installing a sub board

[electrical19]

I have to provide power for 10 appliances.Each appliance will be provided with own mcb.
The distribution board is 50 meter far away. So I will install a sub board in the room where the appliances will go.the sub board will be fed from the distribution board.If I install the subboard with a single phase I have to provide 16mm2 Swa as each machine take7amps and I require 70 Amps.My question is if I feed the subboard with a three phase accordingly with the regs I can use 6mm2 swa clip direct as I require this time only 40Amps (70/1.732=40amp) therefore each machine will be wire with the own radial circuit.is my calculation right with regards the cable sizes and ampege? I need your opinion and expertise Thank you

 

[Leesparkykent]

I have to provide power for 10 appliances.Each appliance will be provided with own mcb.
The distribution board is 50 meter far away. So I will install a sub board in the room where the appliances will go.the sub board will be fed from the distribution board.If I install the subboard with a single phase I have to provide 16mm2 Swa as each machine take7amps and I require 70 Amps.My question is if I feed the subboard with a three phase accordingly with the regs I can use 6mm2 swa clip direct as I require this time only 40Amps (70/1.732=40amp) therefore each machine will be wire with the own radial circuit.is my calculation right with regards the cable sizes and ampege? I need your opinion and expertise Thank you

have you calculated volt drop?

 

[electrical19]

I know the formula for Volt drop in this case I didn't calculate as I want to feed just the subboard and I would also know if I used the right equation for the three phase thank you

 

[telectrix]

if these appliances are all single phase, it's easier to work out the load per phase. put 3 on red, 3 on yellow, 4 on blue ( oops. should now use L,L2,L3. then you'll have 21A, 21A, and 28A respectively. not perfect balance and you'll need at least a 4 core distribution cable to provide TP&N. 6mm is adequate as far as load goes, but then consider VD and don't forget you may need a larger conductor for bonding if extraneous metal is in situ.

 

[electrical19]

Thank you for your advise.
I have few question with regards the bonding conductor and Tp&n. As per regs the conductor for bonding I can use 4mm as the earth will be 6mm same as line and neutral contactor also why shall I use a tp&n if the swa 6mm is protect by 40amp three phase at the distributio board? Please I need you or
everyone advise just for clarification thank you

 

[telectrix]

if your sub is supplying single phase loads, then you need a N. so L1,L2,L3 +N = 4 core. the armour will serve as cpc, but may not be suitable for bonding conductor.

 

[electrical19]

if your sub is supplying single phase loads, then you need a N. so L1,L2,L3 +N = 4 core. the armour will serve as cpc, but may not be suitable for bonding conductor.

The subboard will be three phase +N .with 6mm incoming swa 6mm2 earth and conductor bonding of 4mm.40 amp mcb will protect the cable at distribution board and will terminate at subboard at 63amp 4 pole main switch.As discrimination does not look to good but I have to find out if there is40 amp main switch 4pole for merlin gerin board.

 

[telectrix]

both TP&N isolators can be the same current rating. they are not overload devices, so discrimination is not required. just that their rating is > the load on them.

 

[electrical19]

both TP&N isolators can be the same current rating. they are not overload devices, so discrimination is not required. just that their rating is > the load on them.

Thank you

 

[telectrix]

A quick volt drop calculation tells me that using 6mm, your cable length is limited to 30m if lighting is involved.. therefore i suggest using 10mm SWA, this will also hopefully eliminate the need for a separate bonding conductor.

 

[electrical19]

a quick volt drop calculation tells me that using 6mm, your cable length is limited to 30mif lighting is involved.. therefore i suggest using 10mm SWA. this will also hgoefully eliminate the need for a separate bonding conductor

Thank you very much for your expertise It much appreciated

 

[Darkwood]

My first question is what kind of board are you feeding the submains from?

This is more about achieving discrimination here as you have several machines running the last thing you want is to take out the sub-supply OCPD if a fault developes on a final circuit.
It is sounding like you're not considering discrimination between devices here, you also need to find out the rating of the incoming supply (KVA) and the customer front end fusing (or the DNO's if taken direct of cutout to a DB.)

I would be looking (supply size big enough) at putting front end fuses in the region of 125 - 160amps, this obviously will increase the requirements for the supply cable to the submains but this is how to correctly design such systems, your proposed method is bad design and any final circuit fault can render the whole machine room without power, would you be willing to compensate the owner for losses?.. the only factor that will give you reason to reduce the submain size is if the actually DNO supply is such that you cannot can only achieve partial discrimination and in this circumstance you either still go with the large install with scope that the supply will one day be upgraded (fused down in the mean time) or you discuss with client the limitations of his installation and fit a reduced system - this will still be the largest front end OCPD that the DNO' supply would allow.

I'm gathering you have not done industrial before, you need to consider alot more than say a domestic where the DNO supply means you cannot give full discrimination.

Its usually the case in Industrial that you find out first what fuse requirements are between devices to acheive full discrimination before deciding on a submain size. If you are feeding this from a CU as oppose to a power distribution set-up then it is also unlikely to achieve full discrimination, even with a 63amp front end device.

 

[electrical19]

My first question is what kind of board are you feeding the submains from?

This is more about achieving discrimination here as you have several machines running the last thing you want is to take out the sub-supply OCPD if a fault developes on a final circuit.
It is sounding like you're not considering discrimination between devices here, you also need to find out the rating of the incoming supply (KVA) and the customer front end fusing (or the DNO's if taken direct of cutout to a DB.)

I would be looking (supply size big enough) at putting front end fuses in the region of 125 - 160amps, this obviously will increase the requirements for the supply cable to the submains but this is how to correctly design such systems, your proposed method is bad design and any final circuit fault can render the whole machine room without power, would you be willing to compensate the owner for losses?.. the only factor that will give you reason to reduce the submain size is if the actually DNO supply is such that you cannot can only achieve partial discrimination and in this circumstance you either still go with the large install with scope that the supply will one day be upgraded (fused down in the mean time) or you discuss with client the limitations of his installation and fit a reduced system - this will still be the largest front end OCPD that the DNO' supply would allow.

I'm gathering you have not done industrial before, you need to consider alot more than say a domestic where the DNO supply means you cannot give full discrimination.

Its usually the case in Industrial that you find out first what fuse requirements are between devices to acheive full discrimination before deciding on a submain size. If you are feeding this from a CU as oppose to a power distribution set-up then it is also unlikely to achieve full discrimination, even with a 63amp front end device.

The board will be a Eaton 4 pole mainswitch three phase the only thing is I need to provide a OCPD I always thought that the main switch is OCPD but it is not.

I give you a complete details of the system.
From the intake there are isolators .
The isolator that interess to us is protetct by 3 GG-88 fuses
The SWA is 25mm clip direct to the DB
The DB has like every DB a Main Switch 4 poles and I always belived that is an OCPD but it is not.
Now I will explain my design
40 amp 3 phase MCB 10mm2 swa connect to the New Subboard
I Want to connect the Swa 10mm to the ocpd if exist one for that Subboard the OCPD will be 63amp
This will be my design. An advise will be much appreciated thank you

 

[electrical19]

The board will be a Eaton 4 pole mainswitch three phase the only thing is I need to provide a OCPD I always thought that the main switch is OCPD but it is not.

I give you a complete details of the system.
From the intake there are isolators .
The isolator that interess to us is protetct by 3 GG-88 fuses
The SWA is 25mm clip direct to the DB
The DB has like every DB a Main Switch 4 poles and I always belived that is an OCPD but it is not.
Now I will explain my design
40 amp 3 phase MCB 10mm2 swa connect to the New Subboard
I Want to connect the Swa 10mm to the ocpd if exist one for that Subboard the OCPD will be 63amp
This will be my design. An advise will be much appreciated thank you

I forgot to say the GG88 are100amp three phase
I require maximum 33amp three phase I reckon 40 amp rating is right

 

[SIIVION]

Be aware of the need to separate outlets on different phases where possible, ie group each phase outlets together instead of alternating. Where the two outlets are within reach on different phases label with a warning of 400V.
Label all outlets, including phase, as I have seen too many commercial / industrial works with missing or incorrect labeling.

 

[Darkwood]

This set up will only give load discrimination and not short circuit, if you intend to supply a submains which has spare ways you have to account that other loads are added, if your doing a dedicated board and have discussed this option with client then you need to know what your largest mcb rating is and find a comparison chart to a BS88 fuse that will give full discrimination, where this means your fuse is higher than the DNO cutout then there is little you can do but to cover with the largest BS88 you can fit regarding the KVA of the supply and give partial discrimination.

You seem to be trying to design without taking into account for discrimination of fusing, this can be costly to the end user if a fault occurs on a final circuit as there is no gaurantee that the final circuit mcb would trip before the 40amp Bs88 you propose.

Also why are you giving 4pole isolation?.. you will be fine with 3pole, if you do 4pole which is not required then ensure any device has early make/late break N operation or you could blow any electronics and control gear in the machinery (dependant on the machine) and any 1ph equipment could suffer damage too just with the action on switching a 4 pole device that covers multiple circuits.
You only require 4pole Isolation at the origin if it is highly likely that the untrained end user or other person may be using it for isolation purposes, this is why domestic has to have both L/N isolated because untrained ppl use the switch to turn the power off.

 

[electrical19]

Be aware of the need to separate outlets on different phases where possible, ie group each phase outlets together instead of alternating. Where the two outlets are within reach on different phases label with a warning of 400V.
Label all outlets, including phase, as I have seen too many commercial / industrial works with missing or incorrect labeling.

Yes you are perfectly right.
I seen the worse
Last time I seen the light switch with 2 switches 400v .
How can people do these type of mistake

 

[Darkwood]

Be aware of the need to separate outlets on different phases where possible, ie group each phase outlets together instead of alternating. Where the two outlets are within reach on different phases label with a warning of 400V.
Label all outlets, including phase, as I have seen too many commercial / industrial works with missing or incorrect labeling.

As far as I'm aware this clause was dropped from the regs and also the clause that required say multigang switching that contains more than 1ph used to require labelling with 400v but its been revised to anything with voltages in excess of 230v to earth or where voltage ratings are unexpectedly higher for said accessory or enclosure. At the end of the day, anyone open these points should be qualified and following all safety isolation and testing procedures.

 

[Darkwood]

Yes you are perfectly right.
I seen the worse
Last time I seen the light switch with 2 switches 400v .
How can people do these type of mistake

As my above post implies, there is nothing wrong with having different phases behind multigang switches, its common in commercial and Industrial and is how you acheive balance on phases if running loads direct through the switching.. many domestic installers perceive this to be bad practice or even against the regs - innexperience with larger commercial and Industrial jobs is probably a factor to this misconception but it is widely used practice and done by myself on a few occasions.

 

[telectrix]

any electrician with a brain will realise that if premises have a 3 phase supply, there is possibility of all 3 phases being present in an enclosure. common sense leads to h&s.