cable calculations

[assegayer]

Hey guys just practising my cable calculations,

could you tell me if this is right I am doing it according the onsite guide as instructed in college...

this is the example


11kw oven (i know its unrealistic just making things up)
16 metres from CU
using MCB
single phase 230V
cable will be in wall plastered over for 2m then in ceiling with less then 100mm insulation for 14m.

So I calulated

Ib= 47.82A

MCB size should be = 50A

Cable size = 16mm2

Voltage drop = 2.14V

Am I correct or have I missed something out along the calculation?

 

[telectrix]

way off. what about cooker diversity?

edit: unless it's an industrial oven and actually runs @ 11kW.

 

[assegayer]

way off. what about cooker diversity?

lets just say the cooker is running everything all the time..i know its far fetched but just experimenting

 

[Darkwood]

They work on rheostats so the element switch on and off to maintain a given temp' this gives a certain amount of diversity, it is pointless learning about it if you don't apply, if it was a shower then running it has a fixed constant load so rather than ignore diversity you should change your load in question, its a bad habit to get into and you will trip up if you done follow the correct cable calcs for the question.

 

[davesparks]

lets just say the cooker is running everything all the time..i know its far fetched but just experimenting

Diversity still applies!

 

[malcolmsanford]

Though I'm not a great lover of guidance notes unless the guidance is accompanied by a relevant reference to a regulation. The diversity appendix in the OSG isn't too shabby when it comes to cookers/ovens.

Have a read of that and then do your calcs.

 

[assegayer]

I think its my fault I never asked the question properly....forgot the fact that its an oven and its just an applicance that has a running load of 11kw

 

[malcolmsanford]

Let me turn this around, can you show us you calculations and how you come about it. Doncaster cables do an on line calc but if you want to use them.

How did you get to the results

 

[assegayer]

ok firstly I done

11kw divided by 230

= 47.82A

Then since its an MCB I multiplied I by 1

= 47.82A

Went to the onsite guide table (b1 I think) and saw the appropriate MCB is 50A

Then appendix F I Saw the below 100mm insulation and it says 16mm2 when I match the 50amps to the closest value.

Then I went

 

[Richard Burns]

You have got the right values.

The design current must be not greater than the rating of the circuit breaker that must be not greater than the current carrying capacity of the cable.

Ib < In < Iz

Your design current using I = P/V is 47.82 A
Circuit breakers to BS60898 come in standard ratings so a 40 A breaker would be too small but a 50A breaker correct.
The current carrying capacity of a cable varies according to the installation conditions, as more or less heat can escape the cable in different circumstance.
The current carrying capacity of a circuit is based on the most onerous conditions of installation encountered in that circuit.
You have a cable buried in plaster and a cable above a heat conductive surface (plasterboard) but covered in insulation to a depth of less than 100 mm.
The worst case is the insulation covered cable.
The standard reference method (100) for twin and earth cable installed as above gives a CCC of 45 A for 10 mm² and 57 A for 16 mm²; therefore a 16mm² cable is the smallest size that can carry the 50A current, assuming there are no other factors affecting the cable, such as ambient temperature, grouping, burial or maximum operating temperature.

 

[D Skelton]

You have got the right values.

The design current must be not greater than the rating of the circuit breaker that must be not greater than the current carrying capacity of the cable.

Ib < In < Iz

Technically that's not quite correct. Ib must be less than In and Iz, but In doesn't necessarily have to be lower than Iz in the case of fixed loads, which an oven would be.

 

[Richard Burns]

OK, I correct to: when considering protection against overload The design current must be not greater than the rating of the circuit breaker that must be not greater than the current carrying capacity of the cable.