Forums » Electrical Engineering

Branch circuit

    • 2 posts
    August 19, 2019 5:29 AM PDT

    In a family dwelling with 50 gallon water heater what would be the size of branch circuit for this. Power in nameplate is 5kW

    • 37 posts
    August 19, 2019 6:42 AM PDT

    12 AWG is enough.

    • 2 posts
    August 19, 2019 6:45 AM PDT

    Thank you. Forgot to mention the voltage level 240V

    • 148 posts
    August 19, 2019 8:13 AM PDT

    5000/240 x125%= 26 amps

     

    #10 wire with a 30 amp ocpd


    This post was edited by Dennis Alwon at August 19, 2019 8:20 AM PDT
    • 148 posts
    August 19, 2019 8:17 AM PDT

     Overcurrent protective device can be 150% of the rating however the wire size would be larger than a #10.  Thus you could use a 35 amp breaker but the wire would need to be #8.  No reason to do this..

     

    422.11 (E) Single Non–Motor-Operated Appliance. If the branch
    circuit supplies a single non–motor-operated appliance, the
    rating of overcurrent protection shall comply with the following:
    (1) Not exceed that marked on the appliance.
    (2) Not exceed 20 amperes if the overcurrent protection
    rating is not marked and the appliance is rated
    13.3 amperes or less; or
    (3) Not exceed 150 percent of the appliance rated current if
    the overcurrent protection rating is not marked and the
    appliance is rated over 13.3 amperes. Where 150 percent
    of the appliance rating does not correspond to a standard
    overcurrent device ampere rating, the next higher standard
    rating shall be permitted.

    422.13 Storage-Type Water Heaters. A fixed storage-type
    water heater that has a capacity of 450 L (120 gal) or less shall
    be considered a continuous load for the purposes of sizing
    branch circuits.

     

    422.10 Branch-Circuit Rating. This section specifies the
    ratings of branch circuits capable of carrying appliance current
    without overheating under the conditions specified.
    (A) Individual Circuits. The rating of an individual branch
    circuit shall not be less than the marked rating of the appliance
    or the marked rating of an appliance having combined loads as
    provided in 422.62.
    The rating of an individual branch circuit for motor operated
    appliances not having a marked rating shall be in
    accordance with Part II of Article 430.
    The branch-circuit rating for an appliance that is a continuous
    load, other than a motor-operated appliance, shall not be
    less than 125 percent of the marked rating, or not less than
    100 percent of the marked rating if the branch-circuit device
    and its assembly are listed for continuous loading at
    100 percent of its rating.
    Branch circuits and branch-circuit conductors for household
    ranges and cooking appliances shall be permitted to be in
    accordance with Table 220.55 and shall be sized in accordance
    with 210.19(A)(3).


    This post was edited by Dennis Alwon at August 19, 2019 8:20 AM PDT
    • 39 posts
    August 19, 2019 8:41 AM PDT

     Edit to add:  I'm way late. Dennis has all this covered -

    2017 NEC

    Reference:

    422.13 Storage Type Water Heaters

    A fixed storage-type water heater that has a capacity of 450 L (120 gal) or less shall be considered a continuous load for the purposes of sizing
    branch circuits.

    Informational Note: For branch-circuit rating, see 422.10.

    5KW at 240V = 21A  (20.833 ...A)

    Rated continuous  >>  20.8 X 1.25 = 26A, which would require a #10CU.

     

    That being said, recommend checking the water heater nameplate.

     

    carl

     


    This post was edited by Carl Coulter at August 19, 2019 9:33 AM PDT
    • 148 posts
    August 19, 2019 8:58 AM PDT

    Here is something to be careful with these water heaters.  If it is a residential one it is usually not an issue but if the water heater is 5kw rated at 240 volts and you connect to a 208 volt system you would then have about 75% less wattage at that voltage and hence lower ampacity.

     

    5000 watt heater at 240 will be 3750 watts at 208

    AMps= 21 amps without the 125%

    R = V^2 / P

    R= 240 x 240 / 5000=  11.52

     

    P= V^2/ R

    P = 208x 208 / 11.52 = 3755

     

    or 5000 x 75%= 3750--- pretty close