3 phase question

[RavenJoe]

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 ?

 

[resqcapt19]

Joe,
It is per pole in both cases.
Don(resqcapt19)

 

[electricpete]

As per Don, no pole can have more than 20A.

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.

 

[sbc]

simple, you have 3 20A circuts, with a 3 pole breaker
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.

 

[230842]

When mains feed three phase devices in general, such as motors, SBC proposal is not convenient at all as they need three lines, and eventually neutral, to correct operation. Otherwise, malfunctions or damage could occurr. Julian

 

[toma]

To RavenJoe
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.

 

[sbc]

To 230842 & toma,
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.