Whole House Generator Sizing

[hurk27]

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.

I often question this when a load will not come back on automatically such as oven or other appliances that have to be manually restarted when the power comes back on on generator, whats the difference of this and a manual TS, these loads are in fact automatically shed by the virtue that they wont come back on, on a power failure to me it is then up to the home owner to turn off other appliances such as the A/C unit to finish cooking an electric dryer is another appliance that won't come come back on as it has to be manually restarted to me this meets the definition of automatically controlled loads.

 

[gadfly56]

I often question this when a load will not come back on automatically such as oven or other appliances that have to be manually restarted when the power comes back on on generator, whats the difference of this and a manual TS, these loads are in fact automatically shed by the virtue that they wont come back on, on a power failure to me it is then up to the home owner to turn off other appliances such as the A/C unit to finish cooking an electric dryer is another appliance that won't come come back on as it has to be manually restarted to me this meets the definition of automatically controlled loads.

They may be shed now because that's the way manufacturers choose to design them. Nothing prevents an appliance manufacturer from designing an electric range that does come back on automatically. If I wanted to separate my brand from the rest, I might specifically address the issue, maybe a cooking cycle programmed into the oven that sees there's only been a 2 minute interruption of power, let's finish the roast.

I still think the best way, as others have alluded to, is a subpanel with critical loads that the generator can easily handle, then ATS or MTS as suits your budget or design needs.

 

[ceb58]

I often question this when a load will not come back on automatically such as oven or other appliances that have to be manually restarted when the power comes back on on generator, whats the difference of this and a manual TS, these loads are in fact automatically shed by the virtue that they wont come back on, on a power failure to me it is then up to the home owner to turn off other appliances such as the A/C unit to finish cooking an electric dryer is another appliance that won't come come back on as it has to be manually restarted to me this meets the definition of automatically controlled loads.

What you are saying is fine except there are other larger loads that will come back on by them self. A/C units, water heaters, cook tops. You may forget that up until the 2008 code you could put a 10Kw generator on a 200 amp service and transfer all of the load. This is one of several comments from the ROP that was part of why the code was changed.

Submitter: Dale Rooney, Municipality of Anchorage
Recommendation: Add a new last sentence to read:
Where an automatic transfer switch is used without automatic load shedding
the installation shall have adequate capacity for the calculated load being
supplied or the maximum demand per 220.87(1).
Substantiation: The last sentence of this section seems to allow the use of an
automatic transfer switch in such a way that it may transfer in an overloaded
condition. This will, at best, turn it into a manual transfer switch (lighten
the load and restart the generator) and at worst could result in damage to the
equipment.
This was actually done on the residence of a family which included children
with medical needs.
Panel Meeting Action: Accept in Principle
Panel Statement: See the panel action and statement on Proposal 13-168.
Number Eligible to Vote: 14
Ballot Results: Affirmative: 13
Ballot Not Returned: 1 Gustafson, R.

 

[hurk27]

What you are saying is fine except there are other larger loads that will come back on by them self. A/C units, water heaters, cook tops. You may forget that up until the 2008 code you could put a 10Kw generator on a 200 amp service and transfer all of the load. This is one of several comments from the ROP that was part of why the code was changed.

I was just stating that some appliances that do not default to on when a power failure happens should be considered as self shedding loads because it happens automatically, but Gadfly56 makes a good point that newer replacement appliances might not do this and turn back on after a power failure or generator takes the load, so it's kind of a mute point now.

 

[GoldDigger]

I was just stating that some appliances that do not default to on when a power failure happens should be considered as self shedding loads because it happens automatically, but Gadfly56 makes a good point that newer replacement appliances might not do this and turn back on after a power failure or generator takes the load, so it's kind of a mute point now.

Older appliances with just mechanical switches and timers instead of electronic controls will also happily come back on when the power is restored. The main exception to this will be the clothes dryer which has had the requirement for a separate momentary start button for a long time. (I believe that was required by CPSC rather then UL or NEC? Or was it?)

 

[hurk27]

Older appliances with just mechanical switches and timers instead of electronic controls will also happily come back on when the power is restored. The main exception to this will be the clothes dryer which has had the requirement for a separate momentary start button for a long time. (I believe that was required by CPSC rather then UL or NEC? Or was it?)

Even the newer front loaders have a start button to start them back up if the power goes out or even if you open the front door, which I think is a safety requirement so maybe it could be considered as an automatic self shedding load, 5000 to 5500 watts can be significant when talking about 20kw generators, even commercial dryers have this.

This also brings to mind another idea that electric ovens and cooktops should have a feature that won't allow them to automatically start back up after a power failure, lets say the wife was cooking and power goes out, they find out it will be out for 12 hours, they pack up and go to Aunt Jane's for the night never thinking about what was left on, while at Aunt Jane's the power comes back on and now the food they were cooking turns into a fire ball burning the house down, not a good thing, so like dryers maybe ovens have the same requirement as I have never seen one that could come back on, mater of fact, they don't even keep their memory as the clock and timers all default to flashing 12:00, mine even looses the bake cycle you had it in as all the settings are electronic settings, the temp setting also reverts to 0?F,
as far as cooktops go it would take a relay to drop out that would have to be reset like a start stop circuit other wise it would come back on.

We know microwaves won't come back on but most people would at least want the micro to be on the generator during a power failure so they can at least heat something up.

But really thinking about it, there are not many appliances that will stay off, I think most today will try to fire back up once the power is restored to it, and that kind of foils my thinking on this.

Making a simple load shedding circuit would not take much, for the A/C just a small 120 volt relay fed from the utility sense circuit that would drop out the A/C 24 volt control circuit would be enough, but with todays smart load shedding panels that would allow the A/C unit run when other loads were off that actually monitor the load on the generator would make trying to roll your own useless.

 

[kwired]

Even the newer front loaders have a start button to start them back up if the power goes out or even if you open the front door, which I think is a safety requirement so maybe it could be considered as an automatic self shedding load, 5000 to 5500 watts can be significant when talking about 20kw generators, even commercial dryers have this.

This also brings to mind another idea that electric ovens and cooktops should have a feature that won't allow them to automatically start back up after a power failure, lets say the wife was cooking and power goes out, they find out it will be out for 12 hours, they pack up and go to Aunt Jane's for the night never thinking about what was left on, while at Aunt Jane's the power comes back on and now the food they were cooking turns into a fire ball burning the house down, not a good thing, so like dryers maybe ovens have the same requirement as I have never seen one that could come back on, mater of fact, they don't even keep their memory as the clock and timers all default to flashing 12:00, mine even looses the bake cycle you had it in as all the settings are electronic settings, the temp setting also reverts to 0?F,
as far as cooktops go it would take a relay to drop out that would have to be reset like a start stop circuit other wise it would come back on.

We know microwaves won't come back on but most people would at least want the micro to be on the generator during a power failure so they can at least heat something up.

But really thinking about it, there are not many appliances that will stay off, I think most today will try to fire back up once the power is restored to it, and that kind of foils my thinking on this.

Making a simple load shedding circuit would not take much, for the A/C just a small 120 volt relay fed from the utility sense circuit that would drop out the A/C 24 volt control circuit would be enough, but with todays smart load shedding panels that would allow the A/C unit run when other loads were off that actually monitor the load on the generator would make trying to roll your own useless.

There are still basic appliances of all kinds with simple manual controls. Most of which will not open any switches when power fails, and will resume operation when power is restored. Ranges, ovens, cooktops, dishwashers, clothes washers, you maybe can still find microwaves with an electromechanical timer. Most of the ones that had such a timer would need the start button pressed before it would run, but the start button did not reset if the door was never opened. Turn the timer and it would start again without pressing the start button if the door hadn't been opened.

Not certain the reason for the momentary contact start on the dryers, but it does inherently add some safety features even if those features are not the main reason for the design.

 

[goldstar]

Not trying to be argumentative here, just trying to see if I'm understanding this correctly. If I'm sizing a generator for a house with 2 AC units, a pool with 2 motors and a 50 amp spa tub (all of which are wired into the load shedding system) I would have to size the unit to be able to run all these even if the load shedding module may not start them. So far so good. Now, the house also has a 30 amp electric dryer, a 30 amp welder outlet, a 40 amp double wall oven and a 40 amp electric range am I supposed to size the generator to accommodate the running of these units all at one time ? If that's the case the generator market will plummet. No one wants to buy a generator as it is and no one but Bill Gates will buy a generator that size.

 

[mkgrady]

Not trying to be argumentative here, just trying to see if I'm understanding this correctly. If I'm sizing a generator for a house with 2 AC units, a pool with 2 motors and a 50 amp spa tub (all of which are wired into the load shedding system) I would have to size the unit to be able to run all these even if the load shedding module may not start them. So far so good. Now, the house also has a 30 amp electric dryer, a 30 amp welder outlet, a 40 amp double wall oven and a 40 amp electric range am I supposed to size the generator to accommodate the running of these units all at one time ? If that's the case the generator market will plummet. No one wants to buy a generator as it is and no one but Bill Gates will buy a generator that size.

No you do not include these loads in the calculation. Nobody is telling you to. The loads that are connected to load shed modules and the AC that is controlled through the ATS won't turn on unless the generator can handle the load(s).

To size the generator and keep its size reasonable, you do a load calculation like you would a service but don't include the things that will be controlled by load shedding.

 

[goldstar]

No you do not include these loads in the calculation. Nobody is telling you to. The loads that are connected to load shed modules and the AC that is controlled through the ATS won't turn on unless the generator can handle the load(s).

To size the generator and keep its size reasonable, you do a load calculation like you would a service but don't include the things that will be controlled by load shedding.

If I do this am I in compliance with 702.4 ?

 

[ceb58]

If I do this am I in compliance with 702.4 ?

That's what we have been saying all along. Here is a link to a Kohler calculator that will give you a very good idea of what size generator that is needed and load calculation. http://kohlerpower.com/residential/solutions/sizingcalculatorwizard.htm#

 

[goldstar]

That's what we have been saying all along. Here is a link to a Kohler calculator that will give you a very good idea of what size generator that is needed and load calculation. http://kohlerpower.com/residential/solutions/sizingcalculatorwizard.htm#

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.

Where did I go wrong in this conversation ??? Why can't a HO manage his own loads (not the ones connected to the ATS load shedding module) by shutting off the breakers. What have I said that was wrong ?

 

[gadfly56]

Where did I go wrong in this conversation ??? Why can't a HO manage his own loads (not the ones connected to the ATS load shedding module) by shutting off the breakers. What have I said that was wrong ?

I think the issue is that you have two choices as I see it under 702.4

Full Load transfer

Load Management transfer

Under Load Management it says "Where a system is employed that will automatically manage the connected load..." The homeowner's finger is not an automatic load management system. :happysad:

 

[GoldDigger]

Where did I go wrong in this conversation ??? Why can't a HO manage his own loads (not the ones connected to the ATS load shedding module) by shutting off the breakers. What have I said that was wrong ?

As long as the generator can supply the calculated or measured full load (after subtracting automatically shed loads), then the HO is perfectly free to reduce the load on the generator even more by turning off any breakers he wants. Or by turning off the stove or water heater or microwave at its own switch, for that matter.
He just cannot be relied upon to meet the first condition when an automatic transfer switch is used. After all, he might not even be home.

 

[ceb58]

Where did I go wrong in this conversation ??? Why can't a HO manage his own loads (not the ones connected to the ATS load shedding module) by shutting off the breakers. What have I said that was wrong ?

Not being mean but trying to help you out, you didn't read the code.

As long as the generator can supply the calculated or measured full load (after subtracting automatically shed loads), then the HO is perfectly free to reduce the load on the generator even more by turning off any breakers he wants. Or by turning off the stove or water heater or microwave at its own switch, for that matter.
He just cannot be relied upon to meet the first condition when an automatic transfer switch is used. After all, he might not even be home.

Or they may not be able to. I have a customer that he and his wife are in their 90's. When I sized his generator I sized it 5Kw over the load calc. There is no way I want them stumbling around in the dark trying to find a tripped breaker. This is one of the main reasons the code was changed in the 08 cycle.

 

[hurk27]

Where did I go wrong in this conversation ??? Why can't a HO manage his own loads (not the ones connected to the ATS load shedding module) by shutting off the breakers. What have I said that was wrong ?

Because homeowners are not always home when power goes out, and like I said before if you have a generator that will automatically start, and a ATS that will automatically transfer, then the generator will be loaded with the full load of the house, the homeowner will have all the loads on if on normal power right?, do you think the homeowner can react fast enough when power goes out to get to the panel and turn off the loads that will over load the generator? what if the panel is in the basement and the homeowner is upstairs in his bedroom, thats two flights of stairs, or what if the home owner is out in the back yard working and doesn't even notice that the power went out.

Many of these newer generators start and transfer within just a few seconds, but it doesn't really matter, if you have more load connected to the generator that it can handle, it will burn up the alternator, sure some of the more expensive manufactures do have electronic overload detection built into their transfer switch, but your not going to sell that kind of generator to back up his power.

I don't know how to make it any plainer, then this, if the system is automatic it will automatically transfer the whole load that the homeowner would have on when running on utility, which in many cases is everything, if the generator is not sized for this amount of load it will be damaged, again the breaker on the generator is only sized for fault current, it is not there to prevent you from over loading the generator, that job is left up to us when we size the generator for the total demand load that the generator could see when the ATS transfers the load to the generator, this demand load is not like how we figure the size of a service for a house although you can use this method but most likely you will end up with a much larger generator than needed as the NEC builds in quit a bit of head room for safety, but you want to size a generator for the maximum demand load that the generator will see, yes it takes doing some homework but if you want to correctly install generators then its part of the job, I have seen a few try to use the utility bill to size a generator, no you can't do that either as that is an average of a months usage, but the demand load can far exceed that amount, also you must take into account of all motor loads that may try to start back up all at the same time, while motors when running on the utility will have a diversity factor as it would be very unlikely that more then just a few will try to start up at one time as they cycle to there own independent controls, but in the event of a power failure they can all try to start which can bring a generator to its knees, I get away with this by installing delay timers with different times so that no two motors will start up when the generator restores power, always set the largest motor to fire up first then the next size down and so on, this is because the larger running motors can act as a flywheel to help the other motors start.

I did a house that had a 5 ton A/C unit and a 3 ton A/C unit, the homeowner had already bought the 15kw generator and automatic transfer panel on EBay, well if it wasn't a good costumer and friend I would have walked, but using delay timers and current relays I was able to run almost his whole house on that generator, its been in operation for over 10 years without a problem the only loads we could not run was the built in double oven which was as old as the house and had mechanical controls that would allow it to come back on after a power outage, a hot tub and pool equipment, so we left these loads in the main service panel, I used the current relays between two freezers and two refrigerators so that if one was already on the current would keep a contact open and not let the other one start up until the first one finished its cycle, then the second one would run again its current would then keep the first one from running until it cycled off, but would these be a code compliant method of load shedding? I don't know, but the wiring was safe and it was installed in a code compliant manner and it kept the total demand load from exceeding the rating of the generator which was the main goal.

Just remember with a manual transfer switch a homeowner has time to select the loads he want or can run, with an automatic transfer switch he doesn't have the time to get the loads turned off and this can lead to damaging the generator which in most cases the manufacture will not repair under warranty look at any of the warranty disclaimers and most all will not cover the generator if it has been overloaded, so you could end up paying that bill to get it fixed, even with a manual TS if you do not spell out what loads the home owner can run and put it in writing and have him sign it so you have something to fall back on when the homeowner tries to push the generator a little more and don't think they won't they will but maybe not if they know if they damage the generator from over loading it then its their baby.

The story I posted earlier about the homeowner who thought he could get away with adding on his own the A/C unit to the ATS panel found out very fast that I would not be footing the bill, as I quickly showed him the paperwork he signed that included the instructions that installing any more loads to that ATS sub panel would result in a damaged generator and would void my warranty as well as the manufactures, so yes they will try to push the envelope if they think they can get away with it, it was a buddy of his who was an industrial electrician who happen to be at his house one the time when the electric went out on a hot day, that told him I was nuts that it would be no problem to add the A/C to the panel, well his buddy's miss information cost him quite a bit of money, I had even marked the panel with the words "do not add any loads to this panel or damage to the generator could result" which I also kept a photo of in the file on that house which is one reason I don't like doing manual transfer kits on a main panels that allow a homeowner to select which breakers they can turn on, because it requires allot more paper work and instruction to document that they know what loads they can run and what loads they can't.

 

[hurk27]

As long as the generator can supply the calculated or measured full load (after subtracting automatically shed loads), then the HO is perfectly free to reduce the load on the generator even more by turning off any breakers he wants. Or by turning off the stove or water heater or microwave at its own switch, for that matter.
He just cannot be relied upon to meet the first condition when an automatic transfer switch is used. After all, he might not even be home.

OK I guess you can ignore the book I wrote in my last post, as it took to long to write and I see you got the idea.

It's not the problem of depending on the homeowner to correctly not overload his generator, but the problem of a automatic TS that will transfer all the load that will normally be on because an ATS is a dumb switch without automatic load shedding, it will transfer into any load that is connected to it because it doesn't know any better and with todays generators with much shorter time from the time the power goes out to the time the transfer take place it would be impossible for the homeowner to get to the panel to gets loads switched off before damage to the generator could take place.

We ask why don't the manufactures of these generators protect them better, well I only have one word "GREED"

While I know that a generator has a large current difference between its rated output and the short term surge current it can handle sometimes almost 1.5 times it's rating which protection would have to account for, but many times the common rating of generators doesn't fall within a common rating of a breaker of fuses, special breakers or fuses would have to be obtained and this would just add to the cost of the generator, sure there are electronic overload protection that would also work but again it's all about cost and the competition between manufactures, we don't see these kinds of protection until we get into generators costing many thousands of dollars more then your typical home back up generators, even Generac has heavier windings and better electronic overload protection when you get into their larger commercial and industrial lines, believe it or not, many of the portable generators are better protected as I has seen people load them down to the point the engine stalls but never burn up the alternator.

And this won't change until customers start demanding generators that will last as long as most other items on a house, like what happened to roofing shingles which went from a 2 or 3 year warranty to over 50 years as being more common, of course your not going to see that kind of change, but even a generator with a 5 year warranty should be a norm even a 10 year, because engines that run on NG can last a long time but its not the engines we see failing, its the alternators and there is no reason for it.

To me if the alternator was better made, and had better overload protection then 702.4(B) (1) and (2) could go away as the home owner could just turn off a few breakers and try it again, or if they accidentally left the wrong breaker on and the OCPD's tripped, they could just correct it and not have to worry about burning up the alternator.

I remember the old WWII Onan with a Jeep engine that we had out in the Everglades that we used to back up our house out there, it was only rated 9kw, but my dad would overload that thing terribly and it would just keep going and going, it would get so hot, that you could fry and egg on it sometimes, I don't think it ever failed, but that flat head 4 cylinder Jeep engine would give us some problem from time to time and wasn't very efficient but then gas was only 40 cents a gallon back then, at his bar out there we had a single cylinder diesel with a large flywheel with a hand crank, we would put penny's under the exhaust valves and hand crank the flywheel up to speed, then using pliers pull the penny's out, I think it had a 12kw Onan on it, we had a 500 gallon tank and it would run a month straight on that tank of fuel, but having power out there was not very dependable as they had just strung lines across the glades on wooden poles and after a good Everglade fire we could be out for weeks or in one case it took Lee County Co-op a couple months to replace the poles and wire, so many times we not only fed our stuff from them but neighbors also and thats when they would get overloaded, I remember my Dad telling one neighbor if the lights start dimming then don't turn on that load as that was the limit he would use to stop putting anymore load on the generator, it was survival out there.

 

[GoldDigger]

many of the portable generators are better protected as I has seen people load them down to the point the engine stalls but never burn up the alternator.

One more advantage for the inverter type portable generators, since the inverter has very finely tuned over-current and over-temperature protection.
Or you might consider it a disadvantage because you cannot get more out when you really need it and don't mind cooking some eggs.

 

[goldstar]

That's what we have been saying all along. Here is a link to a Kohler calculator that will give you a very good idea of what size generator that is needed and load calculation. http://kohlerpower.com/residential/solutions/sizingcalculatorwizard.htm#

Thanks Ceb. Nice program but they make a lot of assumptions as to what your assumed total connected load will be (i.e. they always figure a 4 ton AC unit irrespective of what size house you have). Try it yourself and insert the same connected load information for each different size house. Every house above a 1500 sq ft Kohler recommends their 20 KW unit. Then try the 5000 + house using electric heat. Nobody is spending $15K+ on a 38 KW unit like this.

 

[goldstar]

Because homeowners are not always home when power goes out, and like I said before if you have a generator that will automatically start, and a ATS that will automatically transfer, then the generator will be loaded with the full load of the house, the homeowner will have all the loads on if on normal power right?, do you think the homeowner can react fast enough when power goes out to get to the panel and turn off the loads that will over load the generator? what if the panel is in the basement and the homeowner is upstairs in his bedroom, thats two flights of stairs, or what if the home owner is out in the back yard working and doesn't even notice that the power went out.

Thanks Hurk and understood. You've provided a lot of useful information that many of us, I'm sure, didn't know or weren't aware of. BTW, I think deep down inside you really enjoy typing.:lol: