how to work out what size commando plug I need.

[chris1983]

Hello. It has been a long time since circuit designing for me as I am a simple maintenance person. How do I work out commando plug sizing, for example a machine rated at 32A, plus I have several machine that need moving onto one board, I have there rated current how do I work out board size. Thanks for the help

 

[DPG]

Hello. It has been a long time since circuit designing for me as I am a simple maintenance person. How do I work out commando plug sizing, for example a machine rated at 32A, plus I have several machine that need moving onto one board, I have there rated current how do I work out board size. Thanks for the help

Surely this is beyond your remit - is there not a sparks on site?

 

[telectrix]

a machine rated at 32A needs to be on a 32A plug/socket. cable 4.0mm or 6.0mm. MCB rating 32A, maybe a type C if machine has motor/s or other components that have a high start-up current. Zs for a circuit must not exceed the stipulated value for the MCB.
as for the board, all depends on what diversity you can apply.obviously eack socket generally on it's own MCB.

 

[Darkwood]

I must agree that this will be beyond your asking, this can be either a simple task to weigh up or a complex calculation that if you get it wrong can cause all sorts of issues like nuisance tripping to overloads.

Example.. A machine with a 3ph 11kw motor star/delta starting (approx 22amp full load current) will take out any 32amp mcb regardless of its type B,C or D rating, this machine may stipulate that its full load is less than 32amp but it will take out the mcb simply due to inrush demands of the motor.

I would seriously consider getting in a qualified electrician who is competent and experienced in designing distribution and supplies for machinery otherwise you may be digging yourself a hole you cannot get out of as well as the potential risks that may arise from incorrect design.

 

[snowhead]

As above, seek out someone experienced in this work.
It could get expensive by undersizing, having nuisance tripping and having to have the work corrected.
But it could also get expensive by unnecessarilly over sizing, "just in case".

 

[ElectroChem]

A machine with a 3ph 11kw motor star/delta starting (approx 22amp full load current) will take out any 32amp mcb regardless of its type B,C or D rating, this machine may stipulate that its full load is less than 32amp but it will take out the mcb simply due to inrush demands of the motor.

Are breaker curves different in the UK? I though D curve breakers were specifically for high inrush inductive loads. A D32 breaker shouldn't trip until inrush hits at least 320A, well above what a 22kw star-delta should draw on startup.

 

[Darkwood]

Are breaker curves different in the UK? I though D curve breakers were specifically for high inrush inductive loads. A D32 breaker shouldn't trip until inrush hits at least 320A, well above what a 22kw star-delta should draw on startup.

It's a little bit more complex than that, the starting duty of a motor has to be take into account as the tripping curve also has a time value to it, it is therefore a reason we would need a 63(D) in the 22kw example and this wouldn't suit every occasion, high inertia loads need extra allowance and more than likely would need a MCCB or HRC fuse suited to the longer start up duty.

Schneider own tables recommend a 50(D) MCB for a standard duty star/delta 22Kw

Hagar own tables recommend a 63(D) for the same.

This hi-lights some operational differences between different manufacturers hence this should be properly designed to ensure no nuisance tripping occurs, this area of design is the one that tends to trip up even the experienced sparks out there who don't delve into power supplies to machineries, more and more of my callouts are domestic/commercial sparks moving into industrial due to competition or lack of work in their own comfort zone and making some costly mistakes, I often utilise a BS7076 clause that allows the overcurrent device to exceed the CCC of the cable it is covering while ensuring ELI tripping times are not contravened, this falls under the fixed load clause which negates the need for overload protection, If I was to match the cable to the protective device rating on half the jobs I do it would be a lot of wasted material costs but I see this all the time when I am called into these factories.
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PS - as I was quoting a 11Kw which somehow became 22Kw (think you mixed amps and KW up and I followed on from it )

The correct tables for a standard duty startup of a 11Kw are

63(B), 63(C) or a 25 (D) as listed by Schneider... noting the 25(D) type and looking at the ELI permitted values it may be the case 'but not always' that a cable is easier to achieve a permitted value by selecting the higher value MCB with a lower trip curve, in this example a 25 (D) would suit and we can allow more room by looking at the 5second trip figures as oppose to 0.4 values but that depends on the machine and its power circuits if we can allow that.
If ever in doubt its best to go with the C type value and design to that IE the 63(c) for an 11kw, weighing up the duty of a motor on startup is not always possible especially when info is lacking, this is where experience comes in, but as you can appreciate this goes well beyond of the OP if he was to come across this situation.