Generator breaker sizing

hhsting

Senior Member
I have 400kW 208V three phase emergency generator in high rise building. There are two main breakers 1600A, 450A.

Total for main breakers is 2050A.

Are the main breakers for emergency generator sized properly per NEC 2017 section700?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Depends on the loads you are supplying. If 2050 is your total calculated load and you are trying to supply the whole load 400 kw would only be about half the needed size.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
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retired electrician
What is the demand load for the actual emergency loads to be supplied? The generator has a full load output of 1100 amps.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
So the breaker size is ok?
Maybe, but you need the generator details to know. The generator has to be protected from overload per 445.12(A). The combination of breakers that you have will not provide the required overload protection for the generator. However the rule in 445 provides for other methods to protect the generator. Without the generator detailed information, the question cannot be answered.
 

hhsting

Senior Member
Maybe, but you need the generator details to know. The generator has to be protected from overload per 445.12(A). The combination of breakers that you have will not provide the required overload protection for the generator. However the rule in 445 provides for other methods to protect the generator. Without the generator detailed information, the question cannot be answered.

We don’t get submittals. What would be either cases it has overload protection and it does not have overload protection?
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
We don’t get submittals. What would be either cases it has overload protection and it does not have overload protection?
No idea how an effective plan review can be completed with out the equipment documentation. The generator needs overcurrent protection.
 

don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
Alright can you see post#1 question? How do you size the overcurrent protection if two main breakers for emergency generator?
You just have not provided the required information to answer the question as we know nothing about what protection, if any is part of the generator itself.
I fail to understand why the OCPDs in that post are so large since you said the load was substantially less than the breaker ratings.
On the face of what you have told us, there is something missing. There is no reason that I can think of that would result in a design where the feeder breakers for an Article 700 system have total rating that is about twice the full load output current of the generator.

Something is missing and without that something the question will forever remain unanswerable.
 

hhsting

Senior Member
You just have not provided the required information to answer the question as we know nothing about what protection, if any is part of the generator itself.
I fail to understand why the OCPDs in that post are so large since you said the load was substantially less than the breaker ratings.
On the face of what you have told us, there is something missing. There is no reason that I can think of that would result in a design where the feeder breakers for an Article 700 system have total rating that is about twice the full load output current of the generator.

Something is missing and without that something the question will forever remain unanswerable.

I still dont get how the missing info can impact. What I asking is the short circuit ground fault protection.

There are two scenarios one if generator has overload and second generator has no overload. What are the consequences and breaker sizes in both cases?
 

Rock86

Senior Member
Location
new york
Occupation
Electrical Engineer / Electrician
I still dont get how the missing info can impact. What I asking is the short circuit ground fault protection.

There are two scenarios one if generator has overload and second generator has no overload. What are the consequences and breaker sizes in both cases?
That is not what you asked at all. You're not providing the proper information to answer the question properly. If you are on a plan review team, you seriously need to go back to the design engineer and ask them for their reasonings. Then come back to us with their feedback and potential code.

As an engineer and former electrician, you need to know what the system is doing in order to understand it. This generator may be supplying a warehouse worth of lights, or four elevators... we don't know, and if you can't tell us, you need to go back to the designer and have a conversation them the questions.

If the moderators feel i am out of line with the rules, you have my permission to remove my post.
 

hhsting

Senior Member
That is not what you asked at all. You're not providing the proper information to answer the question properly. If you are on a plan review team, you seriously need to go back to the design engineer and ask them for their reasonings. Then come back to us with their feedback and potential code.

As an engineer and former electrician, you need to know what the system is doing in order to understand it. This generator may be supplying a warehouse worth of lights, or four elevators... we don't know, and if you can't tell us, you need to go back to the designer and have a conversation them the questions.

If the moderators feel i am out of line with the rules, you have my permission to remove my post.

Ok but I thought Don asked something regarding overload is their or not.

Anyways since you all want details I will coordinate and provide details.However even afterwards I cannot tell you if generator has overload protection or not
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
445.12 Overcurrent Protection.
(A) Constant-Voltage Generators.
Constant-voltage generators, except ac generator exciters, shall be protected from overload by inherent design, circuit breakers, fuses, protective relays, or other identified overcurrent protective means suitable for the conditions of use.
Echoing Don's comment from post 6, with emphasis on the rationalization.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Ok but I thought Don asked something regarding overload is their or not.

Anyways since you all want details I will coordinate and provide details.However even afterwards I cannot tell you if generator has overload protection or not
Then the question cannot be answered.

I guess to be safe you could require that the breakers you are talking about provide the required overload protection for the generator. That would require the use of breakers that would limit the total long time current flow to less than 1110 amps.
 

jap

Senior Member
I would say if the 1600 and 450 amp breakers are the only overcurrent protections devices associated with the output of the generator, they are oversized.

There's nothing to limit an overload of the output windings of the generator at that combined total.

If the generator has overcurrent protection integral or ahead of the 1600 and 450 amp breakers to protect the generators output winding, then, the generator is protected and the 1600 and 450 amp breakers are not oversized,

However,

You wouldn't be able to run the 1600 and 450 amp equipment at its full capacity since the output protection of the generator's secondary is limited to roughly 1100 amps.

If you want to run your 1600 and 450 amp load at their full capacity and use them to protect the windings on the generator, you need a larger generator.

If you want to run your 1600 and 450 amp loads at the calculated load of 1089 amps for any length of time, you can roll the dice, with a protected output winding at 1100 amps, but, in my opinion, your generator is still too small.

If you want to burn your 400kw generator up, leave the 1600 and 450a overcurrent protection as the generators only overcurrent protection and let your calculated load fluctuate and exceed the generators output for a little while.

JAP>
 
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