What is the Actual Current through 2P, 3P Breaker

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David40

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Breakers rarely operate at 100% of their rating but for the sake of this question I'm going to assume 100% to make it easier to understand.

This is something I have thought about for a long time and was never able to wrap my brain around.

When I install a 200 Amp Service panel I am installing wire rated for 200 Amps. Now that's a 200 Amp capacity per leg. Since I have two legs each capable of carrying 200 amps are the breaker and the panel really capable of carrying 400 Amps total capacity?
Same thing with 3 phase. If I had a 3 pole 3 phase breaker rated at 200 Amps isn't the actual capacity of that system 3 legs X 200 @ phase = 600 Amps of possible power available? (Real life continuous 480 Amps @ 80%)
If so, why don't they label the equipment with the actual ratings?

Obviously I'm not an engineer. I would appreciate it if someone could enlighten me.

Thanks :confused:
 

hurk27

Senior Member
Breakers rarely operate at 100% of their rating but for the sake of this question I'm going to assume 100% to make it easier to understand.

This is something I have thought about for a long time and was never able to wrap my brain around.

When I install a 200 Amp Service panel I am installing wire rated for 200 Amps. Now that's a 200 Amp capacity per leg. Since I have two legs each capable of carrying 200 amps are the breaker and the panel really capable of carrying 400 Amps total capacity?
Same thing with 3 phase. If I had a 3 pole 3 phase breaker rated at 200 Amps isn't the actual capacity of that system 3 legs X 200 @ phase = 600 Amps of possible power available? (Real life continuous 480 Amps @ 80%)
If so, why don't they label the equipment with the actual ratings?

Obviously I'm not an engineer. I would appreciate it if someone could enlighten me.

Thanks :confused:

The rating for a service is and has been the connected line capacity, if stated other wise would lead to confusion, each line will have a rating that corresponds to the line current it can handle, to figure how much current on 120 volts it can handle is just a function of converting to KVA the back to amp @120 volts
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
You are experiencing a very common misconception.
Now that's a 200 Amp capacity per leg.
No it is not. It would serve you well to break yourself of the habit of using the word "per" in this context. It has a tendency to induce a person to make the mistake you made in the following line:
Since I have two legs each capable of carrying 200 amps are the breaker and the panel really capable of carrying 400 Amps total capacity?
The amps that leave the breaker on one leg will return to the breaker on the other leg. It is the same amps. Take an apple in your left hand, and transfer it to your right hand. Just because each of your hands touched an apple, does that mean that you have touched a total of two apples? It does not.


The same thing happens in the three phase system, though it is a bit trickier to visualize. Current that leaves the breaker on phase A will return to the breaker on phases B and C. Here again, it is the same amps.
 

Chamuit

Grumpy Old Man
Location
Texas
Occupation
Electrician
. . . When I install a 200 Amp Service panel I am installing wire rated for 200 Amps. Now that's a 200 Amp capacity per leg. . .

The 200A is the capacity of the whole breaker in AMPS like Charlie said.

You use the 200A rated wire because of the possibility of imbalance.
 

Chamuit

Grumpy Old Man
Location
Texas
Occupation
Electrician
The rating for a service is and has been the connected line capacity, if stated other wise would lead to confusion

An example of what Wayne is talking about:

200A 3P 208V = 72000VA
200A 3P 230V = 79600VA
200A 3P 240V = 83200VA
200A 3P 440V = 152400VA
200A 3P 460V = 159400VA
200A 3P 480V = 166200VA

Could you imagine the label needed for all that info!
 

dbuckley

Senior Member
When I install a 200 Amp Service panel I am installing wire rated for 200 Amps. Now that's a 200 Amp capacity per leg. Since I have two legs each capable of carrying 200 amps are the breaker and the panel really capable of carrying 400 Amps total capacity?
Same thing with 3 phase. If I had a 3 pole 3 phase breaker rated at 200 Amps isn't the actual capacity of that system 3 legs X 200 @ phase = 600 Amps of possible power available?

You can think of it in this way, and sometimes you'll be right.

If those panels are loaded exclusively with single phase L/N loads, then you can get 400A of single phase 120V load out of a 200A 240/120 service, and 600A of single phase 120V load out of a 200A 208/120 three phase service. So a 200A service can run (3x200x120) 72KW of heaters. Give or take 80%! It makes the math very quick and easy.
 
This is common termnology in the movie genset business

This is common termnology in the movie genset business

Here is LA where there are a lot of movie generator rental companies, they all list and price their gensets on the total 120V amps of the genset. So if it will put out 200A on each phase to neutral line, they call it 400A at 120V. Of course, it gets very muddy very quickly when it is a 3 phase genset that can have it's output switch turned to 1 phase only output, which is some percentage (i think 63) of it's rated KW at 3-phase.

I do a lot of work for studio lighting companies, and a lot of times they begin with " we need 1200A for the lighting dimmer board", which sounds like a real heavy load, until you get to the real answer, which is how many KW, and is it a 1-phase or 3 phase board.
 
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