Sizing Stand Alone Home Generator

[kwired]

But that would leave an oversized generator. Most home around here with a 200amp service get a 15 or 20kw generator with 80 or 100amp feeders going into a 200amp whole home ATS. Unless the home is all electric heat the generator runs everything during an outage, even an electric range.

Of course this might not be to code, but if it is why not do it? BTW, is there anything in code allowing this?

Load shedding is automatic method of disabling certain loads to ensure you do not go over the capacity the generator can supply, there is more then one way to do this and can be simple isolation relay that doesn't allow selected loads to run when the generator is supplying the power, or it can be more sophisticated equipment that monitors current on the generator and may allow select equipment to run if the current load level is low enough to handle the optional item. Non optional items get priority - if load is too high optional items are locked out until load goes down.

Art 220 load calculations often do result in a supply with higher capacity then one may normally see. People often "round up" service sizes as well. If you did a load calculation on a home and came up with needing 115 amps supply - many will install a 200 amp service though NEC would allow you to install 125. But even then you may hardly ever find that service has more then 30 or 40 amps of load, it may peak out for 15 - 30 minute periods occasionally at 70 or 80 but then drop back to the usual 30 or 40. NEC does allow you to use proper recording methods and you could possibly use that 70 or 80 amp peak for sizing a generator instead of needing one that can supply the full capacity of the 200 amp panel. That maybe puts you at a 20KVA unit , but maybe you can shed some heating appliance or something and get the load down a little further and go with the less expensive air cooled 16KVA unit instead of the liquid cooled 20 KVA unit.

 

[mbrooke]

Load shedding is automatic method of disabling certain loads to ensure you do not go over the capacity the generator can supply, there is more then one way to do this and can be simple isolation relay that doesn't allow selected loads to run when the generator is supplying the power, or it can be more sophisticated equipment that monitors current on the generator and may allow select equipment to run if the current load level is low enough to handle the optional item. Non optional items get priority - if load is too high optional items are locked out until load goes down.

Art 220 load calculations often do result in a supply with higher capacity then one may normally see. People often "round up" service sizes as well. If you did a load calculation on a home and came up with needing 115 amps supply - many will install a 200 amp service though NEC would allow you to install 125. But even then you may hardly ever find that service has more then 30 or 40 amps of load, it may peak out for 15 - 30 minute periods occasionally at 70 or 80 but then drop back to the usual 30 or 40. NEC does allow you to use proper recording methods and you could possibly use that 70 or 80 amp peak for sizing a generator instead of needing one that can supply the full capacity of the 200 amp panel. That maybe puts you at a 20KVA unit , but maybe you can shed some heating appliance or something and get the load down a little further and go with the less expensive air cooled 16KVA unit instead of the liquid cooled 20 KVA unit.

Ok that might explain it then. Some of these installs probably are non-complaint if the home lost power on say thanksgiving day, but it does manage to get people to agree to back up gennies.

 

[iwire]

Of course this might not be to code, but if it is why not do it? BTW, is there anything in code allowing this?

1) For better or worse the rules recently tightened up on sizing generators.

2) If a manual transfer means is used you can connect any size load to any size generator allowing the user to decide what items to use at the same time.

3) If you use an ATS things change. You have to do load calculations to make sure the genertor has enough capacity to run everything connected to it at the same time. You do get the option of locking out or shedding certain loads to bring down the load. Like locking out AC compressors or maybe a steam generator for a steam room.

 

[GoldDigger]

That is not NEC acceptable and would likely result in a larger generator than needed.

You could argue that you are just doing the load calculation using an analog computer instead of pencil and paper.
In practice, the all-on measurement might come up short of the correctly calculated value based on two factors:
1. Motor starting,
2. Continuous loads.

 

[ActionDave]

But that would leave an oversized generator. Most home around here with a 200amp service get a 15 or 20kw generator with 80 or 100amp feeders going into a 200amp whole home ATS. Unless the home is all electric heat the generator runs everything during an outage, even an electric range.

Yep, you are correct.

Of course this might not be to code, but if it is why not do it? BTW, is there anything in code allowing this?

There did not used to be but now there is.

The thing is, the same type of control freaks that want extra inspections of home electrical systems with documented results of exactly how much current it takes to trip a GFCI got the ear of enough CMP members and convinced them that a home with an a backup generator must have one big enough to power the calculated load of the house..... even if no one is home to cook a turkey in the electric oven, while using the electric dryer, while making coffee and toast, while using a hair dryer and curling iron.

 

[mbrooke]

Yep, you are correct.

There did not used to be but now there is.

The thing is, the same type of control freaks that want extra inspections of home electrical systems with documented results of exactly how much current it takes to trip a GFCI got the ear of enough CMP members and convinced them that a home with an a backup generator must have one big enough to power the calculated load of the house..... even if no one is home to cook a turkey in the electric oven, while using the electric dryer, while making coffee and toast, while using a hair dryer and curling iron.

Yes, because guys from Europe pull the strings on the CMPs :roll: Seriously, one guy mentions his country tests GFCIs and everyone takes it as a personnel insult :blink: :blink:

 

[ActionDave]

Yes, because guys from Europe pull the strings on the CMPs :roll: Seriously, one guy mentions his country tests GFCIs and everyone takes it as a personnel insult :blink: :blink:

No! I am not personally insulted. And that is not the point.

I agree with you about standby generators on houses.

I did make a little poke at you to because in the other thread you took the position (my words to follow, not yours) that good enough here in the US was not near as good as what they did in Europe.

Similar thinking, IMO, is what got us to where we are with residential generators and ATS's. There is no safety issue having a smaller generator. Unoccupied homes are lightly loaded. Generators, with or without load shedding, don't need to be sized for the whole house just because they are on an automatic transfer switch. Ten years ago we didn't have to. But we do now because somebody thought the existing standard was not safe enough and a higher, safer standard was worth fighting for. They won, I accept it, but my opinion has not changed.

 

[jap]

1) For better or worse the rules recently tightened up on sizing generators.

2) If a manual transfer means is used you can connect any size load to any size generator allowing the user to decide what items to use at the same time.

3) If you use an ATS things change. You have to do load calculations to make sure the genertor has enough capacity to run everything connected to it at the same time. You do get the option of locking out or shedding certain loads to bring down the load. Like locking out AC compressors or maybe a steam generator for a steam room.

I'm with Iwire.
If I had a customer ask me for a generator that would "run everything" I don't think I'd even take the time to calculate the load.
I'd go outside and see what size the Main OCP devices were and size it per that with an auto xfr switch, then let them decide if they wanted to spend the money.
Once you do less than that your down to load shedding and manual transfers and selective loads and that's not what they originally asked for.

I've been surprised myself when the actual dollar amount didn't affect them. They actually meant what they were asking for making me look like the cheapo instead of them, so, don't be too quick to judge.

JAP>

 

[Tony S]

one guy mentions his country tests GFCIs and everyone takes it as a personnel insult :blink: :blink:

See
http://forums.mikeholt.com/showthread.php?t=170350&p=1656827#post1656827

 

[kwired]

No! I am not personally insulted. And that is not the point.

I agree with you about standby generators on houses.

I did make a little poke at you to because in the other thread you took the position (my words to follow, not yours) that good enough here in the US was not near as good as what they did in Europe.

Similar thinking, IMO, is what got us to where we are with residential generators and ATS's. There is no safety issue having a smaller generator. Unoccupied homes are lightly loaded. Generators, with or without load shedding, don't need to be sized for the whole house just because they are on an automatic transfer switch. Ten years ago we didn't have to. But we do now because somebody thought the existing standard was not safe enough and a higher, safer standard was worth fighting for. They won, I accept it, but my opinion has not changed.

Wouldn't by chance be generator manufacturers that thought we needed to do it the new way?

 

[iwire]

Wouldn't by chance be generator manufacturers that thought we needed to do it the new way?

I believe that to be exactly the case.

I believe they claimed their own equipment could fail dangerously if allowed to be overloaded.

Seems to me that just the addition of a winding temperature sensor set up to kill the engine would have fixed the issue. Heck, maybe just a simple motor overload device fitted to the output could have done it.

 

[kwired]

I believe that to be exactly the case.

I believe they claimed their own equipment could fail dangerously if allowed to be overloaded.

Seems to me that just the addition of a winding temperature sensor set up to kill the engine would have fixed the issue. Heck, maybe just a simple motor overload device fitted to the output could have done it.

Or maybe the fact that most of these smaller residential gensets have an overcurrent device in the output means they will not be overloaded for very long at all.

But still why make a minor change when you can sell higher priced units because of a change in installation requirements?

I need to get into manufacturing - this generator market and AFCI market are already taken any ideas what other items I can force on consumers via codes?

 

[iwire]

Or maybe the fact that most of these smaller residential gensets have an overcurrent device in the output means they will not be overloaded for very long at all.

I may be mistaken but I believe there are conditions of overload that can draw the voltage down to the point there is not enough current to trip the breaker in a reasonable amount of time.

 

[kwired]

I may be mistaken but I believe there are conditions of overload that can draw the voltage down to the point there is not enough current to trip the breaker in a reasonable amount of time.

I guess I can see that, to some extent. These smaller generators have a relatively high internal impedance compared to a similar kVA sized transformer, which would be current limiting, voltage reducing, and then the marginally sized prime mover also slows down when under such load conditions and leaves you with pretty high current but will take some time before the overcurrent device trips if it is similar trip curve to what we use for most premises wiring. Still shouldn't damage anything if designed well IMO, but they do what they can to get by with as little as possible and keep their price competitive in any such consumer products these days.

 

[mbrooke]

No! I am not personally insulted. And that is not the point.

I agree with you about standby generators on houses.

I did make a little poke at you to because in the other thread you took the position (my words to follow, not yours) that good enough here in the US was not near as good as what they did in Europe.

Similar thinking, IMO, is what got us to where we are with residential generators and ATS's. There is no safety issue having a smaller generator. Unoccupied homes are lightly loaded. Generators, with or without load shedding, don't need to be sized for the whole house just because they are on an automatic transfer switch. Ten years ago we didn't have to. But we do now because somebody thought the existing standard was not safe enough and a higher, safer standard was worth fighting for. They won, I accept it, but my opinion has not changed.

Only in terms of GFCI testing I believed the IEC had a better method in place, hence why I took this position.


My mistake though, I mis-took your poking fun :ashamed:

True this thinking is not good in regards to generators. IMO electricians should be given the same powers that POCOs have in the ability to intentionally size smaller equipment knowing the service will never reach its NEC load calc. In many cases the load calc is even lower than the required 100amp service disconnect.


Out of curiosity, playing devil's advocate, couldn't adding a breaker to the generator make this compliant? And accidental over load would trip it?


I agree in this regard, the NEC should not dictate generator size.

 

[GoldDigger]

Only in terms of GFCI testing I believed the IEC had a better method in place, hence why I took this position.


My mistake though, I mis-took your poking fun :ashamed:

True this thinking is not good in regards to generators. IMO electricians should be given the same powers that POCOs have in the ability to intentionally size smaller equipment knowing the service will never reach its NEC load calc. In many cases the load calc is even lower than the required 100amp service disconnect.


Out of curiosity, playing devil's advocate, couldn't adding a breaker to the generator make this compliant? And accidental over load would trip it?


I agree in this regard, the NEC should not dictate generator size.

For an overload or even a short circuit, the tolersnce band of the breaker and possible current limiting in the generator together make it unlikely that the main breaker at a generator will trip stvall.

 

[stickboy1375]

Regardless of that it does not make business sense (IMO) to start with less than they asked for.

Price it up as they asked with a nice profit and see if it sticks, if they have sticker shock you have room to reduce the costs without cutting into profits.

To each their own.

I was working down in Greenwich, CT and although the house was impressive, it really wasn't that big either, but the owner is extremely wealthy, so check this out, someone sold this guy a 1200 amp service and a 1200 amp generator, I couldn't believe it, but hey, someone made some really good money... The house was maybe 4000 sq. ft.... I shit you not. The best part was, the utility company gave the guy a 75kVA transformer...

 

[mbrooke]

I was working down in Greenwich, CT and although the house was impressive, it really wasn't that big either, but the owner is extremely wealthy, so check this out, someone sold this guy a 1200 amp service and a 1200 amp generator, I couldn't believe it, but hey, someone made some really good money... The house was maybe 4000 sq. ft.... I shit you not. The best part was, the utility company gave the guy a 75kVA transformer...

All electric home or all gas?

Pad mount transformer?

That is something :lol:

 

[jap]

I was working down in Greenwich, CT and although the house was impressive, it really wasn't that big either, but the owner is extremely wealthy, so check this out, someone sold this guy a 1200 amp service and a 1200 amp generator, I couldn't believe it, but hey, someone made some really good money... The house was maybe 4000 sq. ft.... I shit you not. The best part was, the utility company gave the guy a 75kVA transformer...

Careful with the wording.......


JAP>

 

[stickboy1375]

All electric home or all gas?

Pad mount transformer?

That is something :lol:

All gas as far as I know, and yes, pad mount transformer... the house is basically on a small island, but still, the load calcs are a joke.