Tn-s 3 phase?

[Ross1]

Evening guys,

I`m doing a small commercial job involving 2 x 3 Phase boards. The secondary board is 15 meters from first and is gonna supply up to potentially four 3 phase machines plus a ring main supplying portable single phase equipment.

Help sizing the supply cable for the second DB. Not used to dealing with start up currents etc. Here is a list of the max amount of equipment to be used. This is info customer has given to me.

Planer: 3 phase, 1.5 Kw, Full Load 3.6A, starting current 21.6 A

circular saw: 3 phase, 4 - 6 Kw, (other details unavailable)

Chopsaw: 3 phase or single, 2 - 3 kw

pillar drill: 3 phase or single, 1 - 2 Kw

Lathe: 3 phase, 3 - 4 kw

bandsaw: single phase, 2 - 3 kw

mortiser: single phase, 1 - 2 kw

cheers

 

[GLENNSPARK]

Hmmm.....got sum recapping to do by the looks of it. Not my everyday domestic single phase stuff!

well the idea of ballancing 3 phase loads is to avoid excessive neutral currents....and so this would also apply to DNO cables....so to ballance each phase out as best you can....but as you say these are all 3 phase loads anyway which is where i went a bit divvy at #12....

 

[GLENNSPARK]

its the ring final for the portables though.....could you not split it into 3 radials....each one on a seperate phase?

 

[Darkwood]

Heres a MCB' rating to motor list for you, this will work for the types of machinery you have listed.

1.1kw = 6amp(b or c)
1.5kw = 10amp(b or c)
2.2kw = 16amp(b) or 10amp(c)
3.0kw = 20amp(b or c)
3,75kw=25amp(b or c)
4.0kw = 25amp(b or c)
5.5kw = 32amp(b or c)
7.5kw = 50amp(b or c)
To note! ... know you mcb rating first then you know the cable size area to look for and apply standard cable calcs to cover mcb rating, these sizes will stop nuisance tripping on start-up, dont be confused by the type b's for motors these are manufacturers recommendations and tried and tested, dont apply this list to all situes as characteristics of motor need to be taken into account but in your case i know there basic characteristics and this table will do fine.
Also note your mcb rating could be higher than you expect hence the need to know them first so as not to fit an undersize cable to machine on a lesser mcb which trips on start-up, in this senerio upgrading the cable and mcb maybe the only solution at your cost and time.

 

[spark 68]

its the ring final for the portables though.....could you not split it into 3 radials....each one on a seperate phase?

It is not good practice to put sockets on different phases in the same room/workshop, as it is (remotely) possible to risk getting a phase to phase shock.

 

[Darkwood]

It is not good practice to put sockets on different phases in the same room/workshop, as it is (remotely) possible to risk getting a phase to phase shock.

Have to disagree here! its common practice to do so to balance loads, if 2 single phase points, sockets etc over differing phases are within arms reach of each other than all that is required is a warning label between to 2 points, same with light switches its common to have 2 or 3 phases in a grid switch bank and all that is required is a warning notice. What you tend to find is that People who are generally domestic based have the idea its bad because they never come across it but its all listed within the regs and complies.

 

[Ross1]

Thanks guys for your time! I`m gonna be on site tmrw and see what the crack is and take all your thoughts into consideration plus work out some equations.

Cheers

 

[spark 68]

Hi darkwood,

Have to disagree here! its common practice to do so to balance loads, if 2 single phase points, sockets etc over differing phases are within arms reach of each other than all that is required is a warning label between to 2 points, same with light switches its common to have 2 or 3 phases in a grid switch bank and all that is required is a warning notice. What you tend to find is that People who are generally domestic based have the idea its bad because they never come across it but its all listed within the regs and complies.

I can only go from past experience, I was part of a crew installing basic electrics in about 30 small factory units (100A per phase), this work consisted of a single 32A TP+N socket for general use, another TP+N socket at height for the roller/up and over door, two 32A TP+N Isolators, and several SP sockets dotted around for general use, and the guy in charge stipulated all SP sockets to be on the same phase in the workshop area (for the reason I gave earlier), and all SP sockets in other rooms/parts of the unit on the same phase(but different to the workshop area).

The SP sockets were still balanced across 3 phases, ie. workshop on one, office on another, reception and toilets etc. on the other.
All lighting in the workshop to be on staggered phases (different phase per bank).

I did not find a reg to say this was the way it had to be done, but the guy in charge obviously knew his stuff as he was the project manager for the whole estate, so who am I to argue.

 

[Darkwood]

Hi darkwood,


I can only go from past experience, I was part of a crew installing basic electrics in about 30 small factory units (100A per phase), this work consisted of a single 32A TP+N socket for general use, another TP+N socket at height for the roller/up and over door, two 32A TP+N Isolators, and several SP sockets dotted around for general use, and the guy in charge stipulated all SP sockets to be on the same phase in the workshop area (for the reason I gave earlier), and all SP sockets in other rooms/parts of the unit on the same phase(but different to the workshop area).

The SP sockets were still balanced across 3 phases, ie. workshop on one, office on another, reception and toilets etc. on the other.
All lighting in the workshop to be on staggered phases (different phase per bank).

I did not find a reg to say this was the way it had to be done, but the guy in charge obviously knew his stuff as he was the project manager for the whole estate, so who am I to argue.

I would in your example do the same when the electrical design allows you it good practice to segregate areas to a phase a piece, but in some designs their may be the need to have more than one phase in an area for reasons of load balancing (items in use are heavy loading within same area) or to give a second option when a circuit is lost to allow business to continue with minimal disruption, the trend in the OP case would be a phase down one side of the workshop and 2nd phase down other on rcbo's.
The guy in charge you mention has i see it done a correct design for the needs of the building he was designing for, but i was responding to your comment that it is bad practice which isnt true but will agree if the needs of the installation allow then it would be the prefered method to section your ringmains to different areas, each case is different and all though i cant vouch for the project manager; i tend to find there are sparkies out there with the misguided view its bad practice, it possible that teachings do suggest you try to create areas but sometimes this is taken literally as the way it should be done and as i said its is the taught prefered method but only if the if the installation design allows it.
Ive designed many installs and do find similar types of install where single phase supplies to sockets and spurs need to be within close proximity but over a different phase - an example would be a commercial kitchen where many heavy load plug in items share a small area.
This is i assume what is explained when an electrician takes the optional 17th installation design course but as im a tad older all this was included in my 4yr course 20yrs ago.

 

[Knobhead]

Can we take a pragmatic look at this, theory is fine but it don’t work.

Go with your original idea of 16mm. You then have some capacity for expansion. I’ve spent to long in workshops, believe me. Two days after you’ve finished your nice new shiny board there will be a wagon with a HiAb unloading another lathe.

As for starting loads, again look at the practical side. No way, no how will more than a couple of motors start at the same time. OK it does happen at times, the drivers of these beasts knew that if they started two together they would trip the 11KV supply.
View attachment 12493
A nice long tea break for them while we drove to the substation to reset the supply. (I’m not saying they did it deliberately, but….).

As for balancing loads again take a pragmatic view. Split the sockets as best you can. I will go with Glenn’s comment about 415V between sockets. But look at it this way, one bench per phase and the big danger is removed. You can’t regulate what extension leads they will use. But that’s not your problem, that would a management problem put down to poor training.

RCD the sockets not the fixed machines. To be truthful they should not be using 230V hand tools. So start looking at a three phase 110V transformer with it’s own small DB.

Sorry to add burden to you problems.

 

[micknew]

A good stating point would be to find out how many people will be operating the machines. That way a max demand at any one time could be estimated.

 

[Darkwood]

Too right there tony just upgraded some machines with 30kW motors on them, they originally had DOL inching and 3 on site, over 15yrs they said they had take the service fuses out 3time as the coincidence of 2 motors peaked together on inrush.

 

[Knobhead]

Darkwood, how do you regulate against that? It’s impossible!

The navies in the picture would take out a 750KVA transformer in the blink of an eye. They were on autotransformer starters to ease the load, fat lot of good they did when two started together.

As I said you have to take a realistic view of things.
No wonder I’m a grumpy old sod, I always look at the Darkside