Formula 3 phase amps from watts

[Takoma Park Volunteer Fire Department Postmaster]

Can someone supply the furmula that will tell me how many amperes of 208
volt current an 80 KVA generator will supply.

 

[Don Kelly]

Can someone supply the furmula that will tell me how many amperes of 208
volt current an 80 KVA generator will supply.

I=KVA*1000/(V*(root 3)) where V is the line to line voltage (208)
This gives 666A

 

[Bill]

Not on MY slide rule!
I get 222A

ARM

Hello,

If the line to line voltage is 208V, then three legs @ 120V will give
666.17 Amps. We should note that this is a total current for the
generator. Namely, each leg can supply a current of approx. 222.05
Amps. So both of you are correct; it depends on what you consider to be
your reference.

Regards,
Bill

 

[daestrom]

Hello,

If the line to line voltage is 208V, then three legs @ 120V will give
666.17 Amps. We should note that this is a total current for the
generator. Namely, each leg can supply a current of approx. 222.05
Amps. So both of you are correct; it depends on what you consider to be
your reference.

Although I agree that each phase can carry 222.05 amps, there is no way to
combine the three phases to supply a 666.17 Amp load. The vector sum of the
three 222.05Amp phases would be zero (ideally).

When rating three-phase equipment, I've always seen the line current used,
not 3X that value. Line-line voltage and balanced line current is the
normal way to describe their ratings.

Most 3-phase generators are also limited on the amount of imbalance they can
tolerate. This unit probably cannot be used to power a single phase load at
222 Amps on only one phase with the others unloaded. Such an unbalance
would likely burn the rotor. I've seen imbalance limits of 5 to 15%
depending on the size of the unit, but others may tolerate more (or less).

daestrom

 

[SQLit]

R-O-T is 2.8 amps/lVA at 208V 3ø >> 80x2.8=222.3 amps

--s falke

don't forget the power factor of the generator. Probably 90% or less.
222.3*.90= 200.07


You have, probably, at full load a 200 amp generator maybe a little less

 

[Bill]

Three phase currents aren't typically specified summed up. There is
nowhere in a 3 phase system where a current of 666 amps can be read with
a clamp-on meter, for example. Breakers and conductors are sized based
upon the phase current (222A).

I certainly agree, but in terms of what the generator is capable of
producing, the total current is 666 Amps. (1.0pf)

Bill

 

[Mark or Sue]

I certainly agree, but in terms of what the generator is capable of
producing, the total current is 666 Amps. (1.0pf)

One thing to realize in 3 phase systems is that three separate 200A single phase loads will produce
a load at each pole of well over 200A (200 * 1.7 in fact, which is 346 amps) . You can't ever get
more than 222A from each phase of this generator. You can run three separate 128A single phase 208V
loads on each phase at one. This will produce a load of 222A for each phase from the generator
because loads on adjacent phases add to each other by a factor of 1.7.

So if you were a marketing guy, you could say this generator will run 128*3 or 384 amps, but that
would be typical marketing misleading. But notice that the VA still comes to 80 KVA when used with 3
separate single phase loads. No matter how you slice it, you shouldn't ever get more than 80 KVA
from an 80 KVA generator.

 

[Don Kelly]

You are absolutely correct. I am embarrassed -I apparently multiplied by
root(3) rather than dividing. - I blame it on the wine

 

[daestrom]

I certainly agree, but in terms of what the generator is capable of
producing, the total current is 666 Amps. (1.0pf)

No 3-phase generator that I've ever seen (and I've seen quite a few,
induction, synch., wound-rotor, motors and generators) would ever have a
current rating calculated as 3X line current. If you add the three phase
currents of the generator while running at 1.0pf you get zero, not 666.

Machine ratings are by the line current in each phase, not some cockymaimee
adding of all three line currents together. If you use your 666 Amps and
the line voltage to figure the machines kVA, you get the wrong answer
(unless you make up your own formula and don't use the ones standard in the
industry).

daestrom

 

[Operator Jay]

I wondered if anyone would jump on that.
The OP stated 80 KVA which is apparent power NOT true power (KW).
Power factor does not come into play.
222.3 amps is the correct answer if it is a 3 phase generator.
ARM

For the benefit of the OP we should mention that the generator will only
supply the rated 222A per phase to certain loads. If he's determining
whether a certain gen size is 'adequate' he needs to talk to a supplier or
engineer because there are issues beyond rated kVA. Of course he's welcome
to ask more Q's here first.
j