# Generator calculations/equations



## Toddiferous (Nov 1, 2019)

Hello all. Had a course recently in operation of a 120kva generator, but am struggling with the course content when it comes to calculating available current.

Unfortunately, I don't have a ton of info on the generator itself (no manufacturer details), though I don't think that will be necessary. My issue:

The course instructor said they have a 120kva, 120/208v generator that can deliver 1000 amps. Outlets on the generator are one 175A set and one 400A set, which makes sense to me (IL = ((120000/(208 x 1.73)) x 1


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## Toddiferous (Nov 1, 2019)

Got cut off...


Hello all. Had a course recently in operation of a 120kva generator, but am struggling with the course content when it comes to calculating available current.

Unfortunately, I don't have a ton of info on the generator itself (no manufacturer details), though I don't think that will be necessary. My issue:

The course instructor said they have a 120kva, 120/208v generator that can deliver 1000 amps. Outlets on the generator are one 175A set and one 400A set (each protected by breakers corresponding to their marked output), which makes sense to me (IL = ((120000/(208 x 1.73)) x 1.73 = 577A, approx. *Assuming Delta output.

My dilemma is confirming this process of calculation, and providing the course admin with some resources to help show that their 120kva 3 phase generator with a total of 475A output cannot give them 1000A.

Am I wrong? 
Do generators normally have a Delta output with neutral/centre tap for 120v loads?
(If so, how does a Delta with neutral not overload if there are a lot of 120v loads on it?)
Is there a comprehensive (legible) sheet with clear calculations/equations on the web that any of you recommend?

Thanks
Todd


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## The_Modifier (Oct 24, 2009)

Toddiferous said:


> Got cut off...


I hear it happens all the time when you don't fill out your profile as per the user sign up agreement. Specifically your electrically related profession and general location. :wink:


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## CoolWill (Jan 5, 2019)

Look at what is being said. You have a generator capable of delivering 208/120... Which is a wye configuration... Which means that each coil produces 120 volts. If you configured it to delta, you would have a 120 volt delta. 

120000÷(120x1.732) = ???


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## Wardenclyffe (Jan 11, 2019)

The square root of 3 is the positive real number that, when multiplied by itself, gives the number 3.


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## splatz (May 23, 2015)

Wardenclyffe said:


> The square root of 3 is the positive real number that, when multiplied by itself, gives the number 3.


And when divided by itself, gives the number 1.


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## Southeast Power (Jan 18, 2009)

Toddiferous said:


> Got cut off...
> 
> 
> Hello all. Had a course recently in operation of a 120kva generator, but am struggling with the course content when it comes to calculating available current.
> ...


Todd,
This is a reference. 
We use references.
I think you are getting hung up on what to do with that "1,000" in the question.

https://www.generatorjoe.net/html/genrate.html


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## Toddiferous (Nov 1, 2019)

Thanks Southwest Power. 

I did see that table, which helps me confirm the 334 amps for 120kva (seeing that 125kva is 347A), but how is the generator able to provide two separate outputs (a.k.a. breakers controlling the output/attachment of load) at 400A and 175A simultaneously without overloading? 

Note: I noticed that 334A x 1.73 gives me close to the total available generator output of 575A, which is what leads me to the confusion - I feel like I'm missing something basic on how a 334A output as calculated/referenced in he table can give 575 availailable amps. Because of this, confusion, I am also struggling to explain to the generator supplier why his 120kva rental cannot provide 1000A. 

I'm just not sure if I am looking at the generator, or the 3-phase calculations, correctly. (Also, I feel that it is obvious that the generator cannot provide 1000A, though I can understand how the supplier could have mixed it up: 334A x 3 because it is 3 phase)


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## CoolWill (Jan 5, 2019)

Toddiferous said:


> Thanks Southwest Power.
> 
> I did see that table, which helps me confirm the 334 amps for 120kva (seeing that 125kva is 347A), but how is the generator able to provide two separate outputs (a.k.a. breakers controlling the output/attachment of load) at 400A and 175A simultaneously without overloading?
> 
> ...


Go back and read my last post. You're missing that this is a 120 volt delta. It's a trick. You're calculating at 208.


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## Southeast Power (Jan 18, 2009)

Toddiferous said:


> Thanks Southwest Power.
> 
> I did see that table, which helps me confirm the 334 amps for 120kva (seeing that 125kva is 347A), but how is the generator able to provide two separate outputs (a.k.a. breakers controlling the output/attachment of load) at 400A and 175A simultaneously without overloading?
> 
> ...


Its not unusual to have two breakers on a generator. We see it all of the time when a fire pump is involved.


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## Toddiferous (Nov 1, 2019)

Great. Thanks all - I'll work with that, and see if I can get 'er to sink in.


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