KVA to Amps

[Alwayslearningelec]

Why am I not coming up with 123A. I used both 35 and 44 kva and still come up differently. Is it because of notes 1,2 & 3?

 

[infinity]

What formula are you using?

44000/208/1.73=122.27 amps

 

[wwhitney]

Why am I not coming up with 123A.

Because you forgot the factor of sqrt(3) which comes with 3 phase power. Of course, nothing in the image says the supply is 208Y/120V 4-wire rather than 208V 2-wire, but since the result is 123A, it must be.

Cheers, Wayne

 

[infinity]

Because you forgot the factor of sqrt(3) which comes with 3 phase power. Of course, nothing in the image says the supply is 208Y/120V 4-wire rather than 208V 2-wire, but since the result is 123A, it must be.

Cheers, Wayne

You're probably correct. Even though it's not mentioned in the truncated version of the panel schedule no one is going to install a 1 phase, 208 volt panel.

 

[ggunn]

What formula are you using?

44000/208/1.73=122.27 amps

Ah, yes, that pesky sqrt(3).

 

[Tulsa Electrician]

You're probably correct. Even though it's not mentioned in the truncated version of the panel schedule no one is going to install a 1 phase, 208 volt panel.

With the exception of multi family dewelings feed from 120/208 three phase.
Not applicable in this case.

 

[infinity]

With the exception of multi family dewelings feed from 120/208 three phase.
Not applicable in this case.

Yes that is one example of where you may see a 1Ø, 208 volt panel.

 

[jim dungar]

Yes that is one example of where you may see a 1Ø, 208 volt panel.

Yep, but you are most likely to see a 120/240V 1PH3W panel fed by a 120/208V 1PH3Wfeeder.

Particularly if the panel is an off the shelf instead of being a factory special.

 

[Alwayslearningelec]

Because you forgot the factor of sqrt(3) which comes with 3 phase power. Of course, nothing in the image says the supply is 208Y/120V 4-wire rather than 208V 2-wire, but since the result is 123A, it must be.

Cheers, Wayne

What is the 1.73 for?

 

[Eddie702]

KVA=44 which is 44,000va/208 volts=single phase amps of 211.53 amps/1.73=122.27 amps which is the three phase amps.

 

[ggunn]

What is the 1.73 for?

It's because the three phases are offset by 120 instead of 180 degrees.

 

[infinity]

What is the 1.73 for?

In short, anytime you use voltage in the formula you multiply the voltage by the square root of 3 or 1.73. ggunn gave you the reason.

So you end up with:
Kva/(208*1.73) or Kva/208/1.73.

 

[wwhitney]

What is the 1.73 for?

There are a number of ways to think about it, but the one I like is:

In a 3 phase wye system, the L-L voltage is sqrt(3) * the L-N voltage. If you want to covert a given identical current I on all 3 line conductors to a kVA, we can start with the case where the currents come from 3 identical L-N loads connected in a wye pattern. [Note that the neutral currents will cancel to give 0 neutral current.] We know how to calculate the kVA for a 2-wire load, it's just I * V. So since you have 3 identical loads, you end up with 3 * I * V_L-N. Given that V_L-L = sqrt(3) * V_L-N, that's equivalent to sqrt(3) * I * V_L-L. Then it turns out that the same formula works even for the 3-phase 3-wire case, where there is no neutral.

Cheers, Wayne

 

[ggunn]

What is the 1.73 for?

Three phase power was pretty weird for me at first; I transitioned from semiconductors where everything is DC and data to PV where AC is a major player fifteen years ago, and commercial PV is nearly always three phase on the AC side. Here is a pretty good read to get started: https://en.wikipedia.org/wiki/Three-phase_electric_power

 

[kelvin makoa]

i think three phases are offset by 120 instead of 180 degrees

 

[ggunn]

i think three phases are offset by 120 instead of 180 degrees

You think correctly.

 

[kelvin makoa]

You think correctly.

I was sure but iam