Whole house generator and panel question

[hoosierdoc]

I had a company out today to quote a whole-house generator option. We have two 200a panels and currently have a transfer switch to allow selective circuits to be powered by a 30A feed from a generator. He was saying a 36kW generator for 150A or so would do both panels, or a 20kW at 83A would do one and we can choose which circuits to put on the panel backed by the generator. With that we would not be able to power the ovens, drier, hottub (obviously), or one of our two AC units. I asked about having the ability to selectively power circuits so we could run downstairs AC during the day and upstairs AC at night, or turn off AC to use the oven. He said they would not wire it in that manner as it is a setup to harm the generator from drawing too much power. He said there are some contractors that will do it but it's not a good idea. The good news is they can connect it to our disconnect at the meter outside rather than needing direct access to our panel for the generator wire.

I'm looking for advice on how to proceed. I get what he's saying on the potential to overload the generator but at the same time it seems like an easy thing to remember, especially if the circuit is off for the oven and has to be manually switched on before using it.

 

[Butch627]

I have a genrac with a load shedding transfer switch that prioritizes predetermined circuits. It would seem that using that same technology with a timer might get you where you want to be.

 

[Tactically Fat]

Is it worth the step-up in price to go from 20kw to 36kw?

A quick glance at just one web retailer and the 36kw was more than twice the price of the 20kw.

Would something mid-way between the two even be a viable option? Like keeping the upstairs A/C off the backup all together - yet let it run everything else? Kind of a best-of-both-worlds thing?

 

[GodFearinGunTotin]

The problem is you are installing an automatic system and wanting to control it manually. Another way to state what your contractor is telling you is is at the moment of bringing your generator on line, you cannot guarantee that all those other loads won't be there as well. If you happen to catch it while both AC units are going, the oven(s) are cooking July 4th dinner, the freezer(s) are in their cooling cycle, and your water heater(s) are catching up after everybody's taken a shower, the smaller generator will not be able to handle it. And let's face it, if everything is automatic, when the lights go out, you'll likely not be able to get to the breaker panel and get all of those other loads switched off by the time the generator tries to go online.

Now there may be some sort of apparatus that will automatically shed loads, like Butch mentioned up above. I am not familiar with them but that might be an option.

 

[remauto1187]

I would go with mid sized generator instead of shelling out the bucks for the 36kw genset and not to mention all the fuel in the future. You need a smart transfer switch(Generac calls it digital power management). Here is a link to one that will shed loads by prioritization and only allow enough loads to be powered at any given time dependent on size of genset.
http://gens.lccdn.com/GeneracCorpor...switches/0198240SBY-Complete_Smart-Switch.pdf

Here is what the DPM(Digital Power Management) equipment looks like. http://gens.lccdn.com/GeneracCorpor...SBY-B-Complete_Smart-Switch_Power_Man_Mod.pdf

Essentially the transfer switch makes decision based on total loads at any given moment and allows or denies loads to be powered based on those decisions. For it to allow, the transfer switch will enable a power contactor that is nothing more than a switch in line to the loads (Typically big ones like A/C, Electric Water Heater, etc.). For the transfer switch to deny a load power, it just removes the power contactor 24vac coil signal which opens the "switch" to that individual load so it can NOT load down the genset. All priorities are programmed in the transfer switch. Me personally, if Generac is using actual power contactors (relays made for high current) then I would replace those with SSR's(Solid State Relays) which have NO moving parts and failure rate is much lower than a power contactor.

 

[The Keymaster]

I would go smaller, and select the circuits to back up. They can relocate circuits from one panel to another quite easily, so all the circuits could be in one panel. The bottom line...this is emergency power. Prioritize the circuits you will need to supply power to the necessary devices. If you are going to include an AC unit, select the downstairs one. In a power down emergency that will likely be the floor where you will spend most of your time. It looks like he is basing his calculations for current on 240 volts, your 120 volt current availability will be more like 166 amps peak.

I am running a 20K to power my 5000 square foot home. We installed load shedding in order to power both AC units. We do have an advantage, as our dryer, and range/oven and water heater are all gas.

 

[GodFearinGunTotin]

I think I would be disappointed in a contractor, that is in the business of installing backup generators, if he couldn't tell me about these for my consideration. In an emergency/temporary basis, I would gladly do without the benefit and conveniences of "unlimited" power to save a considerable sum of money for such a huge generator.

I would go with mid sized generator instead of shelling out the bucks for the 36kw genset and not to mention all the fuel in the future. You need a smart transfer switch(Generac calls it digital power management). Here is a link to one that will shed loads by prioritization and only allow enough loads to be powered at any given time dependent on size of genset.
http://gens.lccdn.com/GeneracCorpor...switches/0198240SBY-Complete_Smart-Switch.pdf

Here is what the DPM(Digital Power Management) equipment looks like. http://gens.lccdn.com/GeneracCorpor...SBY-B-Complete_Smart-Switch_Power_Man_Mod.pdf

Essentially the transfer switch makes decision based on total loads at any given moment and allows or denies loads to be powered based on those decisions. For it to allow, the transfer switch will enable a power contactor that is nothing more than a switch in line to the loads (Typically big ones like A/C, Electric Water Heater, etc.). For the transfer switch to deny a load power, it just removes the power contactor 24vac coil signal which opens the "switch" to that individual load so it can NOT load down the genset. All priorities are programmed in the transfer switch. Me personally, if Generac is using actual power contactors (relays made for high current) then I would replace those with SSR's(Solid State Relays) which have NO moving parts and failure rate is much lower than a power contactor.

 

[remauto1187]

I think I would be disappointed in a contractor, that is in the business of installing backup generators, if he couldn't tell me about these for my consideration. In an emergency/temporary basis, I would gladly do without the benefit and conveniences of "unlimited" power to save a considerable sum of money for such a huge generator.

Absolutely. Obviously the contractor will make more money off the "huge generator" vs. accessory kits for load shedding....Hmmm wonder what the contractors motivation is? Id toss him........next!

 

[hoosierdoc]

He said he wasn't in favor of a 36kW for our application because the last one he did was $27k installed. I was a bit surprised he didn't mention the load shedding device when I mentioned several times wanting the ability to select which circuits to run. Thanks for that advice.

 

[CountryBoy19]

He said they would not wire it in that manner as it is a setup to harm the generator from drawing too much power. He said there are some contractors that will do it but it's not a good idea.

Sounds to me that he is trying to upsell you because he knows how crappy that would be and he is banking on you paying for the extra generator.

FWIW, I doubt there is a single reasonably sized family home in this state that REQUIRES 36 kw... 20 kw will be plenty to power your entire house as long as you don't run all the big loads at once. I say find another guy to hook it up, the generator has it's own built-in circuit breaker to prevent damage from occurring, if you overload it by pulling that much power the generator breaker will trip...

 

[hoosierdoc]

the generator has it's own built-in circuit breaker to prevent damage from occurring, if you overload it by pulling that much power the generator breaker will trip...

I mentioned smething along the lines of "I'm surprised the generator wouldn't have circuitry in it to prevent an over draw". He said it's not good on the generator to go to max load and then abruptly halt the engine.

 

[The Keymaster]

I mentioned smething along the lines of "I'm surprised the generator wouldn't have circuitry in it to prevent an over draw". He said it's not good on the generator to go to max load and then abruptly halt the engine.

I do not believe it shuts down the engine. The breaker trips like any other breaker, and has to be reset manually just like any other breaker. Once the load disappears because of the tripped breaker, the engine will automatically slow down because Generac has load sensing technology that regulates engine speed based on load. Eventually the generator will recognize a no load condition and shut down.

 

[Tactically Fat]

I mentioned smething along the lines of "I'm surprised the generator wouldn't have circuitry in it to prevent an over draw". He said it's not good on the generator to go to max load and then abruptly halt the engine.

Sounds to me like you need to look into another installer...or three.

 

[CountryBoy19]

Sounds to me like you need to look into another installer...or three.

I agree, this installer sounds like he doesn't even know the product he is selling...

FWIW, yes, it can potentially be hard on an engine that is fully loaded and then instantly dropped to zero load. It can actually over-speed for a split second until the governor/throttle has a chance to react. But I don't see that being a major issue. A split second over-speed that is within reason is completely normal for an engine and will not harm it. The mass of spinning copper inside the generator windings helps to add a sort of "buffer" to the system that will damp out the over-speed and keep it reasonable.

If it was such a problem for this to occur with generators the industry would quickly adapt and include some other form of load interrupter that will not cause a momentary over-speed. Seeing as how generators have functioned like this for decades I think you're in the clear.

 

[Lectric102002]

I had a company out today to quote a whole-house generator option. We have two 200a panels and currently have a transfer switch to allow selective circuits to be powered by a 30A feed from a generator. He was saying a 36kW generator for 150A or so would do both panels, or a 20kW at 83A would do one and we can choose which circuits to put on the panel backed by the generator. With that we would not be able to power the ovens, drier, hottub (obviously), or one of our two AC units. I asked about having the ability to selectively power circuits so we could run downstairs AC during the day and upstairs AC at night, or turn off AC to use the oven. He said they would not wire it in that manner as it is a setup to harm the generator from drawing too much power. He said there are some contractors that will do it but it's not a good idea. The good news is they can connect it to our disconnect at the meter outside rather than needing direct access to our panel for the generator wire.

I'm looking for advice on how to proceed. I get what he's saying on the potential to overload the generator but at the same time it seems like an easy thing to remember, especially if the circuit is off for the oven and has to be manually switched on before using it.

PM sent

 

[remauto1187]

I agree, this installer sounds like he doesn't even know the product he is selling...

FWIW, yes, it can potentially be hard on an engine that is fully loaded and then instantly dropped to zero load. It can actually over-speed for a split second until the governor/throttle has a chance to react. But I don't see that being a major issue. A split second over-speed that is within reason is completely normal for an engine and will not harm it. The mass of spinning copper inside the generator windings helps to add a sort of "buffer" to the system that will damp out the over-speed and keep it reasonable.

If it was such a problem for this to occur with generators the industry would quickly adapt and include some other form of load interrupter that will not cause a momentary over-speed. Seeing as how generators have functioned like this for decades I think you're in the clear.

It is true the circuit breaker trips when the load demand exceeds the generators capable output. This breaker is an OUTPUT breaker. The generator does not shutdown when this breaker is tripped (load exceeds genset output). It is true that very old generators will slightly raise speed during an abrupt shed of ALL loads but most generators have an electronic governor (instead of the former mechanical kind) and the sudden decrease in loads is barely noticed by the generator as far as engine speed regulation. I would hardly call it an "overspeed". All generators have an overspeed switch built in to prevent an overspeed condition. I maintain a (50kw) and (100kw) Kohler genset and they get a sudden decrease of loads in the 20-40a range any given time when the engine(s) are running. (208vac 3 Phase) They instantaneously decrease engine speed within microseconds. You just dont see the sloppy regulation of the old mechanical systems with the newer electronic regulated gensets.


I would seek 2 or more contractors for estimates on a package to fit the OP's needs. Heck, call generac themselves and i am sure they can turn you on to a proven contractor (by generacs standards) or any other brand like kohler, etc.

 

[scott delaney]

I would suggest a power load survey be done. it might be that under full load 2 a/c, lights,ref., oven might be in the 80 amp range.
a/c 17 amps*2
oven 14 amps
ref. 4 amps
this is average amps add up to 54 amps leaving 30 amps or so for everything else.
just because you have 2 200 amp panels does not mean you use full capacity ever. something to think about

 

[KLB]

You have two 200amp feeds or two panels?

 

[Brian Ski]

Just curious what kind of fuel the 36kw is running??? I set up a natural gas generator and the rule of thumb is 10,000 btus per hour per HP. My generator is 6kw (8kw max) and is 11 Hp. 36kw is going to need about a 70 hp engine. 700,000 btus of NG or about 6 gallons of diesel an hour. Buying power from the pole is a pretty good price. It may cost 10 times that or more to generate your own. Natural gas is best for the cost. At that rate you are going to need a Big gas meter, new lines to the house. But NG would be 1/3 the cost of other fuels.

Now my numbers may be off, depends on load and efficiency etc... Just tossing some numbers out to think about.

 

[femurphy77]

I would suggest a power load survey be done. it might be that under full load 2 a/c, lights,ref., oven might be in the 80 amp range.
a/c 17 amps*2
oven 14 amps
ref. 4 amps
this is average amps add up to 54 amps leaving 30 amps or so for everything else.
just because you have 2 200 amp panels does not mean you use full capacity ever. something to think about

Don't forget in-rush calculations!