Cable size for single phase motor control panel

[mark_]

Hi all,

Your help would be appreciated. We are mainly domestic sparx so I havn't much experience with cable sizing for motors.

On site, we need to supply a motor control panel which operates the sewage pump. There are in fact two pumps but I think they switch between periodically to prolong the life span and offer back up incase one goes down.

Anyhow, wiring diagram shows a 230v single phase supply, each pump is shown with a FLC (running i think) of 9.9 amps or 1.5kw. Each pump has an overload set between 7.5 and 11 amps. Ive spoken to the pump supplier who has asked for a D40 mcb as the protective device.

My question is do I size the cable for the D40 mcb as normal (which seems unlikely) or for the 11a maximum that the over load provides. If the latter is the case I would presume that the MCB would only be offering short circuit protection so we would need to be sure the circuit would provide the required short circuit current to trip the mcb in 0.4sec. According to BS7671 thats 800amps (am I wrong) so I may need an RCD to help here.

Hopefully I have explained the situation enough for someone to respond.

Lastly the cable length is max 60m

Many thanks

 

[plugsandsparks]

A D40 is going to present you an issue trying to meet disconnection times as the Zs is around 0.23, check on-site guide. I suspect after a 60M run you may well be up to 0.50 and more.
You will also have to deal with volt drop as well. So choice of cable will be less about amps and more about VD and ZS.
BTW - why D40 , small motors like that run on much smaller breakers?

 

[telectrix]

IMO a C20 would be more than adequate, using 2.5mm cable.

 

[mark_]

Thanks for your replies. I have rang him up again to check and he has confirmed only one pump will run at any one time. He has changed the D40 to a D32, reason being there is a fuse protecting his internals at 25a. So for discrimination he would like a 32a mcb. He said he has had problems with C types tripping in the past so would prefer a D.
Going back to the design aspect, am I correct in saying I can size the cable for the overload rating?

Of course I realise my zs will be determined by the rating of the MCB. If im stuggling to acheive the zs, anything wrong with an RCBO or would nuisance tripping be an issue ?
I will make sure the volts drop is ok.

 

[telectrix]

first , you select the cable according to current and volt drop. then select you OCPD so that Ib<In< Iz, and to get an acceptable Zs value. if the Zs value is above that max. quoted in BGB, the you need to increase cable size.

 

[Plonker 3]

Why not install a larger cpc to bring the Zs down rather than relying on a RCBOS for disconnection times?

 

[Outspoken]

RCBO on a motor...welcome to the wonderless world of nuisance tripping.

If you installed a D16 or a D20 you would meet the needs of all requirements and the designer (I use that term loosely). I have installed many sewage pumps over the years and never installed one on a breaker larger than a D20 (Max Zs=0.57) for 2.5mm², D25 (Max Zs=0.46) for 4.0mm² or D32 (Max Zs=0.36) for 6mm² (Unless the local board was close enough to meet other requirements but this is not common) Those figures are good for a run of approximately 50m.

For the 60m run I would use 4.0mm² on a D20 as this will provide the discrimination required, the disconnection times required and the level of protection required in regard to all aspects of this installation.

 

[Engineer54]

Seems to me, an inappropriate sized and typed MCB is required for discrimination purposes only, with that of the 25A BS88 fuse incorporated in the supplied MCC panel. ....Unless i'm missing something

 

[Outspoken]

Seems to me, an inappropriate sized and typed MCB is required for discrimination purposes only, with that of the 25A BS88 fuse incorporated in the supplied MCC panel. ....Unless i'm missing something

Always ignored the internal BS88's, calculate the correct size of MCB to comply and do the job required and they never blow..

 

[mark_]

I agree we seem to be making life very difficult for ourselves because of the discrimination element. If we fit an RCBO there will defiantly not be any discrimination. I think ill go with the D20. Thanks all.

 

[Engineer54]

Always ignored the internal BS88's, calculate the correct size of MCB to comply and do the job required and they never blow..

That could be a bit of an inconvienience where the upstream protection is not readily accessable, but then again, so would having to change out the BS88 when it blows!!

 

[Outspoken]

That could be a bit of an inconvienience where the upstream protection is not readily accessable, but then again, so would having to change out the BS88 when it blows!!

To be honest I have always installed a DIN rail into the MCP and installed a local MCB on that for resetting/local control purposes with a rotary isolator beside the panel to allow LOTO for maintenance works.

 

[D Skelton]

so we would need to be sure the circuit would provide the required short circuit current to trip the mcb in 0.4sec.

I only have a small point to make here due to the fact that you've pretty much had all the advice you need from the other members. Outspoken hits the nail on the head in post 7.

You need to remember that there is NO required disconnection time for short circuit. A mere triviality in your situtaion but on larger projects it becomes very easy to over spec when the installer isn't aware of this. I haven't got my regs book to hand to quote specific regs but chapter 43 is where you need to be looking

 

[Darkwood]

What brand is the MCB you are using ....

For a 1.5kw 230v DOL motor the recommended breakers are as follows...

32(b), 32(c) or a 20(D)

This is for a 2HP motor with running current of 10.5amps.

This selection is specific to merlin C60H range and other brands may vary slightly.

Remember to adjust permitted zs values for the ambient Temp' and although this may cause a stink with members you can increase the mcb rating over the CCC of the cable as long as you meet short circuit protection requirements.... as unless you tell us different your load is classed a fixed load.... but i stress it is unlikely in your case you will need to go down this route - if you have tripping issues after install we can discuss that option but ill leave it at that for now.

Just to add ...dont get confused by discrimination just fitting the next size fusing upstream will not afford you discrimination and may only afford partial discrimination at most....it is unlikely you can give full discrimination on such a small project without re-educating yourself on cable calcs and install design which is probably more advance than you need here ...not having discrimination shouldn't be an issue in this set-up.