Installation of a 90kva CNC Machine

[Hutch118]

Hi Lads,

Need advice on the installation of a CNC Turning Centre. The Full Load of this machine according to the plate rating is 90kva but in reality, when the machine is running, it takes a fraction of this load.

What I need is recommended cable size, fuse/breaker size. Length of cable ruins 45m clipped direct and running beside a few other cables. I've already done my calculations but the cable size seems excessive for the load I know is going to be taken off the supply.

Can anyone help me out on this please?

Thanks. Hutch

 

[Knobhead]

What size cable have you calculated?

The manufacturers have stated 90KW, do you know something the manufacturers don't.

 

[Hutch118]

What size cable have you calculated?

The manufacturers have stated 90KW, do you know something the manufacturers don't.

Hi Tony, the manufacturers plate states 90 kva which is 72kw, I know from past experience on very similar machines that in reality, they will run at a fraction of the stated power requirements, I suppose it's diversity within the machine?
Cable calculated is 35mm 4c but wondered if I could go down to 25mm. I suppose I answer my own question when I realise the cable has to be able to carry the FLC stated on the machine. (Eg, a machine rated at 60kva takes 30 amps when running in the workshop just now.)

Hutch

 

[Knobhead]

You’ve got me fascinated, just how does 90KVA equal 72KW?

 

[Hutch118]

According to the conversion table I have 90 kva is 72 kw not taking into consideration a 0.8 pfc

 

[pottymotty]

Power formula watts = V I COS Phi
Where cos Phi is power factor so 72 kw seems right then split over three phases about 25 kw per phase divide by 230 gives just over a hundred amps .not checked the regs for vd and current capacity but its a fair length run so i would go with the 35 mm . The 25 mm would as you say probabaly do so its useful to look at a similar machine and measure its current doing work and feel the temperature of the cable while doing work. On and off time ratio has a big effect on the cooling of the cable also as does the ambient room temp so common sence could come into play if you find a similar machine in the same shop takes 70 amps for a couple of minutes then stops for 2 minutes whilst the operator does another operation then maybe the 25 mm but if its basically running continuously at full load then i would opt for the 35 mm but i'd do the calcs for vd and correction factors to be sure

 

[Darkwood]

The 90KVA will take in to account for working the heaviest job the machine can handle and diversity shouldn't be applied here as you cant be sure the use of the machine or the work done won't change, the very nature of the CNC machine means its versatile and adaptable to new work loads and programmes, you never can predict the demand put on this machine as new customers and orders may come in at any time.

Ive just fitted 5 CNC supplies of varying sizes and would never dream of applying diversity to a machine of this nature unless exceptional circumstances meant i could.

 

[Darkwood]

Can you tell us the front end fusing device as this too will have a bearing on the cable size ...
e.g. If your using HRC fuses you are fixed to choosing the available fuse ratings and this could add additional CCC to your cable to ensure your fuse rating is less than the CCC of your cable.

 

[Hutch118]

The 90KVA will take in to account for working the heaviest job the machine can handle and diversity shouldn't be applied here as you cant be sure the use of the machine or the work done won't change, the very nature of the CNC machine means its versatile and adaptable to new work loads and programmes, you never can predict the demand put on this machine as new customers and orders may come in at any time.

Ive just fitted 5 CNC supplies of varying sizes and would never dream of applying diversity to a machine of this nature unless exceptional circumstances meant i could.

Probably not what I meant Darkwood in respect to 'diversity', wrong use of the term. The machine is rated 90 kva, when this runs, the load measured even during heavy cuts is relatively low. I obviously realise that the cable I need to use to supply it has to take the FLC.

 

[Hutch118]

Can you tell us the front end fusing device as this too will have a bearing on the cable size ...
e.g. If your using HRC fuses you are fixed to choosing the available fuse ratings and this could add additional CCC to your cable to ensure your fuse rating is less than the CCC of your cable.

Will be supplied from a fused switch at 200A rating thru 125A HRC fuses, motor rated.

 

[Hutch118]

You’ve got me fascinated, just how does 90KVA equal 72KW?

Tony, explain to me where this doesn't work out. You've got me fascinated!

Hutch

 

[Darkwood]

As i calculate it 90KVA 3phase is 130amps im unsure where you work out 125amps is big enough as front end devices your front end fuses may have to be 160amps depending on switch/fuses typical fuse ratings hence i brought this up when i spotted 35mm cable been considered.

 

[Hutch118]

As i calculate it 90KVA 3phase is 130amps im unsure where you work out 125amps is big enough as front end devices your front end fuses may have to be 160amps depending on switch/fuses typical fuse ratings hence i brought this up when i spotted 35mm cable been considered.

Its 160A on the front end and 125A at the switch inside the machine. Apologies.

 

[Darkwood]

Well you need to leave as is for discrimination in event of overload but although i dont entertain running parallel supplies on anything less than 95mm you could do so as this will reduce cable costs at the present (without working out ) i suspect your looking at a 70mm 4c but running 2x 25mm 4c with satisfy the front end fuse rating and reduce copper csa needed.
Im a little confused as you already preposed 35mm but then you say the front end switch fuse protecting the cable will have 160amps fuses it dosnt make sense????

 

[Hutch118]

Good advice 'Darkwood' it's making me think on this in a different way now.

Thanks. Hutch

 

[Darkwood]

Try not to change your story too much it makes you look unproffesional (not an accusation), you originally asked about reducing 35mm to 25mm and this raised my front end fuse question to which you said 160amps... well i think you know where im coming from as your CCC rating of the cable must be larger than the device protecting it (excluding exceptions for this conversation).