Diversity and max load

[danny124]

I done some calculations regarding diversity for max load and also determine my main switch rating for my board....

can someone tell me if i done my calculations right... i have trouble getting my head round this, got my ELECSA assessment coming up, want to make sure im not getting things wrong. Feels like my brain has gone to marmalade...

Supply to workshop will be 16mm 3 core buried, worked out the Current Carru Cap, and Volt drop already....


Circuits needed: 4 Sockets 32amp
2 Florescent Lights 6amp
1 Possible Circuit in future for Planer machine. 32amp

Max demand Load
& Diversity
(With future Circuit):
Onsite Guide Calculations
Possible Planer Circuit 32amp = 32amps
Ring Circuit 40% (32/100x40) = 12.8amps
4ft Florescent Lights (x2)… Likely be 36w each.
72/230 = 0.31amps
66 % of 0.31 amps = 0.20amps

TotalDemand = 45amps

Rule Of Thumbs Calcaulation

Largest Circuit (Planer) = 32amps
40% of Lighting and Ring Circuit = 15.2amps

Total Demand = 47.2amp


Cheers guys....

 

[scotsparky]

remember that flourescent fittings have a power factor of 1.85 so factor that into your total design current. you need to do it for cable calcs as well.

 

[danny124]

hey scotsparky

Thanks for your reply, and thank you for heads up

Apart from all that, does everything else looks ok to you

Cheers

 

[scotsparky]

is ther only 1 socket circuit?
If its a workshop then it may be classed as commertial which will affect your lighting demand so go for 100% lighting and 50& sockets.

 

[danny124]

hi mate, its just a little hobby workshop for my grandad, he was a joiner so like to keep himself busy by making things for the house...

basically its.....

ring with 4 double sockets
(possible) radial for the planer but its not likely. (but wanting to add this into calculations just in case
2 florescent IP(protection against sawdust mainly) rated lights for lighting.

With regards to my main switch which for the SWA cable suppling shed. Was going to make it 40amp. Its got a 40amp at moment. But Max demand is about 47amps so was going to go with the 60amp option.

And main switch RCD, on CU in shed, does this also have to be 40. someone told me today it may have to be, but i dont think this guy is right?

What you think.

Have i got a few things wrong here.

Just worring about my assessment, and all the simple things im forgeting...

Thanks for your reply again Sir

 

[scotsparky]

will get back to you soon. my books are in van so will have to double check.

 

[jase]

In>Ib i remember somekinda calc like this , cant really remember it though, but surley if max demand is 47A then the overcurrent protection needs to be 50A?

50A switchfuse, armoured to workshop DB, 63A 30mA main switch and then your breakers, or just a 63A main switch and RCBO's to allow for division of installation BRB pge 39

 

[jase]

In>Ib i remember somekinda calc like this , cant really remember it though, but surley if max demand is 47A then the overcurrent protection needs to be 50A?

50A switchfuse, armoured to workshop DB, 63A 30mA main switch and then your breakers, or just a 63A main switch and RCBO's to allow for division of installation BRB pge 39

 

[scotsparky]

jase as its the min switch that is 40 A then the final circuits are seperate MCBs its ok because its backfused. as long as the small busbar in the D.B is covered.
Backfusing is how you get away with taking 25mm tails from 400A busbars to local isolators. remember ithe 3meter rule though.

the formula you remember is Ib>In>Iz
Ib= design current
In = overcurrent rating
Iz = current carrying capacity of cable.

 

[Noddy67]

"the formula you remember is Ib>In>Iz "
__________________


The 'greater than' symbols should be the other way in the calc Ib<In<Iz (and to be very pedantic should be "less than or equal to"' but I can't find the symbol for that on my keyboard either)


(I now remember it as < looks a bit like an L as in 'Less than')

 

[professional]

remember that flourescent fittings have a power factor of 1.85 so factor that into your total design current. you need to do it for cable calcs as well.

A power factor of greater than unity is an impossibility!

 

[ezzzekiel]

multiply the wattage by 1.8 to make up for losses in control gear etc, powerfactor will be unity or below.

 

[jase]

jase as its the min switch that is 40 A then the final circuits are seperate MCBs its ok because its backfused. as long as the small busbar in the D.B is covered.
Backfusing is how you get away with taking 25mm tails from 400A busbars to local isolators. remember ithe 3meter rule though.

the formula you remember is Ib>In>Iz
Ib= design current
In = overcurrent rating
Iz = current carrying capacity of cable.

Backfused?

 

[scotsparky]

backfufusing means that the supply cable is protected by the upstream device. I cant remember the reg of the top of my head and regs are in van which is in garage after a dopy woman hit me.

basically as the load is protected by an overcurrent device. then supply cable to that overcurrent device if under 3 meters is also protected as long as it is capable of carrying the maxumum load of the device. So if load exeeds protective device rating the device trips thus disconnecting load to supply cable.

It means that on a big busbar chamber then the supplys to the local isolators dont have to be able to take the maximum demand of the busbars.

 

[jase]

Thinks me got it, so main switch on workshop DB only has to cover max demand of installation, and switchfuse of the submain just has to be big enough to cover the current the submain is carrying,

 

[scotsparky]

thats about it but the switchfuse of submain has to me within 3meters of main switch and interlink cable if within 3metres has to carry the maximum submain load.

 

[scotsparky]

A power factor of greater than unity is an impossibility!

maybe powerfaqctor is not the correct terminoligy but due to the ballasts and control gear in flouresant fittings a fator of 1.85 has to be addded to show true load.

 

[ezzzekiel]

yeah we know what you meant

splitting hairs

 

[professional]

There is a world of difference between a power factor and a correction factor .........

 

[scotsparky]

well done clever boy you spotted a wording mistake ! drop it!

 

[ezzzekiel]

There is a world of difference between a power factor and a correction factor .........

actually theres only the first word, the second are identical

 

[danny124]

Hey guys just reading though all your replies to my thread....
I will look at the regs as well, thanks for heads up.
The submain cable will be going on a Isolation, as stated in regs over 3 metre it is needing one.

So basically in a nutshell what you fellas are saying is...

1. An isolation switch for submains the Rating of it just needs to cover the Current Carry Capacity (after factors applied etc etc).

2. The Main/RCD switch rating needs to cover the Max demand of the circuits etc etc.

3. I need to mutiply the florescent lighting wattage by 1.8 then do diversity. I take it this is because of the start up current pulled by the module.


Cheers guys,
Big thanks for all your time...