Circuit Breaker for Air Compressor

[satxx]

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.

 

[kerching]

Why are you quoting Yank regs?
What do MIs say?
Is it star delta or softstart or what?
I would have thought a D type may be better

 

[apprentice87]

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.

Ok, so you have 22kw load. Ignoring funny american things, this is what i would do..

22kw/400/1.732 comes out at 31.75A Then we have power factor, lets say 0.8 and efficiency, again, say 85%. By the time we account for these, we have just under 47A This is the figure to use..

To answer your questions;

1, Yes, as this gives the actual current your compressor will pull
2, forget the american stuff
3, Yes

You now need to decide on the type of breaker, as in "C" curve or even "D" curve, calculate whatever the max EFLI is, and volt drop too, and then decide what cable you are going to use.. You might have a shock here..

john..

 

[Sidewinder]

Forget any US nonsense.
If the unit is new, the legal requirement is that the current draw for the unit must be stated on the unit or in the equipment data.
Therefore you forget about any calculations and you use manufacturers' data.

 

[satxx]

Ok, so you have 22kw load. Ignoring funny american things, this is what i would do..

22kw/400/1.732 comes out at 31.75A Then we have power factor, lets say 0.8 and efficiency, again, say 85%. By the time we account for these, we have just under 47A This is the figure to use..

To answer your questions;

1, Yes, as this gives the actual current your compressor will pull
2, forget the american stuff
3, Yes

You now need to decide on the type of breaker, as in "C" curve or even "D" curve, calculate whatever the max EFLI is, and volt drop too, and then decide what cable you are going to use.. You might have a shock here..

john..

Thank you for the detailed explanation. In some equipments rated current or nominal current is mentioned sometimes. Can we use that for finding value of MCB?
Also do we have to decide the cable before deciding on circuit breakers or this is the right way to do it?

 

[apprentice87]

Thank you for the detailed explanation. In some equipments rated current or nominal current is mentioned sometimes. Can we use that for finding value of MCB?
Also do we have to decide the cable before deciding on circuit breakers or this is the right way to do it?

If the makers state a current, then yes use that..

No, you decide what OCPD you need for the load you are installing and the type of load. So, a load with a large inrush current like a welder or a motor would need a different type of OCPD than say a resistive load like a heater.

Once you have decided on the type of OCPD you have to ensure that it will operate corectly in the required timescale, the required "disconnection time" This means that you need to check the EFLI for the circuit to make sure that in the event of a fault enough current will flow to trigger the OCPD in the required time. You then select a cable that enables you to get the EFLI and volt drop down to the required levels.

This means that you could very well have a load that pulls 10 amps, but you need a cable with a nominal rating of perhaps 50 amps just to get the EFLI and volt drop down. You also need to check by means of the "adiabatic equation" that the heat generated in the event of a fault will not damage the cable.

Volt drop and EFLI are the usual deciders on cable size, not the load current at all..

I was once asked to look at an air compressor on a farm. I told the farmer that the cable feeding it was nowhere near the correct size [Usual farmer 2.5 flat twin and earth does everything] The cable run was VERY long... Anyway, a few days later i was asked to go and have another look. The compressor had failed to start as volt drop too high. Even worse, as the cable run was so long, the resultant overload [basically a dead short] was not enough to trigger the OCPD. The motor had literally melted into a blob of aluminium and copper... Mr Farmer replaced the motor, and a few days later the same thing happened again!!

If you cannot get EFLI low enough, you can resort to things like an RCD to acheive disconnection times. This will deal with earth faults, but what about phase/neutral faults?? The adiabatic equation is used to calculate cable heating, but this is based on a max disconnection time of 5 seconds. If the circuit will not flow sufficient current [so the disconnection time is not achieved] then you could very well end up with cable damage..

I am not a proper electrician mind you, but i know a bit.. Hopefully the experts will be along to guide you more on cable sizes etc..

john..

 

[Sidewinder]

Thank you for the detailed explanation. In some equipments rated current or nominal current is mentioned sometimes. Can we use that for finding value of MCB?
Also do we have to decide the cable before deciding on circuit breakers or this is the right way to do it?

1. Ascertain current requirements from OEM data (preferably).
2. Select a suitable breaker at or just above the required current.
3. Decide on the breaker characteristic, e.g. B, C or D, probably a C or D for a load like an air compressor.
4. Work out the cable run length.
5. Select/calculate the cable size to suit the current, volt drop and required EFLI for the run length and the circuit current.
6. As has been said EFLI or VD will probably be your limiting factors.
7. Install, terminate, test, inspect and certify, and test the circuit functions as intended and its job done.