breaker sizing and amp.per leg

[iceman77]

I went and did my first load test that was a 300A square D
breaker with 350 on line side and 4/0 on load right away i say under size i take voltage and its a highleg system at the breaker its 240v on c phase 124v on a & b phase then goes to ATS and stays on c phase until it leaves ATS on B phase its all 4/0 to gen ATS & panel so I did load test and range 100 a on each phase so i tell my boss its under sized and maxed out and i can't hook up a temp 100A service for a trailer.
well 4/0 is good for 230A and you can put 300A on each leg and the trailer will only use 40 to 50A if thats true why don't we call it a 900A service
I remember asking my teacher why can't I run 3 #3 to 300Amp breaker and he said because it has the potential to put it all on one phase. so if he is right if i add the 30A to this panel it will trip if my boss is right i can put 900A on this panel before it trips or am i lost?

 

[charlie b]

I am sorry, but I do not understand your description.

But I will say that on a three phase system, any current that leaves the source on Phase A will return to the source on either Phase B, or Phase C, or both. It is essentially the same current going out and coming back. So if you have 300 amps on Phase A, 300 amps on Phase B, and 300 amps on Phase C, you do not have a "total" of 900 amps, because it is the same amps. Does that clarify anything, or does it make it muddier?

 

[Pierre C Belarge]

Try to think of it this way:

There is a 3 phase, 208 volt panel (service or subpanel) that has a 300 amp overcurrent disconnecting means.
Phase A - can carry up to 300A
Phase B - can carry up to 300A
Phase C - can carry up to 300A

Each leg can carry 300 amps, so we call it a 300 amp service or subpanel. These values are not additive, in other words we do not add 300 + 300 + 300 = 900

Each conductor that is terminated to the lug of the Circuit breaker (in 3 phase, there are generally 3 lugs) will be required to be sized as per the rating of the overcurrent device.

 

[iceman77]

so if i have a 300amp breaker and i take an ampreading on A phase 100A B phase 100A C phase 100A is this system maxed out or can you put 200 more amps on each phase.

 

[charlie b]

You can put 200 more amps on each phase.

 

[iceman77]

ok i was looking at it as one when its Just like fuses. I was looking at the rating of the fingers witch is 150V witch means i could only hook up a 150A sub panel unless i come off the bus bar witch is 400A panel so each buss is rated 400A

 

[LarryFine]

Ice, this can be confucing to a newbie to 3-phase power, but it's not all that different from 1-phase, as far as your questions are concerned. With a 120/240v 1ph system, 100a at 120v is half as much power as 100a at 240v is.

When you have a greater voltage supply, or two hot wires instead of one (or three hot wires instead of two), you have more overall voltage, but the current is the same. 100a at 120v or 240v is still 100a, it's merely 100a at a higher voltage.

The idea is to deliver a given amount of power (watts, which translates to work done) economically. Raising the voltage (or "adding more hot wires") can allow you to deliver more power without having to deliver more current.

Why that matters is that insulation is cheaper than conductor. In other words, for a given power, higher voltage can cost less to deliver than higher current. That's why power is distributed at the highest practical voltage.

 

[LarryFine]

. . . the bus bar ... is 400A panel so each buss is rated 400A

That's exactly right.