Generator Info

[cgorham_rei]

I hope this is appropriate to reply here as I have searched the forums for this article number without finding any discussions on the exception.

to me overload protection can be afforded in two ways:

1. Generator controllers have integral current sensors and protective relaying for self preservation purposes. So if the 50 and 51 relays are set to a certain definite and time current characteristic the feeder
from the generator terminal cabinet to the first OCPD should be considered protected if the conductor damage characteristic is coordinated with these, and a shunt trip signal opens the remote overcurrent protective device.

2. Temporarily ignoring the generator controls and relaying, if the trip setting of a downstream circuit breaker is set to a value less than or exactly equal to the nameplate ampacity of the generator to me the generator conductors are protected from long time overload conditions by the circuit breaker.

3. To me the only application of the 115% sizing would come into play if the first overcurrent device or combined size of multiple grouped OCPD's exceeds the nameplate of the genset the 115% sizing requirement would be required.

can someone please provide feedback on this.

 

[Dennis Alwon]

I moved this to its own thread

 

[iceworm]

I hope this is appropriate to reply here as I have searched the forums for this article number without finding any discussions on the exception. ...

I can't tell what you are looking for. What "article number"? There are a bunch of threads on NEC 445, which is the article that covers generators.

It would help to know what level of generation you are concerned about.

Is is 15kw residential, 100kw construction or backup, 1000kw+ backup or prime? Protective relaying practices vary considerably.

to me overload protection can be afforded in two ways:

1. Generator controllers have integral current sensors and protective relaying for self preservation purposes. So if the 50 and 51 relays are set to a certain definite and time current characteristic the feeder
from the generator terminal cabinet to the first OCPD should be considered protected if the conductor damage characteristic is coordinated with these, and a shunt trip signal opens the remote overcurrent protective device.

2. Temporarily ignoring the generator controls and relaying, if the trip setting of a downstream circuit breaker is set to a value less than or exactly equal to the nameplate ampacity of the generator to me the generator conductors are protected from long time overload conditions by the circuit breaker.

3. To me the only application of the 115% sizing would come into play if the first overcurrent device or combined size of multiple grouped OCPD's exceeds the nameplate of the genset the 115% sizing requirement would be required. ....

The three items you list don't fit my understanding of generator protective relaying norms.

I'll give an example:
Item 3, 115% sizing: The only time the 115% conductor sizing comes into play is with Article 445.13. If the generator did not come with an OCPD (CB usually), then the conductors from the alternator terminals to the first over current device are allowed to be sized as low as 115% of the nameplate rating.

If the gen came with a CB, then article 445.13 does not apply to any field installed conductors.

Item 2 Downstream CB setting: Take a look at 445.12.A Smaller generation is protected by inherent design. The driver does not have enough power to overload the alternator.

The first CB is usually set to at least 125% of the nameplate current. One normally does not want the gen CB to trip on motor starting inrush, transformer inrush. It definitely should not trip on extended 100% power (continuous). The conductors are sized for continuous (125% nameplate) and the CB is sized to protect the conductors.

One might say this CB is protecting the gen against short circuit - not overload

Larger generation, there will be additional protection.

Item 1 - Ok. Sounds like larger generation. Onboard protective devices, CB protective relaying, additional protective relaying are all interrelated/coordinated to protect the alternator from damage due to several (many) out of spec operating scenerios. For larger generation most will trip the gen main CB and shut down the driver. I can't tell where you are going with this.

Rather than go any further - tell us the area of concern. And, I promise, there are a lot of threads about generation, protective relaying, generator conductors, and generator CBs.

And there are a lot of threads about residential generation - maybe not so helpful.

ice

 

[cgorham_rei]

This is a commercial installation with a generator that has no integral circuit breaker, a downstream generator circuit breaker 100% rated 3000A. The nameplate rating of the generator is 2500KVA/3007A@480V but the load served is less than 3000A.

Per 445.13 it calls for ampacity of conductors to be 115% of nameplate, or in this case 3458A. I am just trying to confirm my interpretation of the exception that "design and operation of the generator prevent overloading". With a 3000A 100% rated circuit breaker set to 3000A LTPU the generator in my mind is protected from overload and the phase conductors from the gen to the first breaker would be the same as the first circuit breaker to the downstream panel.

Hopefully this clarifies the request.