GENERATOR OVERCURRENT PROTECTION

We are installing a 120/240 volt single phase 100 kW generator. The gen has a maximum output of 417 amps per the manufacturer and is equipped with a 500 amp breaker. The service and the SE rated automatic transfer switch are 400 amp. The conductors from the generator to the transfer switch are 600 kcmil (420 amps). Is the the 500 amp breaker on the generator compliant with the NEC or should it be changed to a 400 amp breaker?
 

ceb58

Senior Member
Location
Raeford, NC
We are installing a 120/240 volt single phase 100 kW generator. The gen has a maximum output of 417 amps per the manufacturer and is equipped with a 500 amp breaker. The service and the SE rated automatic transfer switch are 400 amp. The conductors from the generator to the transfer switch are 600 kcmil (420 amps). Is the the 500 amp breaker on the generator compliant with the NEC or should it be changed to a 400 amp breaker?
The breaker is fine. It is sized up to handle inrush. Its your conductors that you have a problem with. The wire between the gen. and ATS is too small for the OCP on the generator. Also the ATS doesn't have a large enough ampacities.
 

hurk27

Senior Member
I'm not sure but if the rating of the generator is only 416.67 amps, 445.13 requires us to size the conductors at 115%, but the exception allow 100% if the design will not allow the conductors to be overloaded? it really doesnt go by the breaker size on the generator but the rating of the generator, since it is feeding a 400 amp service that should have a 400 amp main the exception should allow the 600 kcmil's? we know under a fault condition the 500 amp breaker will still trip which is all the breaker on a generator is for, 240.3 tells us to use the article that pertains to the equipment needing protection if it is listed in table 240.3 which generators are listed so then we must go by 445.13, but I'm open to others opinion, with a 400 amp main I don't see how the conductors can be overloaded? if we do have to size to the 115% then that will push the conductors rating to 479.2 amps and the 600's will be to small?

445.13 Ampacity of Conductors. The ampacity of the
conductors from the generator terminals to the first distribution
device(s) containing overcurrent protection shall not
be less than 115 percent of the nameplate current rating of
the generator
. It shall be permitted to size the neutral conductors
in accordance with 220.61. Conductors that must
carry ground-fault currents shall not be smaller than required
by 250.30(A). Neutral conductors of dc generators
that must carry ground-fault currents shall not be smaller
than the minimum required size of the largest conductor.

Exception: Where the design and operation of the generator
prevent overloading, the ampacity of the conductors
shall not be less than 100 percent of the nameplate current
rating of the generator.
 

augie47

Moderator
Staff member
Location
Tennessee
My opinion would be 445.13 covers the conductors to the 1st overcurrent device. In this case his 1st OCP is the generator 500 amp breaker so it would be up to the mfg. to meet 445.13.
His 600 kcmil have a load limit due to the 400 amp breaker so he meets the load ampacity and the next higher size rules.
I see no problem.
 

eHunter

Senior Member
My opinion would be 445.13 covers the conductors to the 1st overcurrent device. In this case his 1st OCP is the generator 500 amp breaker so it would be up to the mfg. to meet 445.13.
His 600 kcmil have a load limit due to the 400 amp breaker so he meets the load ampacity and the next higher size rules.
I see no problem.
Except the genset breaker is 500A.
 

templdl

Senior Member
Location
Wisconsin
Take a look at the breaker time current trip curve and the instantaneous trip point and compare it to the generator's capability to see if it is capable of producing enough over current for a long enough period of time in order to trip the breaker or enough short circuit current to trip the breaker instantaneously. It may end up being just an expensive disconnect switch.
 

hurk27

Senior Member
Except the genset breaker is 500A.
Again the breaker supplied by the manufacture is there to protect the generator from a fault not overload, I believe we are talking about the conductors that leave the generator to the transfer switch and from the transfer switch to the main service OCPD, if he has a 400 amp main, and the generators output current is 416 amps, I can't see how the conductors can be overloaded? no different from conductors feeding a motor that may have a 500 amp breaker on the supply conductors but the motor has 400 amp overloads, the conductors inside of the generator are not even covered by the NEC, and many times they will be even smaller then the NEC would allow.
 

augie47

Moderator
Staff member
Location
Tennessee
Again the breaker supplied by the manufacture is there to protect the generator from a fault not overload, I believe we are talking about the conductors that leave the generator to the transfer switch and from the transfer switch to the main service OCPD, if he has a 400 amp main, and the generators output current is 416 amps, I can't see how the conductors can be overloaded? no different from conductors feeding a motor that may have a 500 amp breaker on the supply conductors but the motor has 400 amp overloads, the conductors inside of the generator are not even covered by the NEC, and many times they will be even smaller then the NEC would allow.
I agree, From an overload standpoint the 600s are protected by the limits on the generator output and by the 400 amp breaker at termination (the same way a tap conductor is protected). In the event of a short prior to the 400, the 500 will obviously provide adequate protection.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Again the breaker supplied by the manufacture is there to protect the generator from a fault not overload, I believe we are talking about the conductors that leave the generator to the transfer switch and from the transfer switch to the main service OCPD, if he has a 400 amp main, and the generators output current is 416 amps, I can't see how the conductors can be overloaded? no different from conductors feeding a motor that may have a 500 amp breaker on the supply conductors but the motor has 400 amp overloads, the conductors inside of the generator are not even covered by the NEC, and many times they will be even smaller then the NEC would allow.
While that is all logical and technically correct, I don't see anything in the code that permits the 420 amp rated conductors to be supplied by a 500 amp OCPD. I would agree that the conductors are large enough for the load as they have an ampacity greater than the output current of the generator, but 450 amp is a standard sized OCPD, and the maximum size that the 420 amp conductors can be connected.
 

augie47

Moderator
Staff member
Location
Tennessee
While that is all logical and technically correct, I don't see anything in the code that permits the 420 amp rated conductors to be supplied by a 500 amp OCPD. I would agree that the conductors are large enough for the load as they have an ampacity greater than the output current of the generator, but 450 amp is a standard sized OCPD, and the maximum size that the 420 amp conductors can be connected.
dag-blame-it.... who snuck that 450 into the "standard" size :D
Teach me once again to look first, post 2nd.
Now that you caused me to re-think the whole situation... if the conditions were right, we could qualify the install with one of the 240.21 tap rules, correct ?
 

templdl

Senior Member
Location
Wisconsin
I agree, From an overload standpoint the 600s are protected by the limits on the generator output and by the 400 amp breaker at termination (the same way a tap conductor is protected). In the event of a short prior to the 400, the 500 will obviously provide adequate protection.
Protection from what? The questions remains does the generator have the capability to generate enough current for a long enough period of to to trip a breaker thermally or a high enough instantaneous current to trip the breaker magnetically?
A 500a breaker's instantaneous trap most likely is adjustable between 5-10x the breakers rating +-20% commonly.
The breaker may be required to protect the cable but is the generator capable of producing enough current to even trip the breaker?
 

augie47

Moderator
Staff member
Location
Tennessee
I understand and appreciate what you are saying, but from a dum**ss inspector standpoint my outlook would be simplified.
The generator and it's overcurrent device I would leave up to the listing agency. From the load side of the 500 amp breaker I would be governed by 240.4 etc. I have no idea what the generator is capable of suppliying for what duration.
240.4 (B) allows me to go to the next larger OCP device, which in the case of a 600 kcmil is a 450. I would be looking for a 450 amp OCP device or something in Art 240 that allows me to accept a larger breaker such as a tap rule.
 

GoldDigger

Moderator
Staff member
Protection from what? The questions remains does the generator have the capability to generate enough current for a long enough period of to to trip a breaker thermally or a high enough instantaneous current to trip the breaker magnetically?
A 500a breaker's instantaneous trap most likely is adjustable between 5-10x the breakers rating +-20% commonly.
The breaker may be required to protect the cable but is the generator capable of producing enough current to even trip the breaker?
+1 !
The breaker is there as a reminder and an on/off switch for all practical purposes. It protects neither the generator nor the wire unless maybe you are looking at a bolted fault that hits enough current while the generator is noisily coming to a stop. There is another thread about load control when using a generator with an ATS which has concrete examples of generators destroyed because of "small" overloads.
 

templdl

Senior Member
Location
Wisconsin
I understand and appreciate what you are saying, but from a dum**ss inspector standpoint my outlook would be simplified.
The generator and it's overcurrent device I would leave up to the listing agency. From the load side of the 500 amp breaker I would be governed by 240.4 etc. I have no idea what the generator is capable of suppliying for what duration.
240.4 (B) allows me to go to the next larger OCP device, which in the case of a 600 kcmil is a 450. I would be looking for a 450 amp OCP device or something in Art 240 that allows me to accept a larger breaker such as a tap rule.
What you said is exactly to the point. Knowing what the generator is calls of if in fact a mute point. The regardless if you use a 450a or 500a breaker it serves no practical purpose other than you have to have a breaker there because of code . It does nothing for the generator other than serving as a very expensive disconnect switch.
As such you are looking at this do nothing breaker in regard to the cable it is intended to protect. All that is being do is to play by the rules an the NEC requires, that is to protect the cable as if the power was being supplied by a utility and forget the generator.
You can rest assure that oversizing generators that has the capability to overload the cables is not going to happen because of of their expense. They are limited in there output capability unlike utility.
 

templdl

Senior Member
Location
Wisconsin
+1 !
The breaker is there as a reminder and an on/off switch for all practical purposes. It protects neither the generator nor the wire unless maybe you are looking at a bolted fault that hits enough current while the generator is noisily coming to a stop. There is another thread about load control when using a generator with an ATS which has concrete examples of generators destroyed because of "small" overloads.
Bingo. What Augie47 pointed out is simply following what the NEC requires. When you consider that the goal is to size and purchase the generator based upon the load to be supplied, then size the cable to supply the load, place a circuit breaker there that protect the cable knowing that the generator does not have the capability of overloading the cable anyway.
As you pointed out generators can be damaged because of an overload. There must be other generator protection methods in place other than the breaker.
The generator may provide a means to trip the breaker by activating a shunt trip in the breaker as a means of protecting the prime mover because of low oil, overheating, or generator should the field start to collapse for some reason.
 

ceb58

Senior Member
Location
Raeford, NC
But now on to other problems. The OP did not say specifically, but what about the amp rating of the ATS?
The gen has a maximum output of 417 amps per the manufacturer and is equipped with a 500 amp breaker.
Even by the name plate rating it would exceed the ATS IF it is RATED at 400 amps.
The service and the SE rated automatic transfer switch are 400 amp.
 

templdl

Senior Member
Location
Wisconsin
But now on to other problems. The OP did not say specifically, but what about the amp rating of the ATS? Even by the name plate rating it would exceed the ATS IF it is RATED at 400 amps.
What is diving all of this, that ATS, the output of the generator? Is the objective to supply power to the load?
Shouldn't the size of the load be know first? Should the cable be sized to carry the load? At that point wouldn't you then select breaker based upon protecting the cable?
Now, wouldn't the ATS also be sized upon that also?
Since I avoided the generator would it be safe to assume that you have sized the generator to power the load.
Now you way say that the genset is smaller which you may justify a smaller ATS which may seem to make sense. But what about the load and the cable that has been sized based upon that.
My point being is that you have now undersized the genset and went with a smaller transfer switch to feed a larger load.
Isn't the cart leading that horse?
 

hurk27

Senior Member
As I said I may be in error in my thinking but why is a generator any different then a motor, we protect for faults at the supply end (500 amp breaker) and we protect from overloads at the load end (400 amp service main breaker) we do the same for motors and AC units, 240.3 and table 3 both allow generators to be sized differently from what we use in 240.4, 445.13 exception allows if by design the conductors can't be overloaded then you can use the 100% rating, but even though I don't agree with 445.13 requiring us to use the 416 amps output of the generator if the conductors are protected by overload at 400 amps, but that is my opinion, not what the code says, to me just because you want a little headroom instead of buying the next size down generator which will be a 80kw, which will only give you 333 amps, to me as long as the conductors are protected from overload I do not see a problem, if the NEC can allow it for motor loads why not generators? why drive the price of a job higher then it needs to be without any safety benefit, as long as the conductors and ATS can't be over loaded by the fact it has a 400 amp breaker at the panel it feeds then the load on the conductors is controlled.

Again the breaker on the generator is not sized for overloads, it is only there as a disconnect and to protect the generator from bolted faults, I would expect that a generator of this size would also have electronic overload protection to protect the generator, but that would be set at the 416.67 amps the generator is rated with a delay curve to allow the generator to supply short term surge for motors starting up.

Remember 445.13 does not require us to size the conductors for the breaker on the generator, it clearly states that we size the conductors at the rating of the generator + 15% of that rating unless you can use the exception which is to me very vague and makes no sense if you can install a smaller breaker as in this case a 400 amp breaker that will limit the generators ability to overload the 600's, but the exception still says we must use the current rating of the generator??? Why? to me someone at NFPA wasn't thinking correctly when they put this in the code, to me it needs more flexibility when the next size down generator is to small for the load but the right one is just over what is needed.

Ok guess since I don't have the time on hand to submit a change I guess I'll leave it alone LOL:p
 

templdl

Senior Member
Location
Wisconsin
Again the breaker on the generator is not sized for overloads, it is only there as a disconnect and to protect the generator from bolted faults, I would expect that a generator of this size would also have electronic overload protection to protect the generator, but that would be set at the 416.67 amps the generator is rated with a delay curve to allow the generator to supply short term surge for motors starting up.

Remember 445.13 does not require us to size the conductors for the breaker on the generator, it clearly states that we size the conductors at the rating of the generator + 15% of that rating unless you can use the exception which is to me very vague and makes no sense if you can install a smaller breaker as in this case a 400 amp breaker that will limit the generators ability to overload the 600's, but the exception still says we must use the current rating of the generator??? Why? to me someone at NFPA wasn't thinking correctly when they put this in the code, to me it needs more flexibility when the next size down generator is to small for the load but the right one is just over what is needed.

Ok guess since I don't have the time on hand to submit a change I guess I'll leave it alone LOL:p
Over the years there has been tremendous improvements to the NEC. What we are discussing is really minor when compared to other issues that have been greatly improved upon. I would like to think that there are more import and topics yet to be addressed than this one. It's a matter on understanding what is practical as compared to what is actually needed in order to follow the rules of the NEC. When you think about it if the conductors are sized to carry the load, does the generator even have the capability to overload the conductors at all. Essentially the generator will be over sized wasting capacity if one were to opt for a 400a breaker.
This has been a good discussion.
 

hurk27

Senior Member
Over the years there has been tremendous improvements to the NEC. What we are discussing is really minor when compared to other issues that have been greatly improved upon. I would like to think that there are more import and topics yet to be addressed than this one. It's a matter on understanding what is practical as compared to what is actually needed in order to follow the rules of the NEC. When you think about it if the conductors are sized to carry the load, does the generator even have the capability to overload the conductors at all. Essentially the generator will be over sized wasting capacity if one were to opt for a 400a breaker.
This has been a good discussion.
As far as the generator being oversized if it was set to only supply a 400 amp load, I would think this is not an issue being were only talking about 16.67 amps of being over sized, I could see your point if we were talking about a 100kw feeding a 200 amp load but generators are over sized all the time in backup installations as many houses don't come near requiring the load the service is sized at and many generators are over sized even to the demand load, even if they are sized more closely there will be times like at night the loads might be very little but the generator will still be running, maybe an 80k would have been fine at 333.33 amps if the demand load of the building was less then this but we don't know if a demand load calculation was ever done, if this is commercial then it is possibly it could have been over the the 80kw rating which would have required the 100kw, so yes its a waste that many live with to make sure they have enough power to meet their loads.

As far as the generator having enough current to overload the 600's well sure it does, how be it for how long is another question, most all generators will produce enough extra kw's to handle surges cause by motors starting up, but this is only for the short term and is not something that should be done for the long term, as unlike a transformer they do not design the alternators to handle this over current for very long to keep things smaller and cheaper, but can it damage the conductors very possibly if nothing shuts the generator down such as electronic overloads, most smaller generators don't have this and can easily be damaged if the over current last to long, but they still can damage the conductors depending upon how long the over current last and the alternator lets go, as far as short circuit current goes I have seen a 15kw trip the 100 amp breaker on the generator with no problem without causing damage to the alternator, not something I would keep doing but yes they can have enough available current for a short time.

Also unlike many portable generators which more closely size the prime mover closer to the rating of the the generator, most backup generator manufactures will use the same size prime mover for a series of generators, Generac uses a 1 liter twin air cooled engine all the way up to there 20 kw units, so a 15kw is not going to stall the engine when overloaded, so it could produce enough current to overload the conductors.

I just went through some of the online specs and see they have changed the size of the breakers they are putting on their generators, as they are more closely sized to the output or even undersized which I find very strange, these spec say the 14kw has a 60 amp breaker which is just above the current a 14kw will produce, but the 17kw unit has a 65 amp breaker which is below the 70.8 amp it would produce , before the breakers were much larger, I looked at a few pdf's I have on the old specs and see they have changed a few things, they still use the 1 liter but the fuel usage is kind of strange, the 17kw uses more NG then the 20kw does at full load, but on propane it switch's the other way around, so either a misprint or something is strange? maybe they did away with the surge rating after to many alternators burned up, who knows I see they no longer have the surge rating listed in the specs?

Heres a link to the specs:

Specs for the 14kw,17kw,20kw


Wonder what happen with Generac?

But anyway, I still believe that a generator can produce enough over current to damage conductors if not protected for long term overloads and with the new specs and smaller breakers I would think it would be harder, but in the OP case with the 500 amp breaker on the generator it would be much easier, as a matter of fact I looked up the latest specs on a Generac 100kw which now shows a 450 amp breaker for the 120/240 single phase model, looking at the lock rotor rating which is the available fault current @ 35% voltage dip, show the available output is 200kva which is 833.33 amps, more then enough to damage the 600's?
 
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