Forums » Electrical Engineering

CB or OCPD rating of the feeder.

    • 2 posts
    October 4, 2019 12:15 PM PDT

    I'd like to just comment on the CB or OCPD rating of the feeder. The ampacity is determined by the method described, 125% of the anticipated full load. The OCPD then can be selected base on that. For feeders, the standard size value for OCPD can be rounded down from that ampacity rating, not up as required for branch circuits.

    • 141 posts
    October 4, 2019 3:20 PM PDT

    The 125% is not always needed as it would depend on the loads served.  For instance, a sub panel with a calculated load of 50 amps would not need to have 125% added to it unless the load was continuous.  So you could have a non-continuous load calculated at 60 amps and still use a 60 amp overcurrent protective device.

     

    Section 240.4(B) allows us to use the next size overcurrent protective device if the load is not a standard cb size as long as the load isn't larger than the conductors serving the load.

    240.4(B) Overcurrent Devices Rated 800 Amperes or Less. The
    next higher standard overcurrent device rating (above the
    ampacity of the conductors being protected) shall be permitted
    to be used, provided all of the following conditions are met:
    (1) The conductors being protected are not part of a branch
    circuit supplying more than one receptacle for cord-andplug-
    connected portable loads.
    (2) The ampacity of the conductors does not correspond with
    the standard ampere rating of a fuse or a circuit breaker
    without overload trip adjustments above its rating (but
    that shall be permitted to have other trip or rating adjustments).
    (3) The next higher standard rating selected does not exceed
    800 amperes.

     

     

    215.2(A)(1) General. Feeder conductors shall have an ampacity not
    less than required to supply the load as calculated in Parts III,
    IV, and V of Article 220. Conductors shall be sized to carry not
    less than the larger of 215.2(A)(1)(a) or (b).
    (a) Where a feeder supplies continuous loads or any
    combination of continuous and noncontinuous loads, the
    minimum feeder conductor size shall have an allowable ampacity
    not less than the noncontinuous load plus 125 percent of
    the continuous load.

    • 34 posts
    October 4, 2019 9:20 PM PDT
    Stacy Littleton said:

    I'd like to just comment on the CB or OCPD rating of the feeder. The ampacity is determined by the method described, 125% of the anticipated full load. The OCPD then can be selected base on that. For feeders, the standard size value for OCPD can be rounded down from that ampacity rating, not up as required for branch circuits.

    You are correct the feeder must be sized to have an ampacity at least (minimum) 125% of the load, if it is a continuous load, as stated in 215.2(A)(1)(a). The overcurrent device must also be sized to at least 125% of a continuous load as stated in 215.3. You might be able to round down from the conductor ampacity but it cannot be rated less than 125% of the load, whether or not it is a standard size. These rules are the same for branch circuits in 210.19 and 210.20.

    • 73 posts
    October 4, 2019 10:34 PM PDT

    Article 215 of the NEC covers Feeders.  The method you described will always result in a safe installation,  but not always the smallest OCPD & conductors. 

    Where a feeder supplies loads or any combination of continuous and noncontinuous loads, the minimum feeder conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.  This is after any adjustments or correction factors.

    As already stated you can round up until you reach 800A if the ampacity of the conductors does not correspond to the standard size. You can round up, but you are not required to round up.

     


    This post was edited by Steve Ward at October 4, 2019 10:37 PM PDT