kw vs kva

[mstrlucky74]

Was told today that these work out to be pretty close when calculating to amps for the same kw or kva. so if you have a 45kw load and make that 45kva whatever amperage you get would be close to if you have calculated in kw. true?

 

[jaggedben]

Yes, kind of, if you ignore power factor. At power factor of 1.0, kva=kw. If the load is mainly resistive, e.g. lighting and heat, power factor will be 1 or very close to that. When you have motors or other less simple loads, then the waveforms of volts and amps may not line up. That's when you get a power factor below 1. KVA is then more than kW. In extreme cases it could be a problem where amps is actually more than kW/V. If you use kW/V to size conductors and equipment you could be undersizing for actual amps.

 

[jumper]

Did not really quite get your question.

kW is kVA x power factor.

A purely resistive, like baseboard heat, load would have a power factor of ~1, so kW =kVA.

A fluorescent light may have a typical pf .9

A motor perhaps .8/.85, if loaded correctly. PF in motor varies a lot for a few reasons.

So not all loads in kW are going to equal kVA.

kW is less then or equal to kVA.

 

[mstrlucky74]

thanks

 

[LarryFine]

To add, the difference is caused by capacitive or inductive reactance.

See:

https://en.wikipedia.org/wiki/Power_factor
https://www.rapidtables.com/electric/Power_Factor.html
https://www.allaboutcircuits.com/ht...ting-current/chpt-11/calculating-power-factor

 

[Sahib]

Was told today that these work out to be pretty close when calculating to amps for the same kw or kva. so if you have a 45kw load and make that 45kva whatever amperage you get would be close to if you have calculated in kw. true?

Practically speaking, loads have a power factor of 0.8. So amps, calculated by kw, may differ within 25% from actual value.

 

[Besoeker]

Was told today that these work out to be pretty close when calculating to amps for the same kw or kva. so if you have a 45kw load and make that 45kva whatever amperage you get would be close to if you have calculated in kw. true?

Not a safe assumption to make. Use kVA for calculating current - or amperage as you guys tend to call it.

 

[LarryFine]

In a nutshell, the wiring has to carry more current than the load itself consumes, so it becomes easy to under-size the conductors.

 

[Sahib]

In a nutshell, the wiring has to carry more current than the load itself consumes, so it becomes easy to under-size the conductors.

Really?! I thought load current=circuit current.

 

[kwired]

In a nutshell, the wiring has to carry more current than the load itself consumes, so it becomes easy to under-size the conductors.

Really?! I thought load current=circuit current.

Better wording would have been: In a nutshell, the wiring has to carry more "apparent power" than the "true power" the load itself consumes. But I'm pretty sure that is his understanding.

 

[kwired]

Practically speaking, loads have a power factor of 0.8. So amps, calculated by kw, may differ within 25% from actual value.

True resistance loads are 1.0 power factor.

Motors will vary depending on loading. any power factor stated on nameplate is only true if motor is operating at rated voltage and rated output load.

 

[jumper]

Bad pun alert:

Apparently, one factor in finding the true answer is how the loads react.

 

[Sahib]

True resistance loads are 1.0 power factor.

Motors will vary depending on loading. any power factor stated on nameplate is only true if motor is operating at rated voltage and rated output load.

I think OP referred to a mixture of loads, not load of a single device. In that context, a power factor of 0.8 is useful. Incidentally a generator is rated to supply loads at 0.8 power factor, signifying 0.8 power factor is common for general loads.

 

[Ingenieur]

S = P + jQ
S total power
P real or active power
Q reactive or imaginary power (I don't like that term because it actually exists but does no real work)
|S| = sqrt(P^2 + Q^2)
pf = cos(arctan(Q/P))
P = S x pf

so the closer the pf to 1 the closer they are
utilities strive to maintain > 0.9
although the load served determines it's pf

 

[Jraef]

The OP has indicated that he has the answer he was looking for and since this shows the earmarks of descending into a side discussion of "what I/he meant...", I'm just going to close the thread now.