# wire sizing for 500kW generator



## Gothp (Jan 4, 2012)

First the backround on this topic. I am an electrician in the military, on the outside I do maintenance for large plant. Currently I am deployed and working a project that would normaly be sent to an engineer, but do to the location it is falling on my lap. 
So the scenario. We are trying to eliminate the use of about 12 generators into 2 large generators. There is also a mixture of 50Hz and 60Hz. The plan is to take 2 commercial 500kW gens, run them to a transfer switch to allow one to be serviced while the other is running. From there into a 1200 amp panel. This will distribute to 4 225kVA transformers stepping down from 480 to 208/120. At that point it will go from commercial equipment into military electrical components. 
The issue i have run up against is when i do wire sizing, how do i figure the load for each phase. Example; one 225kVA transformer has a 270 amp capacity (if my math is wrong please correct me and give me the formula), is that 90 amps per phase or is that 270 amps per phase. Basicaly I need the right max amps per phase to get the right wire size. 
I appreciate any help I can get. The engineer that is "helping" with this is a civil guy and is guessing at what the answers are, not giving me a hard answer that I feel comfortable with.


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## Southeast Power (Jan 18, 2009)

If those units have a breaker, I would expect the to be about 800 amps, and size the wire accordingly. 
If not, I would install 110% conductor capacity to the transfer switches.
From that, feed your 1200amp distribution panel.


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## Southeast Power (Jan 18, 2009)

I would be more concerned with the size of the load more than basing my design on the transformer max capacity alone.
As for your calculations, the amperage is phase to phase and or phase to neutral.
100 amp breaker will give you 100amps phase to phase or phase to neutral.
Kinda like using KW for calculating.


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## Gothp (Jan 4, 2012)

I believe it is an 800amp breaker on the generator. But that doesnt mean each phase is sized to carry 800 amps, right. Idealy the 800amps would be spread over the three phases;267 per phase. So i would size the wire to carry 267? I thought I knew the answer to this question, but the more I research the more confused I become. Thank you again.


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## 3DDesign (Oct 25, 2014)

Gothp said:


> I believe it is an 800amp breaker on the generator. But that doesnt mean each phase is sized to carry 800 amps, right. Idealy the 800amps would be spread over the three phases;267 per phase. So i would size the wire to carry 267? I thought I knew the answer to this question, but the more I research the more confused I become. Thank you again.


On an 800 amp breaker, each phase must be rate 800 amps. It's early and I have to get to work but I would guess parallel 500 MCM on each phase.
Correct me if I'm wrong.


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## Southeast Power (Jan 18, 2009)

3DDesign said:


> On an 800 amp breaker, each phase must be rate 800 amps. It's early and I have to get to work but I would guess parallel 500 MCM on each phase.
> Correct me if I'm wrong.


No, more like 3x500


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## ElectricJoeNJ (Feb 24, 2011)

3DDesign said:


> On an 800 amp breaker, each phase must be rate 800 amps. It's early and I have to get to work but I would guess parallel 500 MCM on each phase. Correct me if I'm wrong.


 parallel 600's per phase or 3 sets of 300's


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## Southeast Power (Jan 18, 2009)

Gothp said:


> I believe it is an 800amp breaker on the generator. But that doesnt mean each phase is sized to carry 800 amps, right. Idealy the 800amps would be spread over the three phases;267 per phase. So i would size the wire to carry 267? I thought I knew the answer to this question, but the more I research the more confused I become. Thank you again.


Yes, 800 phase to phase or phase to neutral.
You are running it as prime so, the manafacture might have a different rating for that.
What is your actual load?
Sf of buildings
Hp of motors
Fastened in place equipment
Exterior lighting
Resistive heating
A/C load
More to consider than just the size of the feeder wire and generator breaker.
You may only have 200 or 300 amps worth of load and end up designing an 800 amp distribution.
Conversely, you might have 1000 amps of load and have to parallel both units.


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## wildleg (Apr 12, 2009)

Gothp said:


> First the backround on this topic. I am an electrician in the military, on the outside I do maintenance for large plant. Currently I am deployed and working a project that would normaly be sent to an engineer, but do to the location it is falling on my lap.
> So the scenario. We are trying to eliminate the use of about 12 generators into 2 large generators. There is also a mixture of 50Hz and 60Hz. The plan is to take 2 commercial 500kW gens, run them to a transfer switch to allow one to be serviced while the other is running. From there into a 1200 amp panel. This will distribute to 4 225kVA transformers stepping down from 480 to 208/120. At that point it will go from commercial equipment into military electrical components.
> The issue i have run up against is when i do wire sizing, how do i figure the load for each phase. Example; one 225kVA transformer has a 270 amp capacity (if my math is wrong please correct me and give me the formula), is that 90 amps per phase or is that 270 amps per phase. Basicaly I need the right max amps per phase to get the right wire size.
> I appreciate any help I can get. The engineer that is "helping" with this is a civil guy and is guessing at what the answers are, not giving me a hard answer that I feel comfortable with.



1) you need to design the whole service. draw it out with wire sizes, and the load calc, and post, and you will get all the help you need. Regardless of the simplicity/complexity, it does not make sense that you are not getting the help you need. If nothing else, have the civil engineer sign off on your "suggested" design. Once you put his a$$ on the line, he may very well send it to someone else to get verification.

2) you also need to understand that this may or may not fall under the NEC, and that your ahj (you) may not require it to technically comply (I have no knowledge of this)


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## micromind (Aug 11, 2007)

A 500KVA gen is good for 602 amps at 480 3Ø. 

a 500KW gen is good for 753. 

Both ratings are amps per phase. Almost all electrical calculations are amps per phase. Think of a 3 pole breaker as 3 separate single pole breakers. With a single pole breaker, you'd size the wire to the rating of the breaker. Same with a 2 or 3 pole. 

Wire for a gen is sized per the breaker, not the gen. It is per phase, meaning that if it's 800 amp, each phase must be able to carry 800 amps. 

Even though it's common to see 2 parallel 500s on an 800 amp circuit, it's a violation of 240.4(C). If it's 800 amps or more, you can't go to the next higher rating. 

If you're using copper, it'd be 2 parallel 600MCM or 3 parallel 300MCM, provided each set of conductors is in its own conduit.


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## wildleg (Apr 12, 2009)

micromind said:


> A 500KVA gen is good for 602 amps at 480 3Ø.
> 
> a 500KW gen is good for 753.
> 
> ...


actually, it's not a violation at 800 amps. it's a violation if it is OVER 800 amps.



> C)
> 
> 240.4 C
> Overcurrent Devices Rated over 800 Amperes.
> ...


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## micromind (Aug 11, 2007)

You're right, I don't know how I missed that part........


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## wildleg (Apr 12, 2009)

micromind said:


> You're right, I don't know how I missed that part........


it's ok. you are allowed one mistake. just don't let it happen again.


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## micromind (Aug 11, 2007)

wildleg said:


> it's ok. you are allowed one mistake. just don't let it happen again.


Lol.......

For years I thought it stated '800 amps', not 'more than 800 amps'. 

I work with a lot of stuff 800 amps or more. 

Oh well.....


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## Knightryder12 (Apr 4, 2013)

wildleg said:


> actually, it's not a violation at 800 amps. it's a violation if it is OVER 800 amps.


That's only true if the calculated load is not over 760 amps. by using two sets of 500 kcmil you limit yourself to 760 amps or less. Better to use the 3 sets of 300 kcmil.


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## Southeast Power (Jan 18, 2009)

Knightryder12 said:


> That's only true if the calculated load is not over 760 amps. by using two sets of 500 kcmil you limit yourself to 760 amps or less. Better to use the 3 sets of 300 kcmil.


Rounded up to the next standard OCP size of 800amps? :001_huh:


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## FF301 (Jan 12, 2014)

You really need an engineer to determine generator capacity, they will consider HP, fuel type, operating temp, generator specs such as # of poles, PF, and so on. 
Are you using a transfer switch only allowing one genset at a time or can you parallel units. 
You need to size for your max output based on unit size before OCP then size based on disconnect to transfer switch to If you can parallel units then you must size up from transfer switch to main panel based on both units running ( again sizing for max current) 
Is this for civilian use or military use cause military does not follow the NEC. 
Remember if you intend to parallel the gensets you need to be able to control the voltage regulator and eng speed to balance the load.


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## Carultch (May 14, 2013)

Knightryder12 said:


> That's only true if the calculated load is not over 760 amps. by using two sets of 500 kcmil you limit yourself to 760 amps or less. Better to use the 3 sets per phase of 300 kcmil.


500 kW on the dot, is equal to 602 Amperes per phase at 277/480 V three phase. If it is a continuous load, you thus have to multiply by 1.25 to find the amperes of wire needed, and to find the value you round up to your breaker size.


602A *1.25= 752A, which means an 800A breaker would be used.

The "next size up rule" applies to OCPD's which are 800A (inclusive) and less. As long as the next size up rule also applies this situation for the nature of this particular load and arrangement of wiring, two parallel sets of 500 kcmil in separate raceways would be acceptable. You do need to make sure that your device isn't ever going to operate at more than 500 kW. Not 550 kW.

I don't really work with generators much, so I don't know if there are special rules that apply to them, unlike ordinary loads.

If you are unsure of whether or not you should be using the next size up rule, it is better to just size the wire to the breaker amps, which would be 3 sets of 300 kcmil copper in separate raceways.


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## STLelectrician (Jan 2, 2015)

I am a generator specialist and yes it can handle 752 amps per phase per gen set and that should be determined by doing your load calc, including the largest LRA of any compressor and motor used and ups systems because they draw allot, and as far as running a 1200A service, you should be OK because as you know load calc for your service is almost always more that what you will ever use....just make sure your load calc is correct for sizing generator for what your running


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