customer wants to add 4 more receptacles. Each nameplate states 480v 3ph 18kw, requires mca 150amps.

    The customer also states that they would never run more than one truck at a time do to maintenance and testing at which they run no more than 50 amps with lighting, AC, and essential loads (cooling system for imaging machine).

    How would one calculate new service size?


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  2. #2
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    Quote Originally Posted by Ohms law View Post
    The customer also states that they would never run more than one truck at a time do to maintenance and testing at which they run no more than 50 amps with lighting, AC, and essential loads (cooling system for imaging machine).
    does the service size need to change if the demanded load isn't being changed?

    or simply additional fused discos with receptacle?

  3. #3
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    I worded this wrong. Each truck will run 50 amps max when in storage but only one would ever run the imaging equipment during preventive maintenance/ testing.


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  4. #4
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    Quote Originally Posted by Ohms law View Post
    I worded this wrong. Each truck will run 50 amps max when in storage but only one would ever run the imaging equipment during preventive maintenance/ testing.


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    I might consider this an an electrified truck parking space as defined in 626.1. Does anyone else feel this is accurate?

    Am I the only one confused by the nameplate stating 18kW, but owner states it will run a 50 A during maintenance and has a MCA of 150 A?

    50 A at 480, 3P is 41.6 kVA.

  5. #5
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    That is correct. We put a power log meter on it and that's the most we seen in a week, like I said the customer stated the power is only for storage of the trucks except for occasional PM's.

    The part that confuses me is the nameplate for the truck and the existing service that was installed however long ago is a 400amp, which leads me to believe the previous electrician sized the service to support what the nameplate states (MCA 150amps) for the 2 trucks.

    Not sure what the 18kw is in relation to.



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  6. #6
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    The silence tells me either the situation does not make sense or no one knows which way to take this. My first thought is 400 amps would be enough for storage and doing PM's on one truck at a time. I was thinking would this be kosher since we are not sizing for each truck and if that would sit well with my inspector.

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  7. #7
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    The MCA of 150A makes some sense for radiological imaging. Things like X-ray machines often have _huge_ momentary power requirements, eg a duration of a fraction of a second.

    You need a good stiff supply and fat conductors to avoid voltage drop, but the average power consumption is much lower.

    In your 'power log' what sort of average power consumption did you see, and what was the duration of the peak? Was 'PM' work going on when you saw the peak? What was the time resolution of your log, and is it possible that your peak was even higher but for a short duration?

    -Jon

  8. #8
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    That's all the info I was given from my service manager. He just gave me the info on 50 amps max in storage and to figure it out. Only one truck would ever be tested at once otherwise they are sitting with essentials running (lights, AC and cooling system for x-ray).

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  9. #9
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    Quote Originally Posted by Ohms law View Post
    That's all the info I was given from my service manager. He just gave me the info on 50 amps max in storage and to figure it out. Only one truck would ever be tested at once otherwise they are sitting with essentials running (lights, AC and cooling system for x-ray).

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    i think you should start with Article 626 and see where that puts the service size. Then ask your self if that service size makes sense for what you are trying to accomplish, run 5 truck shore loads with 1 operating at full load.

    Of course voltage drop should be considered-likely that machine can operate within 10% of rated voltage. not sure what type of load an x-ray is tbh.

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