Hello fellow electrical enthusiasts,

I am currently a student studying electrical engineering, currently pursuing an engineering internship and very interested in the field of power systems, especially in low voltage/ building applications.

I have a few questions about some concepts within power engineering.

Let's say we have a 3 phase circuit breaker that is supposed to be delivering power to a building. The power is calculated for each phase (a,b,c) and then the total power is adjusted (using NEC/ other code) based on demand factor, and that allows us to calculate whether or not we're overloading the breaker (for example, some breakers have 225 amp rating).

Let's say I calculate 20kVA per phase (say we're doing something simple like receptacles/ lights). Does this mean the total power being used by the breaker is 60kVA, or is it 20kVA * sqrt(3), which is 34kVA. Is the 225 amp rating per phase, or is it the total current being absorbed at once by the system?

Now let's also assume the breaker is connected to a 3 phase motor. This motor draws 14,411 VA per phase. The strange thing is, that the motor is rated at 20 Horsepower, which is equal to 14,411 Watts. But if you add up all the phases (14,411*3), this gives you 58 horsepower.

Another thing that is confusing me. If we have a panel that isn't connected to anything, let's say it's rated at 225 Amps at 120 volts. Does that mean this panel is using power, even though it's connected to nothing?

Thank you for any help

I am currently a student studying electrical engineering, currently pursuing an engineering internship and very interested in the field of power systems, especially in low voltage/ building applications.

I have a few questions about some concepts within power engineering.

Let's say we have a 3 phase circuit breaker that is supposed to be delivering power to a building. The power is calculated for each phase (a,b,c) and then the total power is adjusted (using NEC/ other code) based on demand factor, and that allows us to calculate whether or not we're overloading the breaker (for example, some breakers have 225 amp rating).

Let's say I calculate 20kVA per phase (say we're doing something simple like receptacles/ lights). Does this mean the total power being used by the breaker is 60kVA, or is it 20kVA * sqrt(3), which is 34kVA. Is the 225 amp rating per phase, or is it the total current being absorbed at once by the system?

Now let's also assume the breaker is connected to a 3 phase motor. This motor draws 14,411 VA per phase. The strange thing is, that the motor is rated at 20 Horsepower, which is equal to 14,411 Watts. But if you add up all the phases (14,411*3), this gives you 58 horsepower.

Another thing that is confusing me. If we have a panel that isn't connected to anything, let's say it's rated at 225 Amps at 120 volts. Does that mean this panel is using power, even though it's connected to nothing?

Thank you for any help

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