what rating of circuit breaker to be used in 100KW load,3phase?

Question

for instance, we have a load of 100 KW,3phase. then we need protection for that load. What should be the rating of the Circuit breaker?

Answer 1

Circuit breakers have three ratings. Voltage, Current, and Fault Duty. The difficulty is that you have not provided any of these three. However I will try to provide answers where you can insert the missing data.

Voltage:
Common 3-phase voltages in the US are 120/208, 120/240, and 277/480 V. I will assume your supply voltage is either 120/208 V or 277/480 V.

Current:
We need to convert form kW to amps. If your equipment does not have a nameplate rating for amperage, we'll have to assume a power factor. I will assume a conservative value of 0.80.

100 kW ÷ 0.80 = 125 kVA

Now that we have the load in kVA, we can compute amps by knowing the supply voltage. Since this is a 3-phase service, we take the load and divide by the phase-to-phase voltage and then by √3. (The √3 can be approximated as 1.73)


125 kVA ÷ .208 kV ÷ 1.73 = 347 amps.
125 kVA ÷ .480 kV ÷ 1.73 = 150 amps.

The National Electric Code (NEC) requires you to size your equipment to include 125% of the largest motor. If the entire load was one motor, we'd need to up the amperage value by 25%.

347 amps x 1.25 = 433 amps
150 amps x 1.25 = 188 amps

The NEC also limits the load on your switchgear to only 80% of its nameplate rating. This means that a 600 amp switch can only be loaded to 600 x 0.80 = 480 amps.

Common switch sizes include 200, 400, 600 and 800 amps.


Fault Duty:
The final piece of data needed to size the equipment is the availabe fault current. This is furnished by the electric utility. Most assume an infinite bus and neglect the length of secondary cables. The resulting fault current is only dependent on the transformer used to serve the customer. The two attributes that determine the fault current are the size (kVA) and the impedance (%Z) of the transformer.


At this point you should have enough information to fill in the missing data and complete the calculations. Good Luck!

Answer 2

Considering the surge of an air compressor of that size, you would need to change the 50 AMP breaker you currently have to a 30 AMP breaker. If the wire that is in place was connected to the air compressor once before it should be sufficient to wire in. I would think 10/2 with a ground would be the best choice. Make sure you shut off the disconnect at the meter base before adding / replacing the circuit breaker. Your more experienced electricians may work on this while it is "hot", but I would not reccomend it with the novice electrician.. Goodluck!

Answer 3

Depends on a couple of factors, most importantly, what is the voltage? The nature of the load also makes a difference (though less than the voltage) power factor should be considered, and for a motor load, starting current should be considered .