Generator sizing

SIERRADMAX said:

I plan on installing a generator at my home this summer. My uncle is a retired electrician that keeps his license by doing side jobs. He said he will help me wire it up but I should start determining how many circuits to run off generator power & this will determine the size of the unit.

My house is already wired for a generator but the generator panel is insufficient for my needs and I believe the current size wire (#8) is sized too small for my future demands which are as follows:

Well pump (240v)
Oil Boiler (120v)
Wood boiler (120v)
First floor AHU (120v)
2nd floor AHU (120v)
Kitchen (120v) includes refrigerator #1
Refrigerator #2 in basement (120v)

maybe master bedroom circuit (outlets/plugs)

Odds are in the event of lost power, I won't have all the items listed above operating. By using the manufacturers (generators) calculators available online, I would need roughly an 11kw generator.

Would metering the above circuitry at the panel get an accurate amperage reading at the "worst case scenario" to make an assumption on a generator size? It would be nice to be able to run the above on a 6500-8000 watt automatic generator.. Maybe diesel or LP.

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I assume you do not have NG available?

I think there are smart transfer switches that intellegently shed load when necessary.
I was really amazed at the amount of juice that my deep well pump needed.

What about an interlock switch for the main panel. Have all breakers switched off and then alternate what goes on, e.g. run the ref and the wood boiler for a few hours, switch them off, and then switch on the well pump and some other things for a few hours, keeping the load under the gen capacity.

SIERRADMAX said:

I guess my question is, let's hypothetically paint a scenario where the well, oil boiler, both AHU's & both refrigerators kick on at the same time drawing the most amperage. This is probably an extremely rare occurrence. Let's say I take readings from the panel and the highest calculated wattage combined (during an event described) equals 10,000 watts. If I have a 5500 watt generator, what happens?

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you probably trip a breaker or it stalls it out shutting the whole operation down.

If you're looking for total everything on, give yourself +25% headroom for startup surges too. Fridges, well pumps, microwaves, etc all have very large initial draw spikes.

If you want to allow for all those things potentially being on, you need that much plus headroom. Typically in a small-ish (i.e. under 10KW) setup you'd expect to run only some breakers on at a time. eg, well pump and hot water to take a shower. Turn those off to run the fridge/freezer and other circuits for a while.

There's some good load estimates and other info here http://members.rennlist.org/warren/generator.html

The trade offs are things like fuel consumption v/s # of circuits on, how much fuel you can store safely, how much convenience you want for auto transfer, load shedding, or can you put up with turning on and off circuits etc.

Remember, this is supposed to be for emergency use, not every day normal existence.

i do not like auto transfer switches. My house would not freeze if the electric was off all day, so i can switch the genset on when i get home. What i do not want, is the genset firing up and using valuble propane or diesel for a 2-3 hr outage, when nothing bad would happen anyway in that time. The freezers and fridge would be ok, cause nobody would be opening them.

Anyway, I use a 10,000 watt gen/welder and have used it before. It ran propane furnace, fridge, lights,tv, and my 3/4 hp well pump. I used to do that on a 6500 genset,,but would turn breaker on for the well pump when needed only, and make sure nothing else was running. I have a 25,000 propane fired genset that I will sell this year or next cause it is too big for what I need.

The 10KW gen/welder is great because it gets used for welding thru the year and is always ready to go.

Your electrician uncle should know everything needed?

How often do you have outages and how long do they last?

To run all of that continuously and automatically you are spending a tremendous amount of mony for somethign that might get rarely used.

• can you turn off the oil and just use the wood boiler for heat in an outage? Or even just the oakwood to not need electric at all?
• For the well pump - can you add some storage so that you only need to run it periodically, and can time turnign off other loads?
• Things like refrigerators only need to run a couple hours a day to stay cold if you are smart about not opening the door unecessarily.
• Similarly there are probably other appliance you can reduce load by just sequencing and not using them all at once. Start up loads are also not a huge issue if you you can plan to only start up one

I bet you can bring that size down quite a lot. A smaller generator running under more load will go farther on the same fuel than a big one that's idling half the time.

6kw might not be enough for that well. 5kw wasn't enough for me, I needed a 7.5kw steady state unit.

Not sure how much your AHU's draw, but FWIW I have a 7550/11000 watt gas generator and I've run everything in the house, 3/4 hp well, fridges, freezer, 8,000 btu air conditioner. Pretty much everything but the dryer or the oven/range.
Of couse I manually stagger bringing the loads online, and I gather you want an automatic system. The genset does labour a little when the pump kicks in but comes right back. I ran that way for 6 days during Irene and 5 days during Sandy. Fired it up in the morning for a few hours then again in the evening.
I think most people go way overboard for the amount time the power is really out. A manual system takes little more work but is a heck of a lot cheaper.

During Irene, I was out from early Sunday morning untill about 0300 Fri morning.

Briggs & Stratton 5500 / 8250 surge "storm responder" model. I was able to start the genny, wait a minute or 2 for it to warm up, then flip the genny breaker and re-power my whole house, well pump, oil boiler, fridge, waterbed heater, couple of lights, etc. No issues at all.

After re-energizing my place, wait 30 - 60 seconds, then plug in the 250 foot cord running over to my neighbor's place feeding his fridge & freezer. No issues.

I burnt < 20 gal of fuel from sunday afternoon untill fri at 0300, that was running the genny as needed for showers, etc and at least 2 long "maintenance runs" to ensure the fridges & freezers cooled down properly.

Best damn 800 bucks I ever spent.

My electric use monitoring of the past couple weeks has led me to the conclusion that a 3000w inverter genny is best for us and should run at half load about 90% of the time. How often and for how long do you lose power?

Running a big generator for the purpose of handling very large start-up loads while base load is low is very expensive, both to purchase the generator and related hardware and fuel cost. I have a 5500 watt generator which plugs in outside to a manual transfer switch, and the generator has plenty of power to cover emergency + convenience power during long outages. One 240v circuit for the well pump, and then just most light circuits (now LEDs, very low draw), refrig, freezer, microwave, and outlets to power the TV, computer and sound system. Short outages, maybe 2-4 hours, maybe once per year; major outages of a day or longer are very uncommon and normally would involve a major windstorm or tornado wreaking major destruction and trees down. That has happened only once in 23 years. No need to cover the electric stove, dishwasher, dryer, dhw, or AC. We easily can get by without those.

A major consideration is having enough fuel on hand to cover a major emergency, as roads may be blocked and fuel replenishment not possible. Therefore, we keep about 30 gallons of gasoline on hand in 5 gallon containers, which we rotate constantly and use to fill the gas tanks of our two cars. Living in a rural area this also means we don't need to drive 15-30 miles round trip just to fill the gas tank, and we can buy gas as needed at the lowest available local rate, both money savers for us.

I regard the generator as an insurance policy that I hope I never need to collect on.

Water isn't even an issue or concern here until we get into maybe day 3 of an outage. The pressure tank & system hold maybe 5 gallons (conservatively). We have 3 toilets. They can each flush once using the water in their tank - I turn their feed lines off under the tank before flushing. I have a 25 gallon plastic barrel under our HRV that catches the condensation drain from it year round. Couple buckets out of that & the toilet tanks are filled again. We keep a big pitcher of water in the fridge for drinking - the water in the tank/system goes for drinking or a quick hand washing or teeth brushing. If it's dark when the power goes out, I grab the booster box I keep charged in the basement & set it up in the living room. It has a small built in light & built in radio. The wood boiler keeps us warm with no power - although do have to keep an eye on it when burning. BBQ is out on the deck with a spare tank beside it. The only critical thing is fridge & freezer, I will wait out a few hours before I get the genny going for those - then when the genny does get out other things will get fired back up like computers routers & TV. I think we've had more 'quality family time' when the power does go out - even if it's semi-forced. That might be a sad commentary.

If our situation was different and required, say, my being away from home overnight frequently during the winter - I would certainly be doing more than I do now & be more heavily invested in something more standby & automated.

maple1 said:

. I think we've had more 'quality family time' when the power does go out - even if it's semi-forced. That might be a sad commentary.
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Yup.

I actually got to meet some neighbors in the past few storms... the usual shut-ins who you never see even came out. I enjoyed being disconnected for a few days. I was roasting a chicken in my fire pit, making backed potatoes in the coals... I had fun.

I was bored because I was living alone at the time though, so having a family unit around would have been nice and I could definitely see the quality family time being something to cherish.

I bought my gen a year before Irene. An ugly storm was predicted, but it blew out to sea south of us.

When Irene hit, the gen was still in brand new. I didn't even put fuel or oil in it untill 12 hrs after the power was off.

Fearing a bad storm, I had 17.5 gal in 3 cans + 25 gal in the 2 snowmobile tanks + 40 gal in the boat tank.

When Sandy hit, we never lost power, but had 40 gal in a poly 55 gal drum.