Circuit Breaker for Air Compressor

[Electrical_]

Hi

I have to decide on a circuit breaker for an air 22 KW, 3 phase 400V Air compressor.
Load current = P / (1.732 * V *PF) =22000 /(1.732*400*0.8)
= 39.69A
Imposing the 125% CB rule in accordance with NEC,
Current = 39.69*1.25 = 49.61A

Therefore a 50 A Type C CB is used.

These are my questions:
1. Do we have to include the PF if not given in the equipment datasheet for current calculation.
2. The 125% used above is according to NEC (American) standards. Do we have to include it while calculating CB size for UK installations following IEC Code.
3. Is this the right way to find the size of MCB?

Thank you in advance for your help.

 

[zbang]

It's not as simple as that. Have you read Article 430 about motors? You need to take into account the starting surge and that, at least in the USA, the overcurrent device (breaker) is more to protect the wires than the motor; an overload device does that.
(and convert KW to HP for the question)

 

[Electrical_]

Hi. Thank you for replying. You are talking about over load relay right. Im talking about the 3 pole MCB used to draw power from the main supply.
Is that calculated correctly?

 

[tom baker]

Few here are knowledgeable about the IEC.
Is this a 30 hp motor?

 

[zbang]

Yes.
Probably not, since a motor is involved (the maths are correct, the basic principle is not).

ETA- you're only going to get USA & National Electrical Code answers from this forum.

 

[don_resqcapt19]

Hi

I have to decide on a circuit breaker for an air 22 KW, 3 phase 400V Air compressor.
Load current = P / (1.732 * V *PF) =22000 /(1.732*400*0.8)
= 39.69A
Imposing the 125% CB rule in accordance with NEC,
Current = 39.69*1.25 = 49.61A

Therefore a 50 A Type C CB is used.

These are my questions:
1. Do we have to include the PF if not given in the equipment datasheet for current calculation.
2. The 125% used above is according to NEC (American) standards. Do we have to include it while calculating CB size for UK installations following IEC Code.
3. Is this the right way to find the size of MCB?

Thank you in advance for your help.

For the NEC you need to use the motor full load currents from the tables at the end of Article 430 for the sizing of the motor circuit conductors and for the branch circuit short circuit and ground fault protection. Note that the NEC sizes motors based on horsepower and not on KW. Also 400 vols is not a voltage that you will find in the motor current tables in the NEC.
If this equipment is to be used in the US and you need 400 volts, a transformer will likely be required. The typical motor voltage here is 480. Also note that you may have a frequency issue because we use 60 hZ here.

 

[alexfox1986]

Hi

I have to decide on a circuit breaker for an air 22 KW, 3 phase 400V Air compressor.
Load current = P / (1.732 * V *PF) =22000 /(1.732*400*0.8)
= 39.69A
Imposing the 125% CB rule in accordance with NEC,
Current = 39.69*1.25 = 49.61A

Therefore a 50 A Type C CB is used.

These are my questions:
1. Do we have to include the PF if not given in the equipment datasheet for current calculation.
2. The 125% used above is according to NEC (American) standards. Do we have to include it while calculating CB size for UK installations following IEC Code.
3. Is this the right way to find the size of MCB?

Thank you in advance for your help.

Hello! I'm happy to help answer your questions about circuit breakers.

Yes, it's important to include the power factor (PF) in your current calculation, since it affects the amount of apparent power (VA) that the compressor will draw from the electrical system. Without knowing the PF, you wouldn't be able to accurately calculate the actual current (in amperes) that the compressor will draw from the system. In your calculation, you assumed a PF of 0.8, which is a reasonable assumption for a typical motor load.

The 125% rule is specific to NEC (National Electrical Code) standards, and it's used to provide a margin of safety for the circuit breaker sizing. In the UK, the IEC (International Electrotechnical Commission) code is followed, and it has its own guidelines for circuit breaker sizing. In general, IEC standards tend to be more conservative than NEC standards, so it's possible that the required circuit breaker size in the UK could be larger than what's required in the US. It's important to consult the relevant code or standard for the specific installation to ensure that the circuit breaker is sized correctly.

Yes, the method you used to calculate the circuit breaker size is correct. You calculated the load current based on the compressor's power rating, voltage, and assumed power factor, and then applied the 125% rule to determine the minimum circuit breaker size required for the application. However, as I mentioned earlier, it's important to consult the relevant code or standard for the specific installation to ensure that the circuit breaker is sized correctly.

I hope this helps! Let me know if you have any further questions.

 

[topgone]

Hello! I'm happy to help answer your questions about circuit breakers.

Yes, it's important to include the power factor (PF) in your current calculation, since it affects the amount of apparent power (VA) that the compressor will draw from the electrical system. Without knowing the PF, you wouldn't be able to accurately calculate the actual current (in amperes) that the compressor will draw from the system. In your calculation, you assumed a PF of 0.8, which is a reasonable assumption for a typical motor load.

The 125% rule is specific to NEC (National Electrical Code) standards, and it's used to provide a margin of safety for the circuit breaker sizing. In the UK, the IEC (International Electrotechnical Commission) code is followed, and it has its own guidelines for circuit breaker sizing. In general, IEC standards tend to be more conservative than NEC standards, so it's possible that the required circuit breaker size in the UK could be larger than what's required in the US. It's important to consult the relevant code or standard for the specific installation to ensure that the circuit breaker is sized correctly.

Yes, the method you used to calculate the circuit breaker size is correct. You calculated the load current based on the compressor's power rating, voltage, and assumed power factor, and then applied the 125% rule to determine the minimum circuit breaker size required for the application. However, as I mentioned earlier, it's important to consult the relevant code or standard for the specific installation to ensure that the circuit breaker is sized correctly.

I hope this helps! Let me know if you have any further questions.

Well, you could use 125% in sizing the circuit breaker for a motor load. But be ready to suffer if the breaker frequently trips on starting! The NEC allows one to use up to 250% on inverse time CBs or 175% for time-delay fuses. Unless the starting method is not DOL, typical circuit breakers will find a hard time not tripping on starting. Been there, done that, and got laughed at when I was green, you know?

 

[electrofelon]

Hello! I'm happy to help answer your questions about circuit breakers.

Yes, it's important to include the power factor (PF) in your current calculation, since it affects the amount of apparent power (VA) that the compressor will draw from the electrical system. Without knowing the PF, you wouldn't be able to accurately calculate the actual current (in amperes) that the compressor will draw from the system. In your calculation, you assumed a PF of 0.8, which is a reasonable assumption for a typical motor load.

The 125% rule is specific to NEC (National Electrical Code) standards, and it's used to provide a margin of safety for the circuit breaker sizing. In the UK, the IEC (International Electrotechnical Commission) code is followed, and it has its own guidelines for circuit breaker sizing. In general, IEC standards tend to be more conservative than NEC standards, so it's possible that the required circuit breaker size in the UK could be larger than what's required in the US. It's important to consult the relevant code or standard for the specific installation to ensure that the circuit breaker is sized correctly.

Yes, the method you used to calculate the circuit breaker size is correct. You calculated the load current based on the compressor's power rating, voltage, and assumed power factor, and then applied the 125% rule to determine the minimum circuit breaker size required for the application. However, as I mentioned earlier, it's important to consult the relevant code or standard for the specific installation to ensure that the circuit breaker is sized correctly.

I hope this helps! Let me know if you have any further questions.

The general "125% rule" in the NEC is for continuous loads and is used to size conductors and OCPD's. For motors, we are required to use 125% of the motor (or the largest motor if there are multiple) full load current as shown in the charts in article 430 to size conductors. As topgone said, you are allowed to go higher for the ocpd and typically would.