Help Me Understand

[ROBOJOE77]

Our facility has a project underway requiring (3) variable reactance power supplies which themselves require an 800 Amp breaker for each power supply. Our company has hired an outside engineering firm who is now trying to figure out where they are going to pull power from now that we're 8 months into the project who states, their exact words, "We originally approached the design using one of the 1200A breakers with a distribution or termination box splitting into the three 800A connections to the power supplies". This is a pre-existing unused 1200 Amp breaker at one of our MDP's. Can somebody explain to me how this works? These are 250 kVA power supplies @ 480VAC single phase = 521 Amps x 125% = 651.25 Amps each. How do you put this kind of load on a main distribution breaker of 1200 Amps? I'm no engineer, merely a 3rd year Plant Journeyman, but this doesn't seem to math out.

 

[david luchini]

250kVA * 3 = 750kVA.

750kVA = 902 Amps @ 480V, 3 phase

I don't see why that could go on a 1200A-3P circuit breaker.

 

[Dennis Alwon]

These are 250 kVA power supplies @ 480VAC single phase = 521 Amps x 125% = 651.25 Amps each. How do you put this kind of load on a main distribution breaker of 1200 Amps? I'm no engineer, merely a 3rd year Plant Journeyman, but this doesn't seem to math out.

Did you really mean single phase and not 3 phase?

 

[CoolWill]

Our facility has a project underway requiring (3) variable reactance power supplies which themselves require an 800 Amp breaker for each power supply. Our company has hired an outside engineering firm who is now trying to figure out where they are going to pull power from now that we're 8 months into the project who states, their exact words, "We originally approached the design using one of the 1200A breakers with a distribution or termination box splitting into the three 800A connections to the power supplies". This is a pre-existing unused 1200 Amp breaker at one of our MDP's. Can somebody explain to me how this works? These are 250 kVA power supplies @ 480VAC single phase = 521 Amps x 125% = 651.25 Amps each. How do you put this kind of load on a main distribution breaker of 1200 Amps? I'm no engineer, merely a 3rd year Plant Journeyman, but this doesn't seem to math out.

Do the calculation for 3 phase. The supplies would be balanced across all three phases. Supply 1: A-B, Supply 2: A-C, Supply 3: B-C. Collectively they form one 750 kVA 3 phase load, which the 1200 amp breaker could handle.

 

[ROBOJOE77]

Did you really mean single phase and not 3 phase?

Yes each power supply is 480 single phase, technically I guess 2 phase because like coolwill illustrates you use A-B, A-C and B-C for each power supply respectively

 

[ROBOJOE77]

Do the calculation for 3 phase. The supplies would be balanced across all three phases. Supply 1: A-B, Supply 2: A-C, Supply 3: B-C. Collectively they form one 750 kVA 3 phase load, which the 1200 amp breaker could handle.

so you are saying that the individual loads being single phase are then calculated as one collective 3 phase load because they are being distributed across a 3 phase supply, you then apply 3 phase calculations?

 

[CoolWill]

so you are saying that the individual loads being single phase are then calculated as one collective 3 phase load because they are being distributed across a 3 phase supply, you then apply 3 phase calculations?

Yes, that is correct.

 

[ron]

so you are saying that the individual loads being single phase are then calculated as one collective 3 phase load because they are being distributed across a 3 phase supply, you then apply 3 phase calculations?

Yes, as David pointed out, it would be ~900A @480V 3 Phase. It would be important to find out if the 250kVA load is continuous, because they are usually not and wouldn't require the 1.25 multiplier. But even if they are continuous, the 1200A breaker can accommodate the load with 3 taps to the smaller 250kVA equipment, requiring protection within tap rules of the tap of the 1200A feeder.

 

[ROBOJOE77]

so you are saying that the individual loads being single phase are then calculated as one collective 3 phase load because they are being distributed across a 3 phase supply, you then apply 3 phase calculations?

What about my 125% for continuous load? Does that not apply upstream to the 1200 amp breaker?

 

[ROBOJOE77]

Yes, as David pointed out, it would be ~900A @480V 3 Phase. It would be important to find out if the 250kVA load is continuous, because they are usually not and wouldn't require the 1.25 multiplier. But even if they are continuous, the 1200A breaker can accommodate the load with 3 taps to the smaller 250kVA equipment, requiring protection within tap rules of the tap of the 1200A feeder.

You posted this answer just as I was posting my question

 

[ROBOJOE77]

And what about 80% load on the 1200

 

[ROBOJOE77]

Yes, as David pointed out, it would be ~900A @480V 3 Phase. It would be important to find out if the 250kVA load is continuous, because they are usually not and wouldn't require the 1.25 multiplier. But even if they are continuous, the 1200A breaker can accommodate the load with 3 taps to the smaller 250kVA equipment, requiring protection within tap rules of the tap of the 1200A feeder.

They would be considered continuous load

 

[CoolWill]

And what about 80% load on the 1200

Look at the breaker. If it has a sticker that says something like "Rated at 100% load only when used in an enclosure..." Otherwise, look up the part number from the manufacturer. You may very well be able to use that 1200 amp breaker at 100%, and the engineering firm may have accounted for it.

 

[david luchini]

And what about 80% load on the 1200

The 1200A breaker is large enough for a 750kVA continuous load.

 

[ROBOJOE77]

Thanks guys!