Generator Breaker

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ceb58

Senior Member
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Raeford, NC
I have been having a discussion with some others on the rules for sizing conductors on a generator. Looking for more opinions.
The situation: Standby generator with a built in 50 amp breaker.

My opinion is that you must size the conductors for the overcurrent device per 240.4 which would be #8 Cu.
Others have an opinion that you can size the conductors per the name plate which is lower in ampacity than the breaker. In this case it is 45.8 ( on LP gas). They are saying that because the controller limits the output they can use the exception to 445.13 and size per name plate. ( in this case it would be #8 ether way)
My argument against this is that the conductors must be sized to the breaker because the generator is capable of exceeding the name plate rating on equipment start ups. But my main argument is because the generator has a built in breaker 445.13 is not applicable, there for you cannot use an exception to a code rule that doesn't apply to the situation.
Comments?
 

augie47

Moderator
Staff member
Location
Tennessee
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State Electrical Inspector (Retired)
I would agree with you also. Without engineering data I would not accept the controller as OCP for the conductors. You really have no way of knowing the time frame the controller might allow a current in excess of the conductor ampacity.
 

ceb58

Senior Member
Location
Raeford, NC
I agree with you.

We've had this same argument at our shop more than once and I was arguing the other side, but I have changed my ways.

I would agree with you also. Without engineering data I would not accept the controller as OCP for the conductors. You really have no way of knowing the time frame the controller might allow a current in excess of the conductor ampacity.

OK, looks like we are on the same page. But what about the fact that 445.13 has nothing to do with the situation, at all and some one trying to use an exception to a rule that doesn't apply
 

roger

Moderator
Staff member
Location
Fl
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Retired Electrician
Hello Curtis, you can put me down as another in agreement with you.

Roger
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
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EE (Field - as little design as possible)
... I have been having a discussion with some others on the rules for sizing conductors on a generator. ...

The situation: Standby generator with a built in 50 amp breaker. ...name plate 45.8 (on LP gas) ...
Assumption: this is a 240V, 1ph, 10kw - 12kw, residential grade, package unit with integral CB, likely a listed unit.

The NEC requirements for generation are strangely silent. Cookie cutter design by the NEC results in a poor job at best. Between the generator and the first overcurrent protective device, there are only two Art 445 code issues: 445.12 and 445.13. All the rest is design. There in nothing in Art 445 that applies to after the first OCP. After the first OCP, the normal NEC rules of 240.4 apply.

445.12 has to do with generator protection - not conductor protection. Limiting the discusson to 445.12(A): The generator is required to be protected from overload by, "inherent design, circuit breakers, fuses, protective relays ..."

For most package generation (yep - right up to multi megawatt stuff) the power output is often limited by "inherent design". The driver is simply not strong enough to put out more than 100%.

The design issue for the first CB is to select it such that it will allow the gen to run continuous at 100% and still trip on short circuit, inside of the generator damage curve.

The one you have is a package unit that included a CB. Both 445.12(A) issues are done.

Side note: 50A CB is pretty small for a gen with a nameplate current of 45.8A. i would have picked 45.8 x 1.25 = 57.25, next size up --> 60A. Just curious, does the gen have a 50A receptacle, or is it hard wired form the gen CB to the premisis wiring?

445.13
... Others have an opinion that you can size the conductors per the name plate which is lower in ampacity than the breaker. In this case it is 45.8 ( on LP gas). They are saying that because the controller limits the output they can use the exception to 445.13 and size per name plate. ...

Assumption: the conductors in question is are after the first OCP.
First off, I don't know what this "controller" is. Generally there are only two generator control devices: The governor and the voltage regulator. The governor controls the throttle to hold frequency. If the load increases to where the frequency drops, the throttle goes wide open. The only controlling factor is when the driver runs out of horsepower. And that is considered "inherent design".

And the VR does not control the gen output. It simply changes the field DC current to keep the voltage within limits.

There is no controller that limits the gen output. But that really does not matter, BECAUSE:

... is because the generator has a built in breaker 445.13 is not applicable, there for you cannot use an exception to a code rule that doesn't apply to the situation. ...
Absolutely true. After the first CB, one refers to 240.4. It is all just ordinary premisis wiring. So, one could calculate the connected load and if low enough, one cut the ampacity back to just above 45A (next size down CB)

I would agree with you also. Without engineering data I would not accept the controller as OCP for the conductors. You really have no way of knowing the time frame the controller might allow a current in excess of the conductor ampacity.
I agree. I would phrase it different, but still, I agree. If I were designing to the normal standards my customers demand (100%, continuous), the gen would have a 60A CB and 60A conductor ampacity.

With a 50A CB installed from the factory, then 50A conductor ampacity. However, I could not guarantee that the 50A CB would not eventually tripat 100% continuous load.

Still curious, is the gen equiped with a 50A receptacle?

Small gen design per

the worm
 
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templdl

Senior Member
Location
Wisconsin
I always have fouild the subject of generator's and breakers to be amusing.
Think about it,the size of a generator to be most cost effective is sized to serve the intended load. So you have a given number of amps by which you cable is sized.
Now for the breaker, regardless, the breaker selected will have a specific TC curve. Given the TC curve, does the genset even have the capability of sustaining enough over current for an extended length if you to even trip the breaker on overload (thermally) or even be capable of producing a high enough fault current to trip the breaker?
It is often overlooked that you are not dealing with a utility power source which has tremendous capability of delivering current both overload and fault.
 

iwire

Moderator
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Location
Massachusetts
Now for the breaker, regardless, the breaker selected will have a specific TC curve. Given the TC curve, does the genset even have the capability of sustaining enough over current for an extended length if you to even trip the breaker on overload (thermally) or even be capable of producing a high enough fault current to trip the breaker?

But either way the conductors are protected from damage yes?

I would think if the genset cannot provide the current to trip the breaker or open the fuse it should not be able to supply enough current to damage the conductors. They may run warm but not above their insulation rating,
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
But either way the conductors are protected from damage yes?

I would think if the genset cannot provide the current to trip the breaker or open the fuse it should not be able to supply enough current to damage the conductors. They may run warm but not above their insulation rating,

Minor disclaimer: My knowledge on small residential generation is not nearly as good as industrial grade generation - but I'm thinking the physics is the same.

I would agree. Taking ceb's example, nameplate current = 46A. I would expect the gen reactance to be 20% - 25%. So the available short circuit current is say 180A - 230A. Using a QO 50A trip curve, the CB would trip in 1s to 2.5s. If the package assembler did their job, the cb will have tripped inside of the gen damage curve and the gen will be okay. And if the installer did their job and supplied conductors per 240.4 - the external conductors will be okay.

Interesting note: If the package assembler had chosen a 60A, the trip times are 1.5s to 3.5s.

2nd interesting note: Example motor starting - Start a 5hp, 1ph, 240V, 30FLA, 4x LRC, on the 11kw gen. The LRC is ~120A. The trip time on a QO 50A cb is ~6s. Might make it - might not. Which is okay, 5hp on 11kw is really a lot.

Just thinking outloud

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... Now for the breaker, regardless, the breaker selected will have a specific TC curve. Given the TC curve, does the genset even have the capability of sustaining enough over current for an extended length if you to even trip the breaker on overload (thermally) or even be capable of producing a high enough fault current to trip the breaker?
It is often overlooked that you are not dealing with a utility power source which has tremendous capability of delivering current both overload and fault.
I don't know how often it is overlooked when selecting a CB for a gen.

If one is buying a package unit with the CB, I would expect the assembler/manufacturer to definitely pay attention. So this case is okay.

As for someone buying a bare gen and specifying the first CB, I would certainly hope anyone doing this would either be knowledgable or hire someone that is.

For Industrial/Commercial, bare gen, separately selected cb, if one has hired a design build contractor, I guess you get what you pay for. If the contractor is not knowledge and won't fess up - tough luck. Personally I blame this on the owner being too cheap to hire a competent owners agent.

For residential grade, the only place I've seen this come up is for off-grid. Following is strictly personal opinion: These people tend to be pretty self-sufficient. Most get the CB specified by the gen mfg. And a few use what they can scrounge.

I'm thinking it is a small percentage that don't get it right - but I don't know that.

So, question: Over looked by who?

Again - not saying you are wrong, just thinking.

ice
 

templdl

Senior Member
Location
Wisconsin
I don't know how often it is overlooked when selecting a CB for a gen.

If one is buying a package unit with the CB, I would expect the assembler/manufacturer to definitely pay attention. So this case is okay.

As for someone buying a bare gen and specifying the first CB, I would certainly hope anyone doing this would either be knowledgable or hire someone that is.

For Industrial/Commercial, bare gen, separately selected cb, if one has hired a design build contractor, I guess you get what you pay for. If the contractor is not knowledge and won't fess up - tough luck. Personally I blame this on the owner being too cheap to hire a competent owners agent.

For residential grade, the only place I've seen this come up is for off-grid. Following is strictly personal opinion: These people tend to be pretty self-sufficient. Most get the CB specified by the gen mfg. And a few use what they can scrounge.

I'm thinking it is a small percentage that don't get it right - but I don't know that.

So, question: Over looked by who?

Again - not saying you are wrong, just thinking.

ice
Overlooked by those who select asnd install breaskers. It is often assume that, based upon the KVA of the genset, if you size the cable to carry the load and then size the breaker to protect the wire the wire will be protected from an overload and provide short circuit protection.
For the hech of it is rou nn numbers a 1ph 6500w genset produces about 50a at 120. This commonly is a 120/240 output so it would be 25a @ 240 or (2) 120v circuits at 25a each.
If you use a 25a breaker for OCP the field test GF or breakers up too 250v that max trip time would be no more than 80 seconds with 300% of the breaker's current rating or 25x3=75a. The question would be does the genset even have the capability of supplying 75a for 80sec?
The instantaneous element my be calibrated at 10x the breaker rating or 25x0=250a. If you short circuit the genset is it capable of supplying an instantaneous current of 250a?
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... If you use a 25a breaker for OCP the field test GF or breakers up too 250v that max trip time would be no more than 80 seconds with 300% of the breaker's current rating or 25x3=75a. ...

... The instantaneous element my be calibrated at 10x the breaker rating or 25x0=250a. ...
I don't know where the 80s came from. I looked up a sqd qo 25A trip curve. At 3x, trip time was 2.5s to 10s. I probably would have picked a 30A - at 2.5x (75A) trip time is 6s to 30s. I would think these would be typical - but maybe not.

As for short circuit current, as discussed earlier, I still would expect an impedance of 20% - to 25%, so on the order of 100A to 125A. So, no I would not expect a 6500W gen to trip a 25A CB in the instantaneous range

As for, can the gen stand the time-overcurrent, that is what generator damage curves are for. Can't say I've ever seen one for a residential grade, bare gen (no CB supplied with package) - Of course, I also don't recall ever looking for one either. I don't even recall ever seeing a residential grade 6kw without a factory supplied CB. Sound moot to me.

However, I don't see how any of this applies to:
Overlooked by those who select asnd install breaskers. ...
That's a pretty broad group. Do you want to narrow it down a bit? For example:
Are you including mfg/assemblers supplying a package including the cb mounted on the package?
Are you including Industrial/Commercial Contractor design/build/supply bare gen (no CB)?
Are you including residential grade Contractor design/build/supply bare gen (no CB)?
Are you including residential owner bought bare gen?​

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... It is often assume that, based upon the KVA of the genset, if you size the cable to carry the load and then size the breaker to protect the wire the wire will be protected from an overload and provide short circuit protection. ...
First I would ask, Short circuit protection of exactly what?. Cause if it is the conductors, that is a good assumption. I would not tend to use that method - but it is still a good assumption.

But if you are discussing, Short circuit protection of the gen, and gen type you are considering are residential grade bare gens, then I don't know the first CB needs to be selected to protect the gen from Short Circuit or Overload. Yep - truth.

I'm not sure where you are headed. Almost sound like people are too stupid to pick the right CB. If so , I don't agree. Generator system design (bare gen, separately selected CB) is not an NEC cookie cutter installation - not even at the residential grade. Design Decisions Required. Plenty are bright and knowledgable enough to handle it. Yes, highly likely most are specialists -

Still, my guess is when one is spending $500 - $1000 at the hardware store - they come with a mfg supplied CB. Still, I would not be surprised if one could short the gen at the end of a carefully selected extension cord and get it to overload and burn up before the CB tripped. Bad CB selection by mfg? No. You got what you paid for. I also suspect that $10k for a Lister, Yanmar, Petter (or the like) and $3K on protective relaying and that won't be a problem.

Years back I recall giving my brother a hand with a 20KW off-grid Yanmar. I think he had about $20K in the system. Batteries, Inverter, waste heat recovery. Worked really well - until he stuck a backhoe bucket in the feeder. And yeah, the CB tripped. And yeah, still worked well after the feeder was repaired.

ice
 

templdl

Senior Member
Location
Wisconsin
First I would ask, Short circuit protection of exactly what?. Cause if it is the conductors, that is a good assumption. I would not tend to use that method - but it is still a good assumption.

But if you are discussing, Short circuit protection of the gen, and gen type you are considering are residential grade bare gens, then I don't know the first CB needs to be selected to protect the gen from Short Circuit or Overload. Yep - truth.

I'm not sure where you are headed. Almost sound like people are too stupid to pick the right CB. If so , I don't agree. Generator system design (bare gen, separately selected CB) is not an NEC cookie cutter installation - not even at the residential grade. Design Decisions Required. Plenty are bright and knowledgable enough to handle it. Yes, highly likely most are specialists -

Still, my guess is when one is spending $500 - $1000 at the hardware store - they come with a mfg supplied CB. Still, I would not be surprised if one could short the gen at the end of a carefully selected extension cord and get it to overload and burn up before the CB tripped. Bad CB selection by mfg? No. You got what you paid for. I also suspect that $10k for a Lister, Yanmar, Petter (or the like) and $3K on protective relaying and that won't be a problem.

Years back I recall giving my brother a hand with a 20KW off-grid Yanmar. I think he had about $20K in the system. Batteries, Inverter, waste heat recovery. Worked really well - until he stuck a backhoe bucket in the feeder. And yeah, the CB tripped. And yeah, still worked well after the feeder was repaired.

ice
Was I mistaken that the OP was with regard to feeder protection? What I see her is assumptions and innuendos. What would make more sense would to compare a breaker TC to the generators capabilities of generating electric power both its ability to sustain overload current as well as what it's available short circuit is and that hasn't been established yet. What I see is that there are hopes that there will be protection with no documentation that would show any coordination of the OCP and the genset.
You may have missed my point in regard to what it takes to trip a breaker on overload and what short circuit current that the genset is capable of. You may have gotten lucky that the breasker's tripping in you example. But ftom a manufacturers perspective I couid not recommend protection unless I reviewed the coordination between the breaker an the genset.
 

kwired

Electron manager
Location
NE Nebraska
First I would ask, Short circuit protection of exactly what?. Cause if it is the conductors, that is a good assumption. I would not tend to use that method - but it is still a good assumption.

But if you are discussing, Short circuit protection of the gen, and gen type you are considering are residential grade bare gens, then I don't know the first CB needs to be selected to protect the gen from Short Circuit or Overload. Yep - truth.

I'm not sure where you are headed. Almost sound like people are too stupid to pick the right CB. If so , I don't agree. Generator system design (bare gen, separately selected CB) is not an NEC cookie cutter installation - not even at the residential grade. Design Decisions Required. Plenty are bright and knowledgable enough to handle it. Yes, highly likely most are specialists -

Still, my guess is when one is spending $500 - $1000 at the hardware store - they come with a mfg supplied CB. Still, I would not be surprised if one could short the gen at the end of a carefully selected extension cord and get it to overload and burn up before the CB tripped. Bad CB selection by mfg? No. You got what you paid for. I also suspect that $10k for a Lister, Yanmar, Petter (or the like) and $3K on protective relaying and that won't be a problem.

Years back I recall giving my brother a hand with a 20KW off-grid Yanmar. I think he had about $20K in the system. Batteries, Inverter, waste heat recovery. Worked really well - until he stuck a backhoe bucket in the feeder. And yeah, the CB tripped. And yeah, still worked well after the feeder was repaired.

ice

That breaker on the $500-1000 hardware store genset is not a QO breaker or a Siemens Q series breaker, or any other common thermal magnetic breaker found in a panelboard. It is likely a thermal only type of breaker from most of what I have seen, and that is likley all it needs to be on most of those small kVA gensets, it is protecting the genset for the most part and needs to react to heating similar to how a motor overload protector reacts to heating caused by motor current. The impedance of this genset is high enough the short circuit current will not be high enough to get into the instantaneous trip ranges of typical thermal magnetic breakers and they trip on thermal overload anyway. They may or may not select a breaker that has a trip curve that well protects the genset, but I'm guessing most just choose something cheap and it is good enough to get by with.

I do have a 8000 watt unit that I do use for work when there is no power on site. I even use it for welding somtimes, but the breaker on this unit does trip if I weld at too high of setting/too long of duration. I can run same welder via same cordset that is rigged to fit the generator on a 30 amp QO breaker in my shop (from utility supply) and usually weld at higher setting/ longer duration then I can when running on this generator. But that don't mean all similar sized units will handle it same way either.
 

mr.yavapai

Member
Location
Dewey, AZ
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iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Was I mistaken that the OP was with regard to feeder protection? What I see her is assumptions and innuendos. What would make more sense would to compare a breaker TC to the generators capabilities of generating electric power both its ability to sustain overload current as well as what it's available short circuit is and that hasn't been established yet. What I see is that there are hopes that there will be protection with no documentation that would show any coordination of the OCP and the genset.
You may have missed my point in regard to what it takes to trip a breaker on overload and what short circuit current that the genset is capable of. You may have gotten lucky that the breasker's tripping in you example. ....
I don't think I missed your point. You appear to be stuck on the first CB protecting the gen from all damage. if the discussion is about small residential grade, I don't see any evidence that is necessarily true.

As for "gotten lucky" - hummmm .... don't what to say. Maybe, "Verifying generator data against the selected CB TCC data is called 'making design decisions', not 'getting lucky'.

I still don't know where you are going with this. Perhaps, you could read post 13 and narrow the scope a bit.

... But ftom a manufacturers perspective I couid not recommend protection unless I reviewed the coordination between the breaker an the genset.
Good choice. I think that is called, 'making design decisions'.

ice
 
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