# Generator Output Wire Size



## Cletis

#2is my guess. Ill probably be wrong though. Seems more like service entrance cable rules??


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## mkgrady

Cletis said:


> #2is my guess. Ill probably be wrong though. Seems more like service entrance cable rules??


I'm using a 200 amp transfer switch with load shedding features for the AC units if that matters. So in essence the generator will feeds the entire dwelling load but only when it decides it can handle it all.


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## RGH

Why not pipe it and use #2 and #4 (neu) throw that in and 1.5in sch 80pvc and call it a day. You will only be servicing part of the main panel on dedicated circuits Thur internal switching panel 8,10,12circuits etc so don't worry about the 200 amp main. Check that sch80 sizing off the top of my head.


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## RGH

Sorry you got ahead of me didn't see the load shedder but your genny is rated at 100amp and your using a 200amp switch panel?


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## mkgrady

RGH said:


> Sorry you got ahead of me didn't see the load shedder but your genny is rated at 100amp and your using a 200amp switch panel?


Using a 200 amp panel is the only way I can power the whole house, which is what the customer wants. The ATS will cut out any of the heavy loads. This way I don't have to add a load center. I just need to put the ATS in line with the main feeder to the existing panel.


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## Bulldog1

We use 1 1/4 carflex and #2 THHN from the ATS to the generator. IMO SER will look like a hack installed the generator if it looks like I am picturing in my mind. Sorry just my 2 cents.


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## mkgrady

Bulldog1 said:


> We use 1 1/4 carflex and #2 THHN from the ATS to the generator. IMO SER will look like a hack installed the generator if it looks like I am picturing in my mind. Sorry just my 2 cents.


I'm thinking SER because the run through the unfinished cellar and below a deck is around 80 feet. I would put a Junction box out near the generator and switch over to Carflex and THHN/THWN


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## mkgrady

RGH said:


> Why not pipe it and use #2 and #4 (neu) throw that in and 1.5in sch 80pvc and call it a day. You will only be servicing part of the main panel on dedicated circuits Thur internal switching panel 8,10,12circuits etc so don't worry about the 200 amp main. Check that sch80 sizing off the top of my head.


Why would pipe and wire be better than SER?


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## Bulldog1

mkgrady said:


> Why would pipe and wire be better than SER?



Exposed SER would not look as professionally installed as carflex or PVC IMO. Nothing wrong with it just not what we would do. 


Also you need a conductor good for 83 amps in the 75 degree column. We always flex with#2 thhn copper. If the meter is a long distance from the generator we sometimes run ser into a service rated disconnect and flex copper to the generator.


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## mkgrady

Bulldog1 said:


> Exposed SER would not look as professionally installed as carflex or PVC IMO. Nothing wrong with it just not what we would do.
> 
> 
> Also you need a conductor good for 83 amps in the 75 degree column. We always flex with#2 thhn copper. If the meter is a long distance from the generator we sometimes run ser into a service rated disconnect and flex copper to the generator.


83 amps would be the output of the generator but the OCP is a 100 amp breaker on the generator output, so I assume I need wire good for 100 amps. Anybody else have an opinion on whether or not #2 aluminum SER can be used?


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## Dennis Alwon

Well it is a good question but if you think of why we are allowed to use 310.15(B)(6) then I would have to say that you need to use T310.16 in this case.


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## Bulldog1

mkgrady said:


> 83 amps would be the output of the generator but the OCP is a 100 amp breaker on the generator output, so I assume I need wire good for 100 amps. Anybody else have an opinion on whether or not #2 aluminum SER can be used?



You are correct. I posted we use #2 copper which is good for 100 amps. Look in 310.16 at the 75 degree column.


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## Cletis

Bulldog1 said:


> If the meter is a long distance from the generator we sometimes run ser outside on brick into a service rated disconnect and flex copper to the generator.


You run SER outdoor on brick ??


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## Bulldog1

Cletis said:


> You run SER outdoor on brick ??



No we run it in the crawl space to the opposite side of the house.


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## mkgrady

Bulldog1 said:


> You are correct. I posted we use #2 copper which is good for 100 amps. Look in 310.16 at the 75 degree column.


I looked. It's good for 115 amps.


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## mkgrady

Dennis Alwon said:


> Well it is a good question but if you think of why we are allowed to use 310.15(B)(6) then I would have to say that you need to use T310.16 in this case.


So it is your opinion I need to run #1 aluminum SER to get to 100 amps?


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## Cletis

*Slab*



Bulldog1 said:


> No we run it in the crawl space to the opposite side of the house.


What if it's a slab house?


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## mbednarik

I would say #1al or #3 cu. 310.15(B)7 says conductors between the main disconnect and the panel about half way through the paragraph. So i think #2 al and #4 cu is too small.


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## RGH

Dennis Alwon said:


> Well it is a good question but if you think of why we are allowed to use 310.15(B)(6) then I would have to say that you need to use T310.16 in this case.


Dennis, after correction allowance per (t) section, the #2 copper or 1/0 al or copper clad al should be fine. That would give you 125 amp capacity pre-temp correction value. I think condo sizing check per fill requirment aswell if PVC ,yes? or am I overkilling....at 1-1/2" you may want to check 250.6 on your bonding. And for that disc check 225.34 as well. Take some pics when your done!


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## BBQ

Cletis said:


> You run SER outdoor on brick ??


Yes it is done


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## BBQ

mkgrady said:


> We all know that when a wire is used to feed the entire load of a dwelling it can be sized smaller than if it does not feed the entire dwelling. For example if I were installing a 100 amp service I would use #2 aluminim SE cable where #2 aluminum is normally rated 90 amps.
> 
> My issue is how to size the wire from a generator to the transfer switch. The service is 200 amps. The 20KW generator has an output breaker rated 100 amps so I figure I will use 100 amp cable. I plan to use 100 amp SER as I need the fourth conductor. So my question is what size SER aluminum do I need? Is it #2 because while the generator runs it serves the entire home? Or is it #1 because it is not the service?


Because you are load shedding you are not carrying the entire dwelling unit load so IMO you do not get to use the reduced size table.


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## Cletis

*...*



BBQ said:


> Yes it is done


OMG


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## Hairbone

mbednarik said:


> I would say #1al or #3 cu. 310.15(B)7 says conductors between the main disconnect and the panel about half way through the paragraph. So i think #2 al and #4 cu is too small.


:thumbsup:


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## ceb58

RGH said:


> Sorry you got ahead of me didn't see the load shedder but your genny is rated at 100amp and your using a 200amp switch panel?


His switch must be rated for the larger of the two loads. And I hope it is service rated.



Dennis Alwon said:


> Well it is a good question but if you think of why we are allowed to use 310.15(B)(6) then I would have to say that you need to use T310.16 in this case.


I think he can use T310.15(B)(6) for the sizing.
Look at 310.15(A)(2) then read 240.4 which say to size to the overcurrent which in this case would be 100 amp.
So I see no reason why you could not use T310.15(B)(6) to size the feeders which would be #4Cu or #2Al


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## BBQ

ceb58 said:


> So I see no reason why you could not use T310.15(B)(6) to size the feeders which would be #4Cu or #2Al





I do, the feeder the OP is talking about it not the 'main power feeder' for a dwelling unit.

The 'main power feeder' for a dwelling unit supplies 100% of the dwelling unit load. 




> 310.15(B)(7) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services
> and Feeders. For individual dwelling units of onefamily,
> two-family, and multifamily dwellings, conductors,
> as listed in Table 310.15(B)(7), shall be permitted as
> 120/240-volt, 3-wire, single-phase service-entrance conductors,
> service-lateral conductors, and feeder conductors
> that serve as the main power feeder to each dwelling unit
> and are installed in raceway or cable with or without an
> equipment grounding conductor. *For application of this section,
> the main power feeder shall be the feeder between the
> main disconnect and the panelboard that supplies, either by
> branch circuits or by feeders, or both, all loads that are part
> or associated with the dwelling unit.* The feeder conductors
> to a dwelling unit shall not be required to have an allowable
> ampacity rating greater than their service-entrance conductors.
> The grounded conductor shall be permitted to be
> smaller than the ungrounded conductors, provided the requirements
> of 215.2, 220.61, and 230.42 are met


Just because he has load sheded some of the loads does not mean those loads are not part of
or associated with the dwelling unit.


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## ratrod56

Isnt there something about the conductors from generator having to be undergrouund or encased in concrete ??


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## Bulldog1

ratrod56 said:


> Isnt there something about the conductors from generator having to be undergrouund or encased in concrete ??


The NEC does not require that.


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## BBQ

ratrod56 said:


> Isnt there something about the conductors from generator having to be undergrouund or encased in concrete ??


If the generator is an actual emergency generator there could be times where you would be required to protect the feeder.




> *700.10 Wiring, Emergency System.
> (D) Fire Protection.* Emergency systems shall meet the
> additional requirements in (D)(1) through (D)(3) in assembly
> occupancies for not less than 1000 persons or in buildings
> above 23 m (75 ft) in height with any of the following
> occupancy classes: assembly, educational, residential, detention
> and correctional, business, and mercantile.
> 
> 
> Informational Note: For the definition of Occupancy Classification,
> see Section 6.1 of NFPA 101-2009, Life Safety
> Code.
> 
> *(1) Feeder-Circuit Wiring. *Feeder-circuit wiring shall meet
> one of the following conditions:
> 
> (1) Be installed in spaces or areas that are fully protected
> by an approved automatic fire suppression system
> 
> (2) Be a listed electrical circuit protective system with a minimum
> 2-hour fire rating
> Informational Note: UL guide information for electrical
> circuit protective systems (FHIT) contains information on
> proper installation requirements to maintain the fire rating.
> 
> (3) Be protected by a listed thermal barrier system for electrical
> system components with a minimum 2-hour fire
> rating
> 
> (4) Be protected by a listed fire-rated assembly that has a
> minimum fire rating of 2 hours and contains only emergency
> wiring circuits.
> 
> (5) Be encased in a minimum of 50 mm (2 in.) of concrete


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## Salvatoreg02

mkgrady said:


> We all know that when a wire is used to feed the entire load of a dwelling it can be sized smaller than if it does not feed the entire dwelling. For example if I were installing a 100 amp service I would use #2 aluminim SE cable where #2 aluminum is normally rated 90 amps.
> 
> My issue is how to size the wire from a generator to the transfer switch. The service is 200 amps. The 20KW generator has an output breaker rated 100 amps so I figure I will use 100 amp cable. I plan to use 100 amp SER as I need the fourth conductor. So my question is what size SER aluminum do I need? Is it #2 because while the generator runs it serves the entire home? Or is it #1 because it is not the service?


Its based on the generator output. 20KW EQUALS 83.5 amps. So, # 4 copper or #2alum is allowed, even though the breaker on the generator is rated 100amps.


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## Bulldog1

Salvatoreg02 said:


> Its based on the generator output. 20KW EQUALS 83.5 amps. So, # 4 copper or #2alum is allowed, even though the breaker on the generator is rated 100amps.




Where do you come up with this crap? Oh and it's 83.33333 amps


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## sharky1368

*Owner /Operater Of Electric Co.*

# 2 SER would be good.....It will also pass code with NEC...as well as heat ratings!!


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## sharky1368

The orig. residence in ?....was it under 75 ft. high and residential?


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## Bulldog1

sharky1368 said:


> # 2 SER would be good.....It will also pass code with NEC...as well as heat ratings!!


1/0 AL is needed. Your feeder conductor needs to be sized at 125% of generator nameplate.


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## BBQ

Bulldog1 said:


> 1/0 AL is needed. Your feeder conductor needs to be sized at 125% of generator nameplate.


Not in most cases.


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## Bulldog1

BBQ said:


> Not in most cases.


The manufactures normally say to do this in the installation instructions. I assume your saying it is not considered a continuous load. We lose power here in areas for more than 3 hours frequently. The name plate does not account for it being used as a continuous load.


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## Salvatoreg02

BBQ said:


> Not in most cases.


Please explain when it's required to size at 125%?


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## Salvatoreg02

mkgrady said:


> 83 amps would be the output of the generator but the OCP is a 100 amp breaker on the generator output, so I assume I need wire good for 100 amps. Anybody else have an opinion on whether or not #2 aluminum SER can be used?


If its a Generac the breaker is now only 90amps.


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## Salvatoreg02

Bulldog1 said:


> 1/0 AL is needed. Your feeder conductor needs to be sized at 125% of generator nameplate.


Show me where it says that.


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## Bulldog1

Salvatoreg02 said:


> Show me where it says that.



If the generator is expected to run more 
than 3 hours which all installations could you size your feeder as a continuous load. Appliances have the 125% figured into the nameplate generators don't. Read your installation instructions. Many of the air cooled models we have installed tell you this in the installation instructions. I've never seen a liquid cooled model where the instructions called for that.


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## knfuller13

mkgrady said:


> We all know that when a wire is used to feed the entire load of a dwelling it can be sized smaller than if it does not feed the entire dwelling. For example if I were installing a 100 amp service I would use #2 aluminim SE cable where #2 aluminum is normally rated 90 amps.
> 
> My issue is how to size the wire from a generator to the transfer switch. The service is 200 amps. The 20KW generator has an output breaker rated 100 amps so I figure I will use 100 amp cable. I plan to use 100 amp SER as I need the fourth conductor. So my question is what size SER aluminum do I need? Is it #2 because while the generator runs it serves the entire home? Or is it #1 because it is not the service?





mkgrady said:


> We all know that when a wire is used to feed the entire load of a dwelling it can be sized smaller than if it does not feed the entire dwelling. For example if I were installing a 100 amp service I would use #2 aluminim SE cable where #2 aluminum is normally rated 90 amps.
> 
> My issue is how to size the wire from a generator to the transfer switch. The service is 200 amps. The 20KW generator has an output breaker rated 100 amps so I figure I will use 100 amp cable. I plan to use 100 amp SER as I need the fourth conductor. So my question is what size SER aluminum do I need? Is it #2 because while the generator runs it serves the entire home? Or is it #1 because it is not the service?


I am an electrician and I always go by the out of the generator when sizing the conductors for the generator and you size the automatic transfer switch for the size service you have because it becomes the main service panel and the existing panel is now a subpanel. Just remember to isolate the grounds in the existing main panel to make a subpanel and run the ground conductor from the electrodes and water meter to the transfer switch then re-feed the main panel with SER cable sized for the utility service to the transfer switch.


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## VELOCI3

Sent from my iPad using Tapatalk


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