Power calculations help please?

[Darkoo]

Hi All,

First of all apologies for the potentially novice sounding question believe it or not I’ve been a qualified electrician for 10 years but I’ve always worked cards in so calculations have not been something I’ve ever considered I just install the sizes on the spec however I now have to design a small upgrade so looking for clarity on something.

I am installing a 3 phase sub mains to a 24 single or 8 3 phase ways board.

The load on the 3 phase DB is going to be entirely single phase and although balance is going to be as close as possible it will not be perfectly balanced.

I have a total of 38kw of load which will be split 14-12-12 across L1-L3.

Total load is 165A or thereabouts so the question is does the 3 phase cable current carrying capacity refer to the total load (165) or the per phase (approx 55)

Also with that in mind is the 3 phase load then considered 165A or is it 52A applying the calculation to 38kw?

For this I’m not taking into account volt drop or anything I have an app do actually do the calculations but I want to understand how it works.

Thanks in advance

 

[Julie.]

55A

Although this doesn't account for power factor , nor unbalance

So depends on the type of load, and diversity of course.

 

[Darkoo]

55A

Although this doesn't account for power factor , nor unbalance

So depends on the type of load, and diversity of course.

Thanks, do you know the calculation behind this though? So if I put the calculation into any cable calc online when I state that the overall load is 165A it wants a 70mm cable but if I put 55 which would be the per phase it comes down to 16mm im trying to understand why it’s 55 not just outright the answer

 

[cireland]

I would think it would be the load per phase you enter into the calculator, or more correctly the highest of the three.

I would generally consider the possibility of future expansion too.

 

[James]

think of 10 circuits at 10A each
single phase easy,
L= 100A
N= 100A

split them across 3 phases and you have

L1 4 x 10A =40A
L2 3 x 10A =30A
L3 3 x 10A =30A
N = 10A

in simple terms.

 

[Julie.]

Formally

I = P/(3 × Vph) (phase - neutral voltage)

so 38000/(3 x 230) = 55A

I = P/(sqrt3 x V[ph-ph]) (phase - phase voltage)

So 38000/(1.73 × 400) = 55A


Ideally though add up each current that you apply per phase, as stated above, basically if you have 4 10A loads then you would have one phase with 20A whilst the others have just 10A, so you would size for more than 20A

If you just calculated it by power 10A x 230V x 4 = 9.2kW

The calculation would be 9200/(3 × 230) = 13.3A

Which is an average, and not the actual expected current (too low on one phase)

 

[Darkoo]

Thanks for the answers.

So essentially although the total load is the 165 on the cable calcs the cable sizing is based on a single phase load so given than my largest load will be 61A on L1 use a 16mm 4 core to allow for future expansion?

That way I can then utilise 100A per core if required?

Thanks for that though really helpful