Forums » Electrical Engineering

Electric water heater

    • 13 posts
    April 15, 2019 9:02 PM PDT

    I am considering to supply power to a water heater, but I am not sure if I am correct

    Water heater ( 21kW, 3 phases, 400V )

    currently existing the main fuse three times 25A

    can someone tell me if that's enough for the water heater plus 3kW the rest of the house plus a Oven.

  • April 15, 2019 11:07 PM PDT

    I am not familiar with 400V, 3 ph residential services.

    The best I can tell you is:

    400V x 25A x sqrt(3) = 17.3KW

    21kw at 400V, 3 ph gives current is 21,000/400/sqrt(3) = 30.3A

    You say "house", the biggest water heater I have ever seen in a house is 5KW.  Just curious, what is the purpose of the 21kw Water heater?  It certainly is not for heating water for the showers.


    • 160 posts
    April 16, 2019 3:27 AM PDT

    If the water heater meets the following info then you need to take carl's number and multiply by 125% for continuous load= 38 amps or a 40 amp breaker or fuse


    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.

    Obviously 25 amps is not enough especially when you add the 3 kw and ovens.  Hard to know since you did not provide info on the ovens.  Is this a residence?


    • 200 posts
    April 16, 2019 3:55 AM PDT
    Thanks for the suggestion of this topic, Syed. Sorry for the Android edition.

    Let's identify the phases first:
    all the countries in the world are divided in 50 Hz or 60Hz, Japan has both (50 Hz North of Tokyo and 60Hz South of Tokyo). Easy isn't it?
    But when we deal with voltage for Residential, population use, the things become complex, when we say 110V it can be 100V (Japan again, the lowest) until near130V (most common highest is 127V) each phase. When we say 220V it can be 200V until near 260V (most common highest is 240V) each phase or 2-phase depending on the country. Now, when we say 400V or 380V (a common labeling) it can be something near 300V until near 480V (most common highest is 440V, but not sure) 3-phase for 110V countries, or 2-phase for 220V countries.

    When Syed says "3 phases, 400V" we could assume it is a 110V-country, and it will be used all 3 phases to get the required 400V. Also, Water Heaters are usually (pure) Resistive Load meaning in Syed case there are at least 3 heating resistances, one per 110V phase, supposedly. Pure resistive load demands active power (W) and none of reactive power (VAr), the calculations are simpler, no need of sqrt(3), such as:
    I = V/R or V = R × I or R = V/I

    P = V × I or P = V^2/R or P = I^2 × R

    So if the Water heater nameplate says: 21kW, 3 phases, 400V we can calculate as:
    I = P/V = 21kW/400V = 52.5A each phase. Let's use 53A to make it easy.

    Another option would be divide the power equally among phases and divide by local voltage assumed here as 110V: 7kW/110V = 63.6A each phase, but we will work with Nameplate data here as good Practice. Anyway, it is a big stuff as Carl said, a big house in very cold winter... with a tropical climate indoors perhaps.

    So, "plus 3kW the rest of the house plus a(n) Oven" supposedly WELL distributed among 3 phases can be active or reactive power, calculate both WITH and WITHOUT sqrt(3), and choose the HIGHEST current as advice. Assuming the "plus 3kW" (supposedly apparent Power of 3kVA could be estimated by Syed) is well distributed and the "Oven" is single-phase(?, resistive? microwave?):
    Iap = 3kVA/(sqrt(3)×400V) = 4.3A per phase
    I = 1kW/110V = 9.1A per phase.

    The current per phase of "plus 3kW the rest of the house" would be something between 4.5A to 9A each phase. Use 9A as advice, summing with Water Heater power of 53A each phase makes 62A each phase, too high for your current 25A fuses.

    What to do? Increase Fuses rating? NO!

    Again, repeating myself:
    "SAFETY FIRST" motto, taking the risk of be seen as a boring repetitive person...

    The primary purpose of a Circuit Breaker or a FUSE is to protect the CONDUCTOR. Protection for a final equipment is a secondary purpose.

    If the CONDUCTORS WILL NOT BE REPLACED, then your guide will be the CONDUCTORS ALREADY INSTALLED: check the CONDUCTORS of the house, what their size? What their current capacity (ampacity)? Can they handle the new Water Heater? Probably NOT. The Fuses rating can be changed ONLY if the CONDUCTORS allow it.

    If the CONDUCTORS DON'T ALLOW it, a good suggestion is to install a SEPARATED circuit for the Water Heater with new CONDUCTORS (ampacity higher than new Fuses/CBs rating) and new Fuses/Circuit Breakers (rating higher than 53A + 15%)... and leave "the rest of the house" alone.

    I hope this could be of any help. Regards.