Application of diversity

[sparky in training]

good evening all!

sorry it is a late one. I have been doing a college assignment over the past 2 days which involves designing circuits calculating design current, volt drop and all that good stuff. One aspect is applying diversity. Now usually I am fine with this. I know to use table A2 of the OSG etc etc. however the tutor has confused me. He is saying take diversity from the value of your protective device. I am sure i never done that before.
If say I had a lighting circuit and it could draw 3 amps under full load surely I apply diversity of 66% of that value to get Ib design current? My tutor is saying if it is on a 6A MCB take diversity at 66% of the device rating.

This can't be right can it? He is adamant but I am also adamant . Completely confused about the whole thing now

Please help

 

[Richard Burns]

You are attempting to calculate how much current will be drawn on a circuit or an installation in normal use.
This cannot be determined from the rating of the circuit breakers as this does not reflect the actual current used.
In a basic radial circuit supplying one item of equipment then the rating of the circuit breaker should be, by design, only just above the rating of the equipment (though limited by the standard CB ratings) and in that case it might be valid to take the In as the current demand, however once you are supplying multiple points this is not the case.
The current demand of a circuit should be assessed from the actual total load imposed by the connected equipment. As a worst case take account of changes that may be easily made such as replacing LED bulbs with halogen bulbs and diversity is then applied to this total load to get the expected demand on the circuit.
For a socket circuit the loads can change so much, so easily that the In can be appropriate to use as the Ib as a worst case scenario.
Though in reality, not training, it would be appropriate to consider the likely use of the sockets; a four socket ring to domestic bedrooms is different from a 10 socket ring in a busy household kitchen.

 

[Ian1981]

good evening all!

sorry it is a late one. I have been doing a college assignment over the past 2 days which involves designing circuits calculating design current, volt drop and all that good stuff. One aspect is applying diversity. Now usually I am fine with this. I know to use table A2 of the OSG etc etc. however the tutor has confused me. He is saying take diversity from the value of your protective device. I am sure i never done that before.
If say I had a lighting circuit and it could draw 3 amps under full load surely I apply diversity of 66% of that value to get Ib design current? My tutor is saying if it is on a 6A MCB take diversity at 66% of the device rating.

This can't be right can it? He is adamant but I am also adamant . Completely confused about the whole thing now

Please help

On site guide gives a typical light point at 100 w max so if you had 10 x lights at a max of 100 watt then that's 1000 w
Then the 66% is applied to this.
In reality everything is now low energy, LED etc so really the on site guide is a bit dated in that respect

 

[sparky in training]

On site guide gives a typical light point at 100 w max so if you had 10 x lights at a max of 100 watt then that's 1000 w
Then the 66% is applied to this.
In reality everything is now low energy, LED etc so really the on site guide is a bit dated in that respect

Thanks for the response. Yes very dated with todays LED's

 

[sparky in training]

You are attempting to calculate how much current will be drawn on a circuit or an installation in normal use.
This cannot be determined from the rating of the circuit breakers as this does not reflect the actual current used.
In a basic radial circuit supplying one item of equipment then the rating of the circuit breaker should be, by design, only just above the rating of the equipment (though limited by the standard CB ratings) and in that case it might be valid to take the In as the current demand, however once you are supplying multiple points this is not the case.
The current demand of a circuit should be assessed from the actual total load imposed by the connected equipment. As a worst case take account of changes that may be easily made such as replacing LED bulbs with halogen bulbs and diversity is then applied to this total load to get the expected demand on the circuit.
For a socket circuit the loads can change so much, so easily that the In can be appropriate to use as the Ib as a worst case scenario.
Though in reality, not training, it would be appropriate to consider the likely use of the sockets; a four socket ring to domestic bedrooms is different from a 10 socket ring in a busy household kitchen.

Thanks for the reply. I knew it was from the current that could be pulled from connected equipment and not the breaker rating. I needed clarification as this guy has just confused the **** out of me. Another thing he said was if a cable is run down a wall in open ended conduit then heat can escape and that can be classed as clipped direct. Surley not?

 

[Ian1981]

Why would you have open ended conduit is what I would ask?

 

[KJF]

Are you sure your tutor isn't telling you to use the protective device (In) when working out your current rating?

 

[Richard Burns]

Thanks for the reply. I knew it was from the current that could be pulled from connected equipment and not the breaker rating. I needed clarification as this guy has just confused the **** out of me. Another thing he said was if a cable is run down a wall in open ended conduit then heat can escape and that can be classed as clipped direct. Surley not?

That is getting a little more complex on design.
Generally a cable will be able to lose heat more effectively covered in a tube along part of its length rather than fully enclosed in conduit but as Ian1981 says why would you do this.
Whether it wold be classed as installation method C I am not sure as there would be more heat than clipped direct but less heat than in conduit, better to go for the worst case, if the conduit bent over at the top then there would be less chance for heat to escape and so on.

 

[sparky in training]

That is getting a little more complex on design.
Generally a cable will be able to lose heat more effectively covered in a tube along part of its length rather than fully enclosed in conduit but as Ian1981 says why would you do this.
Whether it wold be classed as installation method C I am not sure as there would be more heat than clipped direct but less heat than in conduit, better to go for the worst case, if the conduit bent over at the top then there would be less chance for heat to escape and so on.

Believe me, this task is ridiculously complex. i have to show all cable runs, containment methods, show cable sizing calculations. there are about 25 circuits to coverr. Sadly I can't throw circuits any old route because i have to consider requirements of bs767. thank god its friday. I agree with you go with the worst but it were me I would be adding de rating factors even if heat could escape. like you say. it isn't truly clipped direct and is semi covered.

 

[sparky in training]

Are you sure your tutor isn't telling you to use the protective device (In) when working out your current rating?

Current rating no. He is on about diversity. by his methods his load will still be more than what the full load could possibly draw if he used the MCB's rating

 

[KJF]

What he's saying makes no sense then, which for a tutor that is really bad form.

I can only assume he is getting mixed up with the current rating calc where the rating of the protective device (In) can be used to find Iz. You can also use Ib (design current) to find Iz if overload won't be an issue so I can see how he could have gotten mixed up.

 

[Wilko]

My simple thought is diversity helps set the design current which then sizes the cable and mcb. Tutor method puts cart before horse surely?

 

[hightower]

Believe me, this task is ridiculously complex. i have to show all cable runs, containment methods, show cable sizing calculations. there are about 25 circuits to coverr. .

Really? When I did it I had about 70 circuits. Not sure you're dividing the installation enough if you've only 25.

 

[paul291277]

There are some sparks that become tutors to pass on their invaluable knowledge, whilst others do it because they cannot drum up enough work, I hope your tutor is not one of the latter!