Newbie question abt 3 phase and breaker size

Discussion in 'General Electronics Chat' started by alby1, Nov 5, 2007.

  1. alby1

    Thread Starter New Member

    Nov 5, 2007
    3 phase confuses me. I have a 15kw 3 phase electric furnace that I want to hook-up. The rating says it’s a 208volt, 41.7 amps current pull. I need to buy a breaker for this and don't know what size to buy. My question; is the 41.7 amps the total amps this unit will draw across the three legs? In other words, do I buy a 20amp three phase breaker, 20 amps for each leg? or Do I buy a 60 amp three phase beaker, cause each leg will be drawing the 41.7 amps? I hope this isn't to confusing for those that read this. :confused:
  2. subtech

    Senior Member

    Nov 21, 2006
    Hello alby1

    In this case, each phase or leg will measure 41.7 amperes.

    Line current for a delta connected 3 ph load is as follows:

    3 ph Watts / (volts line-line X sqrt3)

    in your circuit:
    15,000 / (208 X 1.732) = 41.63 amperes (line current)
    Considering the choices given in your post, the 60 ampere 3 pole breaker seems reasonable.
    Be sure to size your wire appropriately and follow any codes that apply in your area.

    I would like to suggest that you verify what I have stated here for yourself by checking here:
    Best of luck with your project.