Whole House Generator Sizing

Status
Not open for further replies.
A/A Fuel GTX

A/A Fuel GTX

Senior Member
Location
WI & AZ
Occupation
Electrician
Have a client that wants a back up generator and ATS. Average house, 200A service. Doesn't want to spend the $$ on what I recommened, a 20KW whole house generator. Is there anything wrong with installing an 11KW generator with a 50A OCPD that would power the 200A service? The owner would realize that he would have to turn off non critical loads manually at the panel or risk tripping the generators OCPD.
 
A/A Fuel GTX said:
Have a client that wants a back up generator and ATS. Average house, 200A service. Doesn't want to spend the $$ on what I recommened, a 20KW whole house generator. Is there anything wrong with installing an 11KW generator with a 50A OCPD that would power the 200A service? The owner would realize that he would have to turn off non critical loads manually at the panel or risk tripping the generators OCPD.
Click to expand...

Yes, you have to size the generator to the load like you would when sizing a service. You do have a couple options:

Use Generac's easy-switch to back up critical loads only. Or use a load shedding switch that shuts off non-critical loads. The shed loads don't need to be included in the calculation. I often find you need to use load shed the AC to make it work even with a 20k on houses around 2500 square feet. Speaking of AC, will a 11K start his AC? I kinda doubt it unless its very small.
 
A/A Fuel GTX said:
Have a client that wants a back up generator and ATS. Average house, 200A service. Doesn't want to spend the $$ on what I recommened, a 20KW whole house generator. Is there anything wrong with installing an 11KW generator with a 50A OCPD that would power the 200A service? The owner would realize that he would have to turn off non critical loads manually at the panel or risk tripping the generators OCPD.
Click to expand...

To do what you want you need an MTS. (to comply with code) customer may not be home on power loss and the whole house will be dropped on the 11 kw unit. It's not gonna like that.
I have a 7.5 kw generator for my home and it will power up the whole house except for the air conditioner and water heater. It is connected through a 200 amp MTS. I just turn off the A/C and the water heater. (and yes everyone has been told what to turn off and it is written on the MTS.) A step by step procedure.
 
qcroanoke said:
To do what you want you need an MTS. (to comply with code) customer may not be home on power loss and the whole house will be dropped on the 11 kw unit. It's not gonna like that.
I have a 7.5 kw generator for my home and it will power up the whole house except for the air conditioner and water heater. It is connected through a 200 amp MTS. I just turn off the A/C and the water heater. (and yes everyone has been told what to turn off and it is written on the MTS.) A step by step procedure.
Click to expand...

Thats' another good option, if he doesn't mind an MTS
 
A lot depends on your customers and their living habits (and not necessarily the size of their electric service) when sizing a generator. If they have electric heat, electric water heater, 3 AC units that they leave on automatic even when they're not home, etc. then you'll have to size the generator to accommodate those loads. If they have gas heat and perhaps one or two AC units you can use the load shedding module in the ATS to manage those loads. If you use a 10 KW unit to power a house with a 200 amp service, the customer is permitted to manage their own loads by shutting off respective breakers and they have to know that up front. But again, you have to know your customer and if they don't have the capability to understand and do that then you have to size the generator accordingly.
 
Non-Service Rated ATS

Non-Service Rated ATS

Do the load calcs and feed only the supportable loads through the ATS. No manual shedding needed. After Sandy I looked at my house, and I can run everything except the A/C on a 7.5Kw generator. I was planning on actually just putting 6 or so critical loads on the ATS and calling it a day. Alas, life happens and I had to shelve the project.
 
jes25 said:
Yes, you have to size the generator to the load like you would when sizing a service. You do have a couple options:
Click to expand...

goldstar said:
If you use a 10 KW unit to power a house with a 200 amp service, the customer is permitted to manage their own loads by shutting off respective breakers and they have to know that up front.
Click to expand...

The two answers above are in conflict. My understanding is that the customer does not have the option of turning off connected loads. Whatever is connected must not be more than the generator can handle. Load shed modules and automatic AC load shedding are not counted because the ATS shuts them down automatically and therefore do not need to be considered connected.
 
mkgrady said:
The two answers above are in conflict. My understanding is that the customer does not have the option of turning off connected loads. Whatever is connected must not be more than the generator can handle. Load shed modules and automatic AC load shedding are not counted because the ATS shuts them down automatically and therefore do not need to be considered connected.
Click to expand...
The ATS will only shut down those loads that come on automatically and you have to wire them into the ATS so that the load shedding module can control them. Other large loads like a double wall oven, spa tub, pool motors, etc. are permitted to be manually controlled by the homeowner. What it boils down to is if you're careless enough to leave 3 AC units, the spa tub and 3 pool motors all on automatic mode (not on the load shedding module) and you leave the house, a power outage will trip the 45 breaker on a 10K unit. Now, you can run all of these during an outage but the generator will probably have to be upward of a 55KW and nobody wants to spend that kind of $$$
 
goldstar said:
The ATS will only shut down those loads that come on automatically and you have to wire them into the ATS so that the load shedding module can control them. Other large loads like a double wall oven, spa tub, pool motors, etc. are permitted to be manually controlled by the homeowner.
Click to expand...
And please tell me where you find this in the code book :?
 
ceb58 said:
And please tell me where you find this in the code book :?
Click to expand...
You are correct to question that approach. An optional standby system connected through an ATS is required to power the entire load connected to it. The option for the customer to manually shed loads is only an available when connected via an MTS.
 
ceb58 said:
And please tell me where you find this in the code book :?
Click to expand...
Please tell me where it says I have to size the generator for the entire connected load. If that were the case none of the manufacturers would sell any unit below 55 KW with an ATS.
 
Smart $ said:
You are correct to question that approach. An optional standby system connected through an ATS is required to power the entire load connected to it. The option for the customer to manually shed loads is only an available when connected via an MTS.
Click to expand...
What does the MTS do ? Allows them to select the loads they want to power. Doesn't a circuit breaker do the same thing ?
 
Smart $ said:
MTS = manual transfer switch

See 702.4(B), then...

...(B)(1) for MTS
...(B)(2) for ATS
Click to expand...

The 2008 NEC it's 702.5(B)(1) and (2)

But having a automatic transfer switch you can not depend upon the homeowner to react fast enough to get to the panel to shut off the loads that would overload the generator, I have seen a few generators burned up because a few tried to do this, the breaker on the generator is only there for short circuits and very short term overloads called surge rating, it will not protect the generator alternator from long term overloads and is why we see them on many generators slightly larger then the output rating of the generator, a 10kw Generac will have a 50 amp breaker, but the output of the generator is only 41.67 amps, a 20kw Generac will have a 100 amp breaker but will only have a rating of 83.3 amps. applying the breaker curves to these breakers one can see that even a 125 amp load on a 20kw Generac would most likely end up with a damaged alternator.

So it is very important that you size the load that the generator must carry to the rating of the generator not the breaker that is on the generator.

This is why the NEC does not allow an automatic transfer switch to transfer a generator into a load larger then what the generator is rated for, also Generac and a few others will void their warranty if it is found that you have connected an ATS to more load then the generator is rated for.

The reason we see smaller generators with ATS's manufactured is because you have many options to use a smaller generator on larger services with higher loads, one of the methods is to use the ATS to supply a sub panel with only the loads needed in an outage in it, the other is to use automatic load shedding systems or panels that won't allow all the loads to run at the same time, these can be set to allow the AC unit to run while the fridge or freezers are kept off and or pumps such as the lift station, if the lift station pump is set for high priority then when it is called to run it will shut off the AC unit, there are many options that will allow you to use a smaller generator system but without it then only a MTS can be used and it is up to how well you set up the system of the owners turning off the other loads as to how well the generator will last.

I had a home owner who though he could push the generator a little more so he decided to hook up his A/C unit to the subpanel I had set up with a ATS, well I got a call that the generator quit working and I found that the alternator was burned up, I checked the subpanel and found that he had run the A/C to it, and I had to tell him that the damage he cause was considerable, luckily he did find a used alternator from a generator that motor was run dry of oil and I helped him change the bad one out but we moved the A/C unit back to the main panel, by the time he paid me for my hours he could have bought a whole house Generac (20kw)

Here most houses are all gas appliances so most calculated demand loads are less then 75 amps, some are even less,
Hot tubs are a big problem as they default to heat when power is removed even for a second.
 
A/A Fuel GTX said:
Have a client that wants a back up generator and ATS. Average house, 200A service. Doesn't want to spend the $$ on what I recommend, a 20KW whole house generator. Is there anything wrong with installing an 11KW generator with a 50A OCPD that would power the 200A service? The owner would realize that he would have to turn off non critical loads manually at the panel or risk tripping the generators OCPD.
Click to expand...

As many pointed out above and I pointed out in my last post, the breaker on a generator is only for short circuit faults, it is not for overloads, a 11kw generator will only have a output of 45.83 amps, the breaker overload trip curve of a 50 amp breaker could sit there with 55 amps on it for way longer then it would take to burn up the windings of the generators alternator, this is why the NEC does not allow a generator with an ATS to have a load that exceeds the rating of the generator.

Even a 20kw could be overloaded with an output of 83.3 amps if the demand loads exceed this rating from a 200 amp service, you have to do your home work to know what the house demand loads are, if the house is all gas appliances and heat, then it is possible that the demand load will not exceed the 83.3 rating of a 20kw generator, but allot will depend upon other electric loads that will automatically restart when the generator kicks on, like hot tubs that default to heat mode when power is lost., they can easily overload the generator.

The most easily method of controlling what loads the generator will supply is using a 100 amp ATS to feed a subpanel then locating only the circuits needed to it that don't over load the generator, other more expensive methods will be using load shedding panels, but many times you will be back to the same price of a whole house generator anyways as the generator is not that much more when you think about it, and I find much more happy customers when they have their whole house working in a power outage.

A load shedding panel can be as high as $1000.00 plus the labor and materials to relocate the circuits to it that need to be shed in a power outage, a 11kw Generac with a 100 amp ATS panel can be around $2400.00 and a 20 kw Generac with a 200 amp transfer switch is only about $1,000 more so it's not allot more for a whole house generator versus a smaller generator with a load shedding panel, even using a small ATS panel can be just as high by the time you figure in the labor and materials to move all the circuits from the main panel to the generator panel.

After the 22kw mark then the generator prices start jumping much higher to the point that makes it more cost effective to use the load shedding method or smaller generator panels.

Also look at the gas supply, below 15kw most houses will have at least a 150k cfm gas meter/regulator, here most newer gas meters are 250k CFM but a 20kw generator will require at least a 350k CFM meter/regulator, and in most cases is not a big problem because the high side line feeding the house is adequate so the meter and regulator is all that has to be changed out and our utility doesn't charge the customer for it but some do and it is a cost you must take into consideration when pricing the install, I don't do plumbing as our state requires a state license and my paper work always states the gas supply to be done by others with the notation of the required BTU's of gas needed for a given installation, I have a plumber who works with me to help keep the prices down for my customers and I do the same for him when he needs an electrician.
 
Smart $ said:
MTS = manual transfer switch

See 702.4(B), then...

...(B)(1) for MTS
...(B)(2) for ATS
Click to expand...
Yes, I've read the same Code section you all are reading. How is Generac selling these : http://www.electricgeneratorsdirect.com/Generac-Guardian-6051-Standby-Generator/p6952.html ? If, in fact, you are all correct in your interpretations then I will go out on a limb here and say that there are literally 100's of thousands of generators out there with whole house ATS's that are improperly sized, have passed all inspections and are working just fine. If you have high amperage, automatic draw items like AC units, a spa tub or a swimming pool they need to be connected to the load management system via relays or contactors. If those loads draw an excessive amount of current the load mgmt. system will not start them. IMHO, if you have a 10 kw generator hooked up to your 200 amp service and you're stupid enough to turn on your double wall oven during an outage then you deserve to have your generator crap out or burn up the alternator as Hurk pointed out. On the subject of "connected load", are you required to size the generator with the possibility of every appliance running at the same time ? Maybe Hugh Hefner would at his mansion.

According to what you are all suggesting, the only proper way to install a gen-set is to use an "essential circuits" system where there are selected loads wired into a sub-panel with the ATS built in. In most cases this is my preferred method however, there are instances where it is impractical and a whole house system has to be installed. BTW, if you look at a Generac 12 circuit transfer panel http://www.electricgeneratorsdirect.com/Generac-Guardian-6437-Standby-Generator/p10690.html coupled with a 10 KW unit it comes with (5) sp-15's, (3) s/p 20's, (1) 2-p 30 and (1) 2-p 40. This is a UL listed panel. What do they expect you to connect to this with a 10 KW generator with a 45 amp breaker ? FWIW, I usually change out the 2- pole breakers and re-install the breakers I want. Did I void the UL listing ?
 
goldstar said:
Yes, I've read the same Code section you all are reading. How is Generac selling these : http://www.electricgeneratorsdirect.com/Generac-Guardian-6051-Standby-Generator/p6952.html ? If, in fact, you are all correct in your interpretations then I will go out on a limb here and say that there are literally 100's of thousands of generators out there with whole house ATS's that are improperly sized, have passed all inspections and are working just fine. If you have high amperage, automatic draw items like AC units, a spa tub or a swimming pool they need to be connected to the load management system via relays or contactors. If those loads draw an excessive amount of current the load mgmt. system will not start them. IMHO, if you have a 10 kw generator hooked up to your 200 amp service and you're stupid enough to turn on your double wall oven during an outage then you deserve to have your generator crap out or burn up the alternator as Hurk pointed out. On the subject of "connected load", are you required to size the generator with the possibility of every appliance running at the same time ? Maybe Hugh Hefner would at his mansion.

According to what you are all suggesting, the only proper way to install a gen-set is to use an "essential circuits" system where there are selected loads wired into a sub-panel with the ATS built in. In most cases this is my preferred method however, there are instances where it is impractical and a whole house system has to be installed. BTW, if you look at a Generac 12 circuit transfer panel http://www.electricgeneratorsdirect.com/Generac-Guardian-6437-Standby-Generator/p10690.html coupled with a 10 KW unit it comes with (5) sp-15's, (3) s/p 20's, (1) 2-p 30 and (1) 2-p 40. This is a UL listed panel. What do they expect you to connect to this with a 10 KW generator with a 45 amp breaker ? FWIW, I usually change out the 2- pole breakers and re-install the breakers I want. Did I void the UL listing ?
Click to expand...

The NEC is not saying that a 20kw generator can't be connected to a 200 amp service what the NEC is saying is it can't automatically transfer into more then 20kw worth of load that is possible to run when the generator comes online, most houses will not even be close to the service rating if they did the electric bill would be very high, areas that don't have NG as a common fuel source might be a problem if the house is all electric but even then its not uncommon to have LP as a fuel source or oil, I have done many demand calculations on houses around here as even though the house has a 200 amp service most houses with all gas appliances don't even come near a 100 amp service much less a 200 amp, here with the price of electric so high it is not very common to find a house with electric heat actually its very rare around here, but unless you know you can't just guess that the generator will not be over loaded, you have to do the homework or you could be setting yourself up with a costly mistake if the alternator should fail from being over loaded as this would not be covered by the manufactures warranty and you could be left holding the bill.

My house has a 100 amp service but my calculated demand load is around 45 amps or 10.8kw I could get away with a 11kw generator but I wouldn't put in anything less then a 14kw just because of the A/C start up current tends to bring a smaller generator to its knees if all the other loads also come on, but I have seen many 200 amp service that only have a 65 or 75 amp demand load that a 17kw or 20kw would have no problem running, so you can see it has nothing to do with the size of the service but is based upon how much load that can start automatically if power was to go out when the home owners are not at home, even if they are at home they most likely will not be able to get to the panel fast enough to turn off loads to prevent the generator from being over loaded, this is why Generac can sell a 20kw with a 200 amp transfer switch or even a 400 amp transfer switch because the demand load will be nowhere near the rating of the service as many home owners who have a house built just want a larger service thinking bigger is better, they just don't realize that if they really used that much electric their utility bill would be very high, at least around here it would be.
 
goldstar said:
Please tell me where it says I have to size the generator for the entire connected load. If that were the case none of the manufacturers would sell any unit below 55 KW with an ATS.
Click to expand...
Wayne has done a fine job of answering the question.
Other large loads like a double wall oven, spa tub, pool motors, etc. are permitted to be manually controlled by the homeowner.
Click to expand...
When it comes down to it you only have a few choices to go by. Do the load calculation as required by 702.4(B) then if they want an ATS you can sell a system that will handle the full load or install an ATS with load management 702.4(B)(2)(a)(b). Or, and this is my preferred method, install a ATS connected to a optional load panel. Or install a manual transfer switch and wish them luck. But no where in 702.4 (B)(2) dose it say any thing about part of the load being able to be shed manually if it is connected to an ATS.
 
Status
Not open for further replies.
Top