kVA and kW

[Robotmannick]

Before I ask this, I should say this is for my understanding only and is completely hypothetical, not for a domestic situation!

If a supply cable is rated at say 1500kVA, does that mean that a 1500kW load can be connected to it? Am I right in thinking that the power factor will mean that the 1500kW load will be too great for the 1500kVA cable?

Or have I got that completely wrong?

Cheers,

Nick

 

[ban-all-sheds]

If a supply cable is rated at say 1500kVA, does that mean that a 1500kW load can be connected to it?

Not necessarily.

Am I right in thinking that the power factor will mean that the 1500kW load will be too great for the 1500kVA cable?

Quite possibly - kVA = kW/PF, and PF is ≤ 1

 

[Robotmannick]

Yeah, I think that's what I meant !

Best case scenario (PF=1) it would just about be ok. Given that the PF will invariably be at least a little less than 1 (quite a lot less if the load is a whacking great motor for instance), then I'd be overloading the cable.

Thanks BAS.

 

[ricicle]

Yeah, I think that's what I meant !

Best case scenario (PF=1) it would just about be ok. Given that the PF will invariably be at least a little less than 1 (quite a lot less if the load is a whacking great motor for instance), then I'd be overloading the cable.

Thanks BAS.

Are you enquiring about a cable loading or transformer/generator rating. If it is the former then loadings for these are not done on KVA but KW or Amps.
Transformer or generator ratings are given in KVA so as it can be seen what the maximum current it can deliver though not all of it neccessarily being used for real power (KW)

 

[westie101]

Are you enquiring about a cable loading or transformer/generator rating. If it is the former then loadings for these are not done on KVA but KW or Amps.

possibly that applies in installations. All the ratings that we give equipment are either in kVA or Amps. kW is rarely used on the supply networks

 

[bernardgreen]

As Ricicle has said a cable cannot be rated in Kva alone. The operating voltage of the load has to be given in order to calculate the current the cable will have to carry

Cables are often stated to be capable of supplying a certain Kva load but with the unspoken assumption that the voltage is 230 volt ( or 415 if 3 phase ). Not good if the supply voltage is less than 230 as the cable may be overloaded by the current at the lower voltage.

 

[JohnW2]

If a supply cable is rated at say 1500kVA, does that mean that a 1500kW load can be connected to it? Am I right in thinking that the power factor will mean that the 1500kW load will be too great for the 1500kVA cable?

As ricicle has implied, it would make no sense for a cable's rating to be quoted in kVA - quite apart from the issue of power factor, the voltage is irrelevant to the 'carrying capacity' of the cable. The relevant thing is the current-carrying capacity of the cable (and, of course, it's ability to handle the voltage concerned).

Having said that, as everyone has said, the current drawn by a 1500kW load can never be less, and will usually be (slightly or appreciably) more than the current that would result in 1500kVA (at the voltage concerned) - so your surmise is essentially correct.

Kind Regards, John

 

[Robotmannick]

Interesting...I've been given the cable rating in kVA - no other specs. Perhaps as you've suggested, the 415 (3-phase) has been assumed. As Westie points to, the specs come from the supplier.

I'm essentially poking my nose into something that is well outside of my skills because I'm interested in learning more. I'm being told by managers who probably know as little (if not less!) than me that connecting a 1500W load to cables rated at 1500kVA is fine.

It's still theoretical at the moment as this is a paper exercise for a project evaluation. If it ever comes to fruition, it will of course be handled by qualified people etc so I have no safety worries whatsoever. My concern was more than no cost provision has been made for up-rating the supply capacity so they're essentially making a false evaluation.

 

[JohnW2]

I'm essentially poking my nose into something that is well outside of my skills because I'm interested in learning more. I'm being told by managers who probably know as little (if not less!) than me that connecting a 1500W load to cables rated at 1500kVA is fine.

As everyone has said or implied, if a cable 'rated at 1500kVA' means that it is rated at X kA and that multiplying X by the voltage supplied to the load gives an answer of 1500, then using that cable for the 1500kW load would theoretically only be 'fine' when PF=1 and would become increasingly less 'fine' as PF reduced below 1.

Kind Regards, John

 

[Spark123]

Unless you are running a whacking great big 1.5MW heater or incandescent lighting (or something else resistive) then the load won't be in KW. You are talking about something up in the region of distribution type engineering with that sort of power, you'd certainly need a big localised transformer!

 

[ban-all-sheds]

As ricicle has implied, it would make no sense for a cable's rating to be quoted in kVA - quite apart from the issue of power factor, the voltage is irrelevant to the 'carrying capacity' of the cable. The relevant thing is the current-carrying capacity of the cable (and, of course, it's ability to handle the voltage concerned).

That was my second thought too. (The first was a non-thought, i.e. I didn't think about what the OP wrote).

But then.....

Ignore any insulation strength concerns, and voltage is relevant, as it's what will drive the current through the resistance of the cable.

The cable's resistance is fixed. The current it can carry is fixed.

Therefore the square of the current multiplied by the resistance is fixed.


If you knew the length of the cable concerned then it would be quite valid to quote I²R.

If you don't know the length then you could quote a kVA/m figure.

 

[JohnW2]

That was my second thought too. (The first was a non-thought, i.e. I didn't think about what the OP wrote).
But then..... Ignore any insulation strength concerns, and voltage is relevant, as it's what will drive the current through the resistance of the cable. The cable's resistance is fixed. The current it can carry is fixed. Therefore the square of the current multiplied by the resistance is fixed. If you knew the length of the cable concerned then it would be quite valid to quote I²R. If you don't know the length then you could quote a kVA/m figure.

You could, indeed, but you would then surely be talking about dissipation in the cable at maximum permissible current, which would not be an indicator of what load the cable could supply, would it? For example, such an I²R figure for a cable quite capable of supplying a 1500W load would obviously be very much less than 1500 kVA and, I would think, not a very useful method of 'rating' a cable! ... or am I misunderstanding you?

Kind Regards, John

 

[ban-all-sheds]

Oh, absolutely - the "rating" takes no account of how much power needs to be dissipated in the load.

So 2.5mm² T/E with a Ct of 27A and an R1+RN of 18mΩ/m (at 70°C), it's "rating" would "clearly" be 13.1W/m.

 

[JohnW2]

Oh, absolutely - the "rating" takes no account of how much power needs to be dissipated in the load. So 2.5mm² T/E with a Ct of 27A and an R1+RN of 18mΩ/m (at 70°C), it's "rating" would "clearly" be 13.1W/m.

Indeed so - but what use would such a 'rating' be to anyone? I think this tangential point is a bit of a red herring!

Kind Regards, John