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Thread: Urgent: question on branch circuits and devices for roadway

  1. #1
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    Urgent: question on branch circuits and devices for roadway

    I am in charge of reviewing illumination and ITS plans for a roadway project. If any mistakes are found, I adjust them.

    The original designer never received the power requirements (from another design firm) in time for the latest submittal of plans i am reviewing. i have since acquired those power numbers.

    example: on a branch circuit, I have a single cabinet (nothing else will be added to the circuit) that needs 600 W of power. By using 120 volts on the service, I know that the current necessary is 600/120=5 amps. The designer, not having this information, went conservative and decided to design for a current of 20 amps. The electrical service ordered used a KVA load and branch circuit load based on 20 amps, but what is really needed is 5 amps.

    I am not familiar enough in electricity and services so i have the following questions:

    i know the branch circuit only needs 5 amps, but 20 is provided. So long as the conductor sizes can carry the load safely, is the design safe and conservative? Or is the extra amps (15 more than required )dangerous to any of the equipment?

    Does the electrical service need to be altered at all for this to work properly, assuming it was designed with a higher KVA load due to the 20 amps? In other words, reducing the amps means reducing the KVA load necessary

    is there anything in the NEC that states the branch circuit must equal what is necessary for the device to work?

    is it possible to adjust the branch circuit from 20 amps to 5 amps after the service has been ordered? Switching to 5 amps (while maintaining the breaker size from the 20 amp design) would help tremendously with conductor and conduit sizes, but I'm not familiar enough with a service to know if the circuit and KVA load can be adjusted.

    if so, what is the time frame?

    I feel like I know enough to be dangerous. I can provide any more information regarding the design as needed if its not clear.

  2. #2
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    Three things come to mind. First, obviously, if you loads are 5 amps vs 20 amps, you can rework your voltage drop calculations and, as long as the conductor is protected at it's ampacity, keep the 20 amp over-current device. This would likely reduce conductor/conduit size. Secondly, oversizing the service would have no effect on safety. Depending on the numbers, possibly no major effect on price. we would need more details on what is proposed.
    Lastly, an over-exaggerated load could result in higher utility install costs and, again, depending on the numbers, a higher available fault current and increased gear costs.
    Can you give us an idea of the service size being considered ?
    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.

  3. #3
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    Quote Originally Posted by augie47 View Post
    Three things come to mind. First, obviously, if you loads are 5 amps vs 20 amps, you can rework your voltage drop calculations and, as long as the conductor is protected at it's ampacity, keep the 20 amp over-current device. This would likely reduce conductor/conduit size. Secondly, oversizing the service would have no effect on safety. Depending on the numbers, possibly no major effect on price. we would need more details on what is proposed.
    Lastly, an over-exaggerated load could result in higher utility install costs and, again, depending on the numbers, a higher available fault current and increased gear costs.
    Can you give us an idea of the service size being considered ?
    Example: type d service (this project is in Texas), 120/240 volts. Three branch circuits. Each circuit at 20 amps (60 total). Each breaker at 30 amps. Main breaker at 100 amps. KVA load is 60x.12=7.2. Run length: 1,000 feet.

    What should really be installed: same service, same volts, same branch circuits. Each circuit at 5 amps. Same breakers. kVA load is 15x.12=1.8.

    the smaller amps mean smaller conductors, which would prevent providing additional conduit as the wire sizes are too large for the installed conduit. But that means the branch circuit rating needs to be lowered to 5 amps from 20. I just wasn't sure if an already installed service could be adjusted in terms of branch circuit size. If so, what's the time frame?

    Note: Only one cabinet is connected from now until the end of time on this branch circuit.

  4. #4
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    Quote Originally Posted by Plano12345 View Post
    i know the branch circuit only needs 5 amps, but 20 is provided. So long as the conductor sizes can carry the load safely, is the design safe and conservative? Or is the extra amps (15 more than required )dangerous to any of the equipment?
    The current that flows is dictated by the load, not the breaker size. Breakers 'allow' current they do not actually provide it.

    There is no need to change the design, unless there are specific limitations from the equipment manufacturer.
    Just because you can, doesn't mean you should.

  5. #5
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    So the services are ok. Essentially, its larger than necessary.

    When doing voltage drop, since the branch circuit is set for 20 amps, I need to use 20 amps for voltage drop? The device really need 5. Should, I therefore, only use 5 amps?

    My worry is that over the length of the run, I have to assume that was is set for the circuit will always be there, which is 20 amps.

    My design concern is the conduit built is too small. If i have to use 20, then my only solution would be to provide more conduit?

  6. #6
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    THe service is obviously way oversized, but with the numbers involved that will cause no problem.
    Is the total load on each circuit only 5 amps ?
    Whatever the load, that is the number you should use for your voltage drop calculations.
    FWIW, for 5 amps at 120 volt, the calculator I use shows using a #4 but there a lot of things that could affect that number.
    Last edited by augie47; 09-11-13 at 10:44 PM.
    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.

  7. #7
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    Basically, only one device, a cabinet, needing 600 W, will be connected to the branch circuit. Using 120 volts, I know the current is 5 amps. The power run goes from the service to the cabinet with no other devices on the run.

    therefore voltage drop should be now done using 5 amps. 20 was done at first to be safe as the power necessary was unknown.

    For clarity, the current on the wire when the cabinet attaches will be what it needs, which is 5. There will be an additional 15 amps available from the branch circuit that will theoretically be never used.

  8. #8
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    Quote Originally Posted by Plano12345 View Post
    For clarity, the current on the wire when the cabinet attaches will be what it needs, which is 5. There will be an additional 15 amps available from the branch circuit that will theoretically be never used.
    Yes.
    Just because you can, doesn't mean you should.

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    Thank you so much for the help. I look forward to contributing when possible and asking questions when necessary.

  10. #10
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    Quote Originally Posted by Plano12345 View Post
    Example: type d service (this project is in Texas), 120/240 volts. Three branch circuits. Each circuit at 20 amps (60 total). Each breaker at 30 amps. Main breaker at 100 amps. KVA load is 60x.12=7.2. Run length: 1,000 feet.
    I'm a bit confused here. Based on the above, the breakers are 30 amps, but other posts in this thread seem to suggest that the breakers are 20 amps.

    In either case, you should check with the manufacturer of the cabinet to see if it has a rated maximum overcurrent protection (MOCP). It would not surprise me to learn that the manufacturer requires a 15 or 20 amp breaker, and connecting to a 30 amp circuit could void a warranty.

    Also, what is in this "cabinet"? Is it hard-wired lighting controls? Is there a receptacle outlet? If there's a receptacle in the cabinet (or anywhere in the circuit), the size of your breaker is going to be limited by the amp rating of the receptacle (e.g. a 20 amp receptacle can't be connected to a breaker larger than 20 amps). I'm sure your engineer knows this, but if he didn't know the load of the cabinet, chances are he didn't know what's inside it either.

    I feel for your engineer. If I had a nickel for every time an architect failed to get me necessary information but said we still had to send out drawings, ...

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