Generator Circuit Breaker

[NEC User]

Is there any calculation I can perform to determine what circuit breaker should be installed at the generator? The generator is diesel 80KW 277/480. Or should I just rely on what the manufacturer says? I thought it would be 80000= 1.732 X I x 480 and solving this I get I=96 Amps. Should I use a 100A circuit breaker to prevent damage to the generator or should I do 125% * 96Amps => 120A and thus use a 125A circuit breaker? The generator manufacturer stated it would come with a 125A circuit breaker but I'd like to know how they came up with this calculation.

 

[jrohe]

Is there any calculation I can perform to determine what circuit breaker should be installed at the generator? The generator is diesel 80KW 277/480. Or should I just rely on what the manufacturer says? I thought it would be 80000= 1.732 X I x 480 and solving this I get I=96 Amps. Should I use a 100A circuit breaker to prevent damage to the generator or should I do 125% * 96Amps => 120A and thus use a 125A circuit breaker? The generator manufacturer stated it would come with a 125A circuit breaker but I'd like to know how they came up with this calculation.

80 kW = 100 kVA at 0.8 power factor. Converting 100 kVA to amps = 100000 / (480 x sqrt(3)) = 120.3 amps, round up to 125 amps.

 

[NEC User]

Asides from protecting the wires from overheating and an electrical fire from occurring. Is the purpose of the breaker to protect the generator which would damage itself if a short occurred? In this case shouldn't we round down however NEC does allow to use the next higher size if practical. Thank you for your reply.

 

[eHunter]

Asides from protecting the wires from overheating and an electrical fire from occurring. Is the purpose of the breaker to protect the generator which would damage itself if a short occurred? In this case shouldn't we round down however NEC does allow to use the next higher size if practical. Thank you for your reply.

What type of breaker is the genset manufacturer using and what is its trip curve?

 

[NEC User]

The generator manufacturer provided me documents regarding the trip curve and circuit breaker type. Does that mean that the 100000 VA = 1.732 * 480 * I is the incorrect approach?

 

[eHunter]

The generator manufacturer provided me documents regarding the trip curve and circuit breaker type. Does that mean that the 100000 VA = 1.732 * 480 * I is the incorrect approach?

The formula is:
KW x 1000/(1.73 x E x PF) = I
80,000/(1.73*480*1.0) = 96.34A @ 1.0 PF
80,000/(1.73*480*0.8) = 120.42A @ .8 PF

Most CAT or Cummins/Onan or Kohler and some other integrated genset control/management systems when properly configured will protect the genset from overcurrent, under/over voltage, temperature violations and a large number of other operational parameters by tripping an alarm and if critical trip a protective relay and/or perform a disconnect and/or shutdown the prime mover.
Most low cost gensets usually do not incorporate sophisticated controllers to protect the genset from damage.

 

[templdl]

Should it be assumed that the generator be able to be able to generate enough over current for a sufficient time to trip the breaker thermally and a sufficient amount of fault current to trip the breaker magnetically? It is important to realize that their is a difference between the limited capacity of a generator and that as supplied by a POCO.

 

[eHunter]

Should it be assumed that the generator be able to be able to generate enough over current for a sufficient time to trip the breaker thermally and a sufficient amount of fault current to trip the breaker magnetically? It is important to realize that their is a difference between the limited capacity of a generator and that as supplied by a POCO.

That would really depend on the individual genset and how it is built and configured.
Many gensets lack prime mover capacity to generate the current long enough to reach catastrophic genhead damage.
Most that I have had experience with will either bog down and never reach max current or they strain so hard they overheat the prime mover coolant and/or the exhaust TC causes an alarm and shutdown.

 

[jim dungar]

Should it be assumed that the generator be able to be able to generate enough over current ...

I would not assume this at all, unless the generator was equal to one used by a utility.

For engine driven generators, I am not aware of any manufacturer that does not include overcurrent and fault protection internal to their 'controller'. In fact most single generator installations I have been involved with, often do not have enough fault current for protective devices to operate in a timely manner. This causes real problems with arc fault inciident energy calculations.

 

[ceb58]

I have one that is 100Kw @ 480v the numbers workout to 150 amp but it came with a factory 175 amp breaker.

 

[GoldDigger]

I In fact most single generator installations I have been involved with, often do not have enough fault current for protective devices to operate in a timely manner. This causes real problems with arc fault inciident energy calculations.

+1
The generator impedance limits the fault current to a greater degree than for POCO because the generator can rely on active regulation to offset voltage drop. That means that the available instant bolted fault circuit current will not be as high. The energy for this can come from the inertia of the generator and engine.
And the prime mover cannot keep the generator spinning against a prolonged overload, so the fault current cannot stay high enough long enough to quickly cross the time-temperature curve of the breaker either.

Do typical generators have integrated thermal overloads for prolonged overload protection, similar to what motors have? Or is it usually external like an external motor overload?

 

[templdl]

That would really depend on the individual genset and how it is built and configured.
Many gensets lack prime mover capacity to generate the current long enough to reach catastrophic genhead damage.
Most that I have had experience with will either bog down and never reach max current or they strain so hard they overheat the prime mover coolant and/or the exhaust TC causes an alarm and shutdown.

Bingo, to my point exactly. It is not common for a gender to be over sized enough such that it would have the capability to trip an OCPD. Generally because of cost a gender is sized just for the anticipated load.
But, if it makes someone feel better to use an OCPD.....
I know that there are OCPDs that care made specifically for gender applications.
I have recommended that the OCPD be provided with a shunt trip which can be activated be the prime mover or generator monitoring devices that when activated will trigger the shunt trip to remove the load.

 

[templdl]

I would not assume this at all, unless the generator was equal to one used by a utility.

For engine driven generators, I am not aware of any manufacturer that does not include overcurrent and fault protection internal to their 'controller'. In fact most single generator installations I have been involved with, often do not have enough fault current for protective devices to operate in a timely manner. This causes real problems with arc fault inciident energy calculations.

That's actually the point that I was trying to make. I've been harping on this for years when it comes to assuming that by simply providing a common UL489 breaker to the output of a generator will protect the gender let alone the cable.
It seams as though it is assumed as though thev gender is like that of a POcO. Often times there is a lack of knowledge of what a breaker's trip curve is.

 

[jim dungar]

...by simply providing a common UL489 breaker to the output of a generator will protect the gender let alone the cable.

The breaker will protect the cable (afterall there isn't enough fault current to damage it). By default all UL489 breakers, and LV fuses, will protect properly applied feeder and branch circuit wiring.

Where does it say that our output protective device is supposed to actually protect the generator?
To my knowledge every commercially available, non-utility, generator includes inherent overload protection. They employ 'on-board' breakers, sophisticated internal controls, or more typically such poor speed regulation that their output votlage falls off during an overload.

This is similar to any discussion involving protecting the output of a transformer. Does the device serve the purpose we think it does?

 

[templdl]

The breaker will protect the cable (afterall there isn't enough fault current to damage it). By default all UL489 breakers, and LV fuses, will protect properly applied feeder and branch circuit wiring.

Where does it say that our output protective device is supposed to actually protect the generator?
To my knowledge every commercially available, non-utility, generator includes inherent overload protection. They employ 'on-board' breakers, sophisticated internal controls, or more typically such poor speed regulation that their output votlage falls off during an overload.

This is similar to any discussion involving protecting the output of a transformer. Does the device serve the purpose we think it does?

Point made. Is is sad that the generator is not capable of producing enough current that would overload the cable should the capable sized 125% of the continuous low + 100% of the non continuous load. The size the breaker based upon the conductor's rated ampacity where you are allowed to use the next higher breaker if the conductor ampacity is not a standard breaker rating which goes back to my previous point of the gen sets capability of tripping the breaker which is supposed to protect the conductor.
I also agree regarding the transformer but since the transformer is dependent on how much it can be overloaded a transformer has the capability of overloading the conductor that is connected to its secondary.
But I agree with you regarding the concept of conductor over current protection. But there is a difference between what their over current capabilities are when it comes to being able trip a breaker.