residential generators
- Thread starter angsoto
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Re: residential generators
As far as the NEC is concerned, you can size it for one 60 watt lamp if that is all you would want to use, see 702.5.
This would be an Optional Standby System and have no minimum or maximum rules except as mentioned above.
Roger
[ September 13, 2005, 12:05 PM: Message edited by: roger ]
As far as the NEC is concerned, you can size it for one 60 watt lamp if that is all you would want to use, see 702.5.
This would be an Optional Standby System and have no minimum or maximum rules except as mentioned above.
Roger
[ September 13, 2005, 12:05 PM: Message edited by: roger ]
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: residential generators
You are not "undersizing" the generator. However the homeowner would have to manage their loads to not cause the generator CB to trip. If they don't want to do that, then size the generator for the connected load.
You are not "undersizing" the generator. However the homeowner would have to manage their loads to not cause the generator CB to trip. If they don't want to do that, then size the generator for the connected load.
bphgravity
Senior Member
- Location
- Florida
Re: residential generators
Except for a few commerical facilities, I have yet to see an optional generator system sized for total load operation. The larger ones in this area for dwellngs usually max out at about 15KW.
Except for a few commerical facilities, I have yet to see an optional generator system sized for total load operation. The larger ones in this area for dwellngs usually max out at about 15KW.
Re: residential generators
I have read this rule (702.5) and agree that the size of the generator should not be an issue so long as the conductors that lead from the generator are rated for the full current output of the generator and the output CB is sized accordingly. I am having a problem convincing the AHJ otherwise; however.
I have a commercial installation with 120/208 3 phase, parallel x 2 feeders of 300MCM. There is no main disconnect as there was the six disconnect rule employed at the time of installtion. Although I have never seen the load calculation for the building, I surmise based on the wiresize that it was intended as a 600A service. The service is fed overhead, via a CT can on the exterior of the building, then enters the builing underground and terminates into a service gutter where the feeders are then tapped to the various disconnecting means. I have logged the load data (maximumum amps, average amps and maximum KW) from he service consumption over a seven day period and come up with an average load of only 100A per phase and the maximum load on any phase over those seven days was 321 Amps. This is a 24/7 operation.
What I submitted to the AHJ was that we interupt the service feeders at the CT can and reroute into a new 600A service entrance rated automatic transfer switch (with 600A MCB for the utility feed only). We would then refeed the existing electrical gutter with the same 300MCM parallel feeders from the load side of the ATS. Based on the load readings that I obtained over the seven days of normal business operation, I have chosen to install a 100KW 3 phase generator with a maximum output of 364A per phase using a 400 A MCB and 2 x parallel 4/0 feeders to the generator input side of the ATS. I know that this generator will support the loads including the starting currents of the motor and compressor loads as the data I collected over seven days of normal operations at the facility falls well below the rated output of my generator.
Tha AHJ tells me that I must size my generator to 600A; however, as that must have been the original calculation for the building. I do not see the issue as he does. If my wire size from the utility remains unchanged and the wire size I have installed from my generator is protected by the generators MCB, I do not see any code violation. As far as the customer is concerned I have dilligently ascertained his load requirements and I believe I have done him a favor by not oversizing a generator that would run with little load and cost him more money. It is my understanding that an underloaded generator is subject to maintenance issues such as "Wet Stacking" and "glazing".
Any comments from engineers, inspectors, journeymen or instructors on this scenario are greatly appreciated.
[ September 13, 2005, 09:26 PM: Message edited by: mikelp ]
I have read this rule (702.5) and agree that the size of the generator should not be an issue so long as the conductors that lead from the generator are rated for the full current output of the generator and the output CB is sized accordingly. I am having a problem convincing the AHJ otherwise; however.
I have a commercial installation with 120/208 3 phase, parallel x 2 feeders of 300MCM. There is no main disconnect as there was the six disconnect rule employed at the time of installtion. Although I have never seen the load calculation for the building, I surmise based on the wiresize that it was intended as a 600A service. The service is fed overhead, via a CT can on the exterior of the building, then enters the builing underground and terminates into a service gutter where the feeders are then tapped to the various disconnecting means. I have logged the load data (maximumum amps, average amps and maximum KW) from he service consumption over a seven day period and come up with an average load of only 100A per phase and the maximum load on any phase over those seven days was 321 Amps. This is a 24/7 operation.
What I submitted to the AHJ was that we interupt the service feeders at the CT can and reroute into a new 600A service entrance rated automatic transfer switch (with 600A MCB for the utility feed only). We would then refeed the existing electrical gutter with the same 300MCM parallel feeders from the load side of the ATS. Based on the load readings that I obtained over the seven days of normal business operation, I have chosen to install a 100KW 3 phase generator with a maximum output of 364A per phase using a 400 A MCB and 2 x parallel 4/0 feeders to the generator input side of the ATS. I know that this generator will support the loads including the starting currents of the motor and compressor loads as the data I collected over seven days of normal operations at the facility falls well below the rated output of my generator.
Tha AHJ tells me that I must size my generator to 600A; however, as that must have been the original calculation for the building. I do not see the issue as he does. If my wire size from the utility remains unchanged and the wire size I have installed from my generator is protected by the generators MCB, I do not see any code violation. As far as the customer is concerned I have dilligently ascertained his load requirements and I believe I have done him a favor by not oversizing a generator that would run with little load and cost him more money. It is my understanding that an underloaded generator is subject to maintenance issues such as "Wet Stacking" and "glazing".
Any comments from engineers, inspectors, journeymen or instructors on this scenario are greatly appreciated.
[ September 13, 2005, 09:26 PM: Message edited by: mikelp ]
jimwalker
Senior Member
- Location
- TAMPA FLORIDA
Re: residential generators
Mine is 5200 watts.No way do i need anything more.By selecting loads we can run anything,including the AC.However it's intended use is for huricanes and at that time gas must be conserved.So it will keep frig going ,water pump as needed,few lights,and charge ups back up.Pointless in trying to take over the job of intire house.We been lucky but ready for an outage should we be as unfortunite as New Orleans.Even at half load it burns 6 gal in 12 hours.That amounts to 42 gal to last a week.Thats a lot of gas to have near a house.Actually we have 7 cans at 5 gal each plus the cars.Could hold out maybe 10 days.Everything depends on what they need.
Mine is 5200 watts.No way do i need anything more.By selecting loads we can run anything,including the AC.However it's intended use is for huricanes and at that time gas must be conserved.So it will keep frig going ,water pump as needed,few lights,and charge ups back up.Pointless in trying to take over the job of intire house.We been lucky but ready for an outage should we be as unfortunite as New Orleans.Even at half load it burns 6 gal in 12 hours.That amounts to 42 gal to last a week.Thats a lot of gas to have near a house.Actually we have 7 cans at 5 gal each plus the cars.Could hold out maybe 10 days.Everything depends on what they need.
allenwayne
Senior Member
Re: residential generators
I like sqd`s 60 amp 4/8 gen transfer panel and yes it is rated as service equiptmrnt 60 amp.internal lock out to keep poco and gen set off together.Price around $45.00 = breakers 2 pole well 3 piggy backs watch what you use and a good cheap alternative to what the big o wants $300.00 for
But who am i just a dumb sparky in a huricane alley
I like sqd`s 60 amp 4/8 gen transfer panel and yes it is rated as service equiptmrnt 60 amp.internal lock out to keep poco and gen set off together.Price around $45.00 = breakers 2 pole well 3 piggy backs watch what you use and a good cheap alternative to what the big o wants $300.00 for
But who am i just a dumb sparky in a huricane alley 
sceepe
Senior Member
Re: residential generators
You may want to reassure him that the gen set's main line OCPD is sized correctly and it will protect the gen set, xfer sw, and feeders from overcurrent if your size calcs are wrong. I would then take him over and show him some really egregious plumbing code violation.
Ask him to site the building code that requires it. Sounds like you have done you homework (more than most would do). You are taking the liability for the size of the gen set. It is not his call.
You may want to reassure him that the gen set's main line OCPD is sized correctly and it will protect the gen set, xfer sw, and feeders from overcurrent if your size calcs are wrong. I would then take him over and show him some really egregious plumbing code violation.
jwelectric
Senior Member
- Location
- North Carolina
Re: residential generators
Originally posted by roger:
Are comments from others not appreciated?
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Re: residential generators
What, no residential romex jockeys?
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Re: residential generators
I was speaking in broad terms, I didn't start checking profiles.
From what I can tell, you're a bit of everything. I thought you were an inspector, but your profile says Electrician.
I was speaking in broad terms, I didn't start checking profiles.
From what I can tell, you're a bit of everything. I thought you were an inspector, but your profile says Electrician.
- Location
- Massachusetts
Re: residential generators
George, Roger's comment passed right by at the speed of light.
[ September 14, 2005, 07:41 PM: Message edited by: iwire ]
ZOOM!Originally posted by georgestolz:
I was speaking in broad terms, I didn't start checking profiles.
George, Roger's comment passed right by at the speed of light.
[ September 14, 2005, 07:41 PM: Message edited by: iwire ]
john m. caloggero
Senior Member
Re: residential generators
The NEC in Section 702.5 that the "optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at the same time." The Code is quite clear in the last sentence of this section that states "The user of the optional standby system shall be permitted to select the load connected to the system." This is quite clear that the AHJ has no jurisdiction over the loads that are going to be supplied.
The NEC in Section 702.5 that the "optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at the same time." The Code is quite clear in the last sentence of this section that states "The user of the optional standby system shall be permitted to select the load connected to the system." This is quite clear that the AHJ has no jurisdiction over the loads that are going to be supplied.
Re: residential generators
The existing installation would not easily allow this separation of selected circuits with out great expense. Furthermore, the owner wants the entire building to be on back up power and that is why his loads were studied over a seven days and the generator was sized to handle his load situation. There is another rule that I am looking to have formal interperation of as well and that rule is 220.87.
Rule 220.87 refers to utilizing an optional load calculation by means of collecting load data over a 30 day period at the service of the installation. If I am correctly interpreting this rule, then wouldn't I be able to take 3 weeks more data in addition to the week already collected and correctly state that the maximum average demand plus 25% would be the rating of the service in question?
Any comments on the interpretation of this rule are appreciated as well. (By Anyone who has an opinion )
Steve,
The existing installation would not easily allow this separation of selected circuits with out great expense. Furthermore, the owner wants the entire building to be on back up power and that is why his loads were studied over a seven days and the generator was sized to handle his load situation. There is another rule that I am looking to have formal interperation of as well and that rule is 220.87.
Rule 220.87 refers to utilizing an optional load calculation by means of collecting load data over a 30 day period at the service of the installation. If I am correctly interpreting this rule, then wouldn't I be able to take 3 weeks more data in addition to the week already collected and correctly state that the maximum average demand plus 25% would be the rating of the service in question?
Any comments on the interpretation of this rule are appreciated as well. (By Anyone who has an opinion
)Re: residential generators
I'm a bit confused..
Why would you need to feed a 15 KW generator @ a residence,w/ parallel 300kcmils..
Added: mikelp,
meaning no disrespect..But you have totally taken this man's thread over bud..
*welcome to forum..
[ September 15, 2005, 10:41 AM: Message edited by: dillon3c ]
I'm a bit confused..
Why would you need to feed a 15 KW generator @ a residence,w/ parallel 300kcmils..
Added: mikelp,
meaning no disrespect..But you have totally taken this man's thread over bud..
*welcome to forum..
[ September 15, 2005, 10:41 AM: Message edited by: dillon3c ]
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