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Questions about using a generator

Posted by swampwiz (My Page) on
Wed, Sep 9, 09 at 14:45

I want my home to be able to use a generator to supply enough power for:

- lights, TV, computer

- refrigerator

- A/C (4 tons) or range or on-demand hot water heater or dryer (these can be done at separate times)

I guess one of the questions is what is the best way to run a generator. I believe that most folks who run a generator simply plug it into an outlet, and it ends up feeding the main panel (how this could run the A/C which is 30A, 230V, I have no idea.) Or perhaps they do it some other way?

I guess the other question is how big of a generator to get for this desired load.


Follow-Up Postings:

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RE: Questions about using a generator

"- A/C (4 tons) or range or on-demand hot water heater or dryer (these can be done at separate times)"

Either one of these is a very large load for a generator.

Look at the nameplate on the condenser.
It should have a LRA (locked rotor amps) and is a measure of the starting current of the compressor, and keep in mind it is a 240 V load.


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RE: Questions about using a generator

There are several very good reasons why 'simply plugging it into an outlet' is NOT the best or safest way to connect your generator : 1) To do this you need a cord with a plug on each end, one for the socket on the generator, and one for the socket outlet inside. Such a cord is, appropriately enough, referred to as a SUICIDE cord because of the exposed, live pins at one end when the other end is connected to a supply; 2) The socket outlet that you plug the generator into, and the wiring between socket and breaker panel will not be rated for the amount of current that you will be feeding through it; 3) Because fallible humans will be connecting it up, sooner or later someone will forget to switch off the main switch to the house before firing up the generator. At best you end up trying to feed power to half the neighbourhood, probably tripping the generator protection. At worst, you electrocute the serviceman who has been sent out to reconnect your power.

The best, and only sane solution is to get a proper transfer switch fitted. This will have an inlet for a connector, so that the generator can be plugged in safely. It will have the correct wire sizes for the job, and will have an interlock to eliminate any possibility of back-feeding.


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RE: Questions about using a generator

IMO, you've got some serious sizing issues to consider.

If you can live with just a few lights lights, TV, computer, refrigerator and maybe a couple of intermittant small appliances, then a small portable generator should work just fine--perhaps something along the lines of 5000-6000W.

If you need to add central air, electric range, clothes dryer and on-demand hot water, that'll really up the ante. Any one of those will single-handedly eat up the full power output of a small portable unit. And most electric on-demand hot water heaters probably draw more juice than even a significantly larger portable genset will produce. (The on-demand electric heaters I've seen pull from 80 to 120 amps, or roughly 20,000W and up, but perhaps you have something much smaller in mind.)

Paulusgnome is absolutely right about the need for a proper transfer switch. Backfeeding a generator into an existing outlet is both dangerous and illegal!

If you opt for a small portable generator and can forego the large-draw items (think microwave instead of oven and clothesline in the garage instead of dryer, for instance), you should be able to put a handful of selected 15- and/or 20-amp circuits on a small manual transfer switch for a few hundred bucks.

If, however, you go with a large permanently installed backup generator, that's typically part of the overall installation and often involves a transfer switch that cuts over everything that's wired to your main panel.

The former can probably be accomplished for $1000, depending on generator quality. The latter can easily run well into five figures.


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RE: Questions about using a generator

Unless you are talking a gas "on demand water heater" forget it. You aren't going to power an electric on-demand heater with a generator.


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RE: Questions about using a generator

A properly sized standby set can run that on demand heater and the rest of the house just fine. Such an installation would probably exceed the budget "swampwiz" has in mind many times over, but it is certainly possible. See the link.

Now to address the OP's question- It is true that many people use generators in the fashion you mentioned. It is also true that such tactics are almost universally dangerous, illegal and more difficult to use than a properly installed transfer switch. The only correct way to connect a generator to house wiring is via a device approved by both your local utility and applicable inspection authority. 99 percent of the time that device will be a transfer switch. If you won't/can't use a transfer switch, you would be well advised to stick with a portable generator and extension cords completely independent of your house wiring.

Here is a link that might be useful: This will probably do


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RE: Questions about using a generator

Wayne440 I was wondering just what you were thinking about a properly sized standby set and then I followed your, this will probably do, link. I agree that would probably be enough to power the house.


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RE: Questions about using a generator

I admit it is overkill, unless the house in question has a 400A service and is being used as a restaurant or hotel. I just didn't have anything else handy. I forgot to mention earlier that many genset vendors will figure out set sizing for "free" or for a fee that can later be applied toward purchase.


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RE: Questions about using a generator

Thanks for the responses. I certainly don't want to do the wrong thing. I am wondering if I get some sort of variable speed HVAC system, that I could keep the load below a certain amount.

Like I said, other than the fridge, a few lights and a TV or a computer, I can run those other items individually when the HVAC is off. And I could stay away from the range and just eat toasted sandwiches and cold beer. :-)

I suppose that I could run the tank hot water heater as normal, or turn that breaker on only when the HVAC is off, and get my hot water that way.

Now as for transfer switch, where would that be? Would that be an add-on to an existing breaker, and I would simply plug in the generator into the circuit that would be feeding that switch? Could I run other items on that switch? How much current, and at what voltage would that be?


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RE: Questions about using a generator

"I am wondering if I get some sort of variable speed HVAC system, that I could keep the load below a certain amount."

Variable speed systems do not necessarily pull less power, depending on how they are implemented.

You will need a very large backup to run a 4 ton AC HVAC system and a whole house electric on demand water heater.

Large systems can be very expensive to install and maintain.
Many require auxiliary starting engines to bring the main set up to speed before starting the main engine to then drive the system.

Smaller 4 cylinder engines are often used for this task, driving the larger engine on its flywheel just like a conventional starter motor in smaller engines.

Even an unloaded alternator (what is really used in the gensets) is large physical load to bring up to speed, and the larger units do not have a clutch between the engine and the alternator (any slip would really cause problems).


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RE: Questions about using a generator

Here is one vendor's FAQ that might help.

Here is a link that might be useful: Gen tran FAQ


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RE: Questions about using a generator

If you are going to power a 4 ton central HVAC system, you are looking at a permanently installed home standby genset with a tranfer switch. Its an additional electrical panel that sits next to your main and has the breakers for the loads you want backed up. Then when power goes out, the transfer switches starts the generator and transferres only those loads to the gen set.

We have a Generac 15kw installed 4 years ago. It got us through 13days of no power after Ike last year and numerous smaller outages. It has no trouble with our two central ac units (one 3 and one 4 tons) along with the rest of the smaller house loads. Mostly powers th whole house except the hot tub, kitchen oven and the garage 220v outlets. Ours is natural gas powered.


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RE: Questions about using a generator

Gee, mine has no problem with the electric on demand heater. Of course it is a 80 KVA unit.


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RE: Questions about using a generator

Hi,

Central air conditioners require a big generator. I have a Cummins Onan RS 20,000 (20 kw) and I power only the smaller of the two central air compressors I have. With the things you would like to power I think you are looking at a $US 20,000 an up installation there.

I am not sure where you are in the USA. The most popular solution for generators is petrol fueled because of cost. This will mean a small lawnmower type appliance possibly in the 250 lbs in weight range. The best one I know of is a Honda EU6500 iSA. This will cost about $US 4,500. Then on top of that will be a transfer switch installation cost / wiring of about $US 1,500 - $US 2,000. So figure on about $US 6,000.

What does this get you. This will power up some lights, fridge, sump pump, well pump and a small A/C unit. It will not power up electrical heating, water heating. For the A/C a popular solution is to buy a window A/C unit of about say 12,000 btu. Then when the power goes out camp out in the one room with the small A.C.

Note you have to be able bodied to wheel out the generator, and move about gasoline.

I have posted some comments in the past on generators and price ranges. The cost of generator equipment has climed alot over the past 4 years or so, mainly I think because of the cost of copper and the general weakness of the US dollar.

Warmest regards, Mike.


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RE: Questions about using a generator

"The most popular solution for generators is petrol fueled because of cost."

A very poor choice for larger units, or if you may need operation for more than a few hours.

Gasoline goes bad, even with stabilizers added.

As has occurred along the gulf coast, once power is out the gas stations cannot even pump unless they have a back power source.

If you are only trying to cover an occasional short outage, gasoline might be effective, but do not fail to replace the stored gasoline (run the old stuff through a car) or you may have no usable fuel when you need it.


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RE: Questions about using a generator

As mentioned gasoline has several disadvantages as fuel for stationary standby sets. The big one is that your gasoline will go stale or end up in some thief's car via a siphon hose at some very unhandy time. The advantage is that if you have your own hose, fuel can often be "reallocated" in similar fashion for your generator.
Gasoline sets are also thirsty, even at light load.

Diesel is a good choice if you need a larger set or have the $$$ for a small one that may cost twice as much as its "gas" counterpart. The same things apply to fuel, but diesel lasts longer, and is usually less desirable to hoodlums. Diesels (generally) use less fuel at light loads than gas or gasoline sets.

"Gas" (meaning natural gas or propane) sets are popular for light comercial and home standby. They are great as long as gas flows from the line or tank, then useless. Note that you cannot take a truck and several cans to pick up 50 gallons of propane. A siphon isn't of much use either.

A prolonged outage earlier this year convinced me that diesel is the right fuel in my case. I can run my house on about 1 gal/hr.


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RE: Questions about using a generator

Hi,

What Mr. Brickeyee and My Wayne says is quite correct. Gasoline / petrol does degrade.

What operating a small petrol genset entails goes something like this. You have about 20 gallons of gasoline stored in 5 gallon containers. Note 20 gallons is about the maximum you can store in home / shed property due to fire risk and local building code regulations. That 20 gallons will be stabilized with some sort of fuel stablizer. e.g. Stabil from Home Depot. It means your petrol will last about a year in storage.

20 gallons of petrol will give you roughly 24 hours run time going pretty much flat out for a 6500 watt gen set. After which you will need to look around for more petrol. Note that you probablly will not need to run your genset 24 /7, but maybe just say 4 hours a day. Cool down the fridge. Take a shower, have some air con, then shutdown for a bit.

Propane will store almost indefinately and is a better fuel choice. It is not such a popular solution due to cost.

Best, Mike.


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RE: Questions about using a generator

The best solution is natural gas if its available. Its very rare that natural gas supplies are interupted at the same time as electric power. There is no fuel degrade issue, the engine runs VERY clean and the fuel is FAR cheaper than gasoline.

If you are installing a standby genset to cover air conditioning load, either natural gas or a very large propane tank are your only real options.


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RE: Questions about using a generator

"Its very rare that natural gas supplies are interupted at the same time as electric power."

Natural gas gets shut down in many cases of large area disasters that damage structures and flooding since it becomes a fire fuel from damaged buildings and equipment releasing it.

It is rarely done in a zoned manner, but the gas flow is simply shut off.


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RE: Questions about using a generator

jcthorne- I respectfully disagree. My diesel sets run the HVAC (and everything else) as well as gaseous fueled sets would. Hourly fuel cost is about half that of propane.


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RE: Questions about using a generator

"Hourly fuel cost is about half that of propane."

When natural gas is available it is far cheaper than propane.


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RE: Questions about using a generator

When making fuel decisions, one has to consider his/her own situation. That is why I avoided making a blanket statement like "diesels are ideal standby sets". If I had natural gas available, I might buy a gas set, but keep the diesel as well. As you mentioned earlier, gas suppliers do shut it off at unhandy times.


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RE: Questions about using a generator

A 250 gallon propane tank full and only hooked to the generator would be a large expense to begin with, however it will sit there for years and still be as good as the day you bought it and you have no worries about it not being available when needed.


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RE: Questions about using a generator

A typical 15kw set uses about 3 gph of propane at full load and 1.5 gph at 1/4 load. Most suppliers fill to 80% so you have 200 usable gallons if it is "full". That is better than a couple of gasoline cans in the garage.


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RE: Questions about using a generator

"Its very rare that natural gas supplies are interupted at the same time as electric power."

Apparently you have never been through a ground zero experience of a tornado or hurricane.


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RE: Questions about using a generator

"Apparently you have never been through a ground zero experience of a tornado or hurricane."

If I were to go thru a ground zero hit from a tornado or hurricane or earthquake the last thing I'd worry about is the NG or generator working! I've lived here since 1969 thru ice storms and floods and the NG has never gone out- maybe just lucky. Hope I don't jinx myself.


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RE: Questions about using a generator

Ah, another thread on religion.

My vote for the best and most rational conclusion?

"When making fuel decisions, one has to consider his/her own situation."

Exactly. If you're on a farm and you have a 1,000 gal tank of diesel fuel for your heavy equipment, diesel is a no brainer. If you have a townhouse with natural gas and you're petrified at the prospect that all 2400 of your tropical fish would die if there was a five-day ice storm, then get a natural gas standby genset.

If you're me (translate: cheap as hell), you get a cheap gasoline-powered genset, two 5-gal jerry cans, a transfer switch (all for $1000) and try to keep both your cars filled up to the brim with gas when bad weather threatens.

Footnote: I can live without air conditioning for a couple of days every three or four years. Honestly, I can.


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RE: Questions about using a generator

250 gal of propane is less than 7 days supply. I have been through 2 direct hits via hurricane. The gas supply was not cut in either case. One time there was even a documneted leak on the main feeding the entire subdivision. Has was left in service until power was restored. An orange cone with a no smoking sign was posted. I did not say it was impossible. Just a very rare occurance to have both failures at the same time and indeed gas supply up time is far higher than electric.

Both hurricanes left power off for much longer than 7 days. Diesel does not store well and is much more expensive to use than natural gas.

I have seen a propane backup for a NG genset installed but that is truely belt and suspenders thinking.


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