3 phase question
3 phase question
(OP)
I have a question about breakers. If I have a 20 amp 3 pole breaker. On a 208v 3 phase 4 wire load. Do I have 20 amps per phase capacity to ground or 20 amps total phase to phase ? Example : I am running a 3 phase 208v 4 wire to 3 1000w heaters. 1 heater per phase and a nuetral. The heaters are 120v single phase. If I use a 3 pole breaker. Does this give me 20 amps per phase to ground ? Just like a single pole breaker would do for the same load for 1 heater ? Does this differ at all from running 3 seperate single pole breakers except for the fact that if 1 would trip it would open all 3 legs.
Also. In a single phase system. House power 240v. Does a branch curcuit of say 30 amps provide 30 amps per pole or 30 amps total ?
Also. In a single phase system. House power 240v. Does a branch curcuit of say 30 amps provide 30 amps per pole or 30 amps total ?






RE: 3 phase question
It is per pole in both cases.
Don(resqcapt19)
RE: 3 phase question
Since your load is 4-wire wye connected, that pole current also corresponds to the current to ground within each leg of the wye.
If you had a delta connection, the pole current would likely be larger than any of the phase-to-phase currents within the legs of the delta. But you probably wouldn't be able to measure those.
If you are feeding resistive loads, your 208v 4-wire 20A system behaves very much the same as three independent single phase 20A 120v systems (except that the balanced return currents from the 3 systems cancel out... only the imbalance flows in the neutral).
If you were feeding motor loads, you can get different performance from your 3-phase 208 system then you could from three single-phase systems. Higher voltage and three-phase construction both offer possibilities for improved motor efficiency.
RE: 3 phase question
if any one circut overloads you loose all 3. better to
use 3 20A breakers. no need to dredge through hours of
learned therory to look at common sense.
RE: 3 phase question
Julian
RE: 3 phase question
In the U.S. if your panel isn`t rated for 100% . You can only load a circut 80%, if it is on for 3 hours or longer.
In 3 ph loads the breaker should have a common trip, so it breaks all 3 phases at once. Since your load is resistive
your 20 amp breaker could safely carry 16 amps per leg.
Your load would be 1000/120=8.33 amps per phase. The way you
put this it would be 16 amps per phase to neutral.
RE: 3 phase question
Ravenjoe has 3 120v single phase heaters, NOT
a 3ph motor. He does NOT have a 3ph load. I
would never recomend single breakers for a 3ph
load. If Rj lives where it gets below freezing,
he wouldn't want to loose all 3 heaters because
of 1 failing.