I am a mechanical engineer with basic understanding of electricity. I am programming an app for ordering 3 phase heaters, and I have an argument to resolve with a coworker. Lets say I have a 208V 3-phase heater (delta connection), with a heating element on each leg L1, L2, L3. The total KW of the heater is 15 KW. My calculation for amps in each leg is 15KW*1000/[208V*1.73] = 41.7 amps. On each leg there is a thermal limit switch with 25 amp rating. Since I1 = I2 = I3 = 41.7 amps in each leg, I need to split into two branches to divide the amps into two because of the 25 amp limit switch.
A coworker is arguing the following: Each element is producing 15KW/3 = 5 KW per element. Treating each leg separately, one can treat it as a 'single phase' circuit with 208V across the element. So amps per leg is 5 KW *1000/208V = 24 amps. So no need to branch for the thermal limit switch.
Obviously one can't come up with two different answers. My thought is that even though on average each element is producing 5 KW of heat, instantaneously it might be producing more than that, as long as all three elements together are producing 15 KW at any given instant. So instantaneous amps would be 24 amps*1.73 = 41.7 amps (to reconcile the two approaches). With a Wye connection my coworker's argument would work because the voltage across each element is 120V. But intuitively I don't know why it works for Wye but not for delta connections.