How to work out the power?

[shocksl]

Hey guys

I can pretty much do the rest of the calcs.

But how do I know what the power is ? Once I got it the rest is just following a process.

Thanks.

 

[Rockingit]

Read the question….. ALL of the question….

 

[shocksl]

Read the question….. ALL of the question….

Is it that the data is missing ? So effectively I can put any power in there?

I was thinking p = I 35A x (400 v Square of 3)?
Will that be acceptable?

 

[plugsandsparks]

That question is almost as bad as situations i get with clients, one of the most important info missing, lol
Anyway as for power, this can be expressed as KVA or KW, however key info missing again, so you can only provide KVA.
Try ChatGPT and ask it to calculate KVA based on a load current of 35A and supply voltage of 400V.
That would have been a great question and a good use of BS7671, shame - good luck

 

[Rockingit]

Sorry - am I missing something here? To me it seems that everything is there with the exception of a distance?

 

[DefyG]

Sorry - am I missing something here? To me it seems that everything is there with the exception of a distance?

Unless we assume unity .............

 

[shocksl]

How would you work out power then?

 

[Rockingit]

How would you work out power then?

It doesn't ask for power. It asks for design current (which is stated at 35A per ph).

So our cable needs to be in a ratio /10m length which allows for the Max Zs of a C40 RCBO - so that's Ze (0.35) + x, where x is informed by the R1R2 value of relevantly sized cables and voltage drop for conductor sizes at load and voltage, less any derate factors. I'm not going to get out the tables as we don't do other people's homework on here....

 

[westward10]

It is asking for stage 1 to select a cable size by using the design current, the rating of the protective device and the installation method. You have been given these three factors to work with things like volt drop, R1+R2, Zs, adequacy of cpc are done after stage 1 to ascertain if the cable selected is adequate.

 

[shocksl]

It is asking for stage 1 to select a cable size by using the design current, the rating of the protective device and the installation method. You have been given these three factors to work with things like volt drop, R1+R2, Zs, adequacy of cpc are done after stage 1 to ascertain if the cable selected is adequate.

I know it isn't asking for power.

But I normally work in by p/voltage. But I don't know what that P is. Then In is greater than IB . And I carry on after that

 

[westward10]

You are complicating things. This is stage 1 of circuit design just concentrate on that you only have one calculation to do and that is applying the correction factor for the buried cable to In, once you have this you can select a cable.

 

[westward10]

I know it isn't asking for power.

But I normally work in by p/voltage. But I don't know what that P is. Then In is greater than IB . And I carry on after that

Stage 1 is asking for Iz, if gives you Ib and In.

 

[Rockingit]

What Westy said.

But if you insist.... let's look at it from another angle, using the traditional beer/head analogy for pf. It's a milking parlour - pick full fat (4%), semi (2%) or skimmed and go with that.

 

[plugsandsparks]

I think thats where i just looked at and thought whats the point, without distance its not going to give the CSA, so i guess from the responses, that the student is asked to consider a CSA based on just one aspect of the cable design... then they tell you its a 400M run, lol
If they had included the distance and made it a nice long run, the student would have benefitted more.
Anyway OP asked for power not CSA