Wiring a 2.2kw compressor

[Sookme]

Hi guys,

Been asked to wire in a compressor to a mates small unit. It’s 3phase 415v 2.2kw.

There was previously a compressor installed... and what I have in place from the board is -
40A MCB with a 20m run of 4mm 4 core + earth SWA, to a non protected 32A isolator switch mounted on the wall.

The issue I have is the compressor has a length of 2.5mm flex with a 3p+e plug. I’m planning to hard wire it in place but concerned about going from the 40A MCB + 4mm cable with a non fused isolator.

Would the best option to be either - replace the MCB in the board with a 32A or - remove the non fused isolator and install a 32A fused isolator to protect the 2.5mm cable supplying the compressor or - replace the 2.5mm cable with some 4mm?

cheers

 

[westward10]

You size the circuit to the flc of the motor but the circuit protection must take into consideration the lrc (starting current) of the motor.

 

[James]

What do you expect the running and starting currents to be?

aproximatley, it’s not a trick question

 

[telectrix]

drop the 40A breaker to a 20A type C.

 

[James]

drop the 40A breaker to a 20A type C.

Could possibly go as low as 6A

 

[Marvo]

I'm with Tel, with a 2.5mm trailing cable I'd fit a 20A C curve MCB or even D curve if the start current makes it necessary. I'd probably check the mechanical off-loader valve is working though before I went to a D curve.

 

[westward10]

And fault protection for the chosen device.

 

[Lucien Nunes]

As others have said, a 32A MCB is unnecessary for a motor with around 5A line current, The nearest equivalent application I have here is a 4kW compressor with 8.6A line currrent that runs on a 16A BS3871 type 3.

Normal reciprocating compressors start against moderate inertia and friction, but not against pressure, so the starting duty is not especially severe. Typical induction motor starting currents are 5-7x full load, so it is reasonable to allow at least 10x FLC before the magnetic trip of the MCB engages, assuming the MCB is at the most sensitive end of the range for its particular curve.

In my example, the 16A type 3 minimum instantaneous trip current is 7x16=112A, 112/8.6=13. I.e. the motor can take up to 13x FLC before any 16A type 3 will react instantly. The thermal element should never react because an overload or excessively frequent starting should trip the overload relay in the starter first.

Applying the same margin to your compressor would suggest a C16 (5x In = 80A = 16x FLC) and presumably your loop impedance will be fine for this. Even a D6 will give you 12x FLC but that's getting a bit peculiar for no real advantage. The manufacturer of the starter or compressor as a whole might specify a maximum upstream protection rating and this should be observed.