Generator sizing help


Old 09-17-09, 05:55 AM
Thread Starter
Join Date: Feb 2009
Location: New Hampshire
Posts: 10
Question Generator sizing help


I've had 2 different companies come to my house - both to remane anonymous for now, for sizing and installing a standby generator for my home. Last winter was brutal and I don't want to go through that again, so I decided to get a standby generator. Both quotes are got were so far off from each toher I don't know what I need vs what the tech is trying to sell me. I've searched and searched for a generator calculator and even those that I find have different recommendatios. So I figured I would try here.

I'm no longer looking at a standby, going to install a transfer switch and get a portable for this year and see how that goes and prepare to upgrade next year to a standby. (whne I have more $ for it).

This is what I NEED to run:
  • Well pump - 1/2 HP I belive the motor is submerresed in the well.
  • Fridge - 22 cu ft I think, no clue on motor size.
  • Harmon Pellet insert - being installed in 2 weeks (Harmon tech highly advised against their own battery backup)
  • Oil buring furnace with force hot water - can't find anythign on a furnace except for the fan motor, don't have's force hot water. (Furnace to possibly be replace with the Harmon HydroFlex 60 later next month if I have the $) - bought insert instead to help with heat.
  • Microwave - we have a baby that needs warm bottles

Nice to include:
  • 1 TV and cable box (underground utilities so cable usual stays running)
  • Washer
  • Electric Dryer - to be swapped out for gas when we get the standby Generator so both run off that fuel.
  • No range needed - we have a grill
  • No ACs needed
  • No lights needed - battery powered lanterns will be used.

I was looking at the Champion 3500 portable, but then read somewhere just the well pump alone will take that?

Some advise would be greatly appreciated.
Sponsored Links
Old 09-17-09, 06:15 AM
bob1908's Avatar
Join Date: Apr 2009
Location: land of lincoln
Posts: 129
Smile generator sizing

i lived in north carolina we had a bad ice storm, the ecetricty was out for a week i went to sears and got a 5500 watt generator it was last one. i was very happy with it. i went got some 4 gage wire and used a breaker in my out building witch it was wired to the house and when the power went out i would turn off the main breaker from the eletric co. and i had lights & water (i was on a well too)hot water also, my house was all elettric i could not run the furance it was 8000 watts i got a gas heater to warm the place up.

if i had the advance notice that i was buing a gen i would have got an gen with eletric start, i converted this gen to an eletric start.

do you have eletric water heater

Last edited by bob1908; 09-17-09 at 06:24 AM. Reason: adding more info
Old 09-17-09, 06:36 AM
Join Date: Feb 2008
Location: usa
Posts: 755
Old 09-17-09, 07:20 AM
Join Date: Jun 2009
Location: Virginia, USA
Posts: 45
keep in mind

when running a gen off nat gas, you'll have 20% less hp from that eng than from gasoline, which translates into 20% less current output. 2nd problem with nat gas, it's going to need major gas supply to feed it - normal house has 175k btu gas meter (what you are used to seeing at most houses) with a 3/4" piping coming into house - when i had my 13KW gen sized for nat gas, plumber told me i'd need a 1,000,000 btu meter, which the city would change to for no charge, but he wanted to run a 1.5" copper line to gen - that meter is the size of a V8 car engine - would have given the house an industrial look i wasn't ready for.

propane decreases engine's power by 10% from what it is with gasoline - fwiw

i'd get a watt meter, fairly inexpensive, i've seen them on ebay for as little as $20 - you plug whatever you plan to run into the watt or current meter and then plug it into the wall socket - turn appliance on and see what current (how many watts) it's actually drawing. do this for all the appliances you plan to run. on a lot of the items, like furnance, you should be able to find the mfgr's plate and it'll show current draw - ie, on the oil blower mtr, it might state 120V, 60 hz, 1.2A or 1.2 amps - if my math is correct 1.2amps about 140-150 watts (100 watt light bulb = .9 amps). My furnance is forced air, so the blower motor in the air handler is 1180 watts, but the little blower that moves combustion air thru vent pipe (it's a high efficient gas) is another w amps or 240 watts). my hot water heater is nat gas, but it has the same kind of blower motor moving combustion gas thru the vent pipe, and again it's 240 watts.

2nd, on items like the refrigerator with a motor, there will be a huge surge load when that motor first starts up - example, my refrig (supposedly an energy star high efficient one) running wattage is 880-900 watts, but surge at start up is 1350 watts. The mfgr's plates i dscribed above, will always show running current, not surge current

allow for items like that, and oversize your gen by 20% for some margin and durability. Don't even dream of powering a 240V electric dryer or central A/C without a major capacity gen

plan on starting that gen, when not in use, every 4-6 weeks and running for 15-25 minutes just to boil out any moisture that collects in the oil from humidity, and to keep the engine's internals coated with some oil - or plan to "pickle" the engine (remove plugs, squirt in some oil into cylinders, drain carb etc

even planning to run every month or so, i'd use some fuel stabilizer in the gas as gas will go bad - 12 months seems to be the accepted life for gasoline but i think it's more dependent on how much air is in the tank when stored - ie, full tank will last longer.

be careful and google the gen you plan to buy looking for chat forum threads on the model you select

i saw a coleman powermax or blackmax at costco last year, $950 for 11 hp honda eng, 6500 continous watts / 8750 surge watts, elec start, huge 10 gal tank, price included battery & wheels - heckuva deal

went online, found some chat strings, one with about 11 posters, complaining that the gen head on this model on their unit had thrown the windings between 2 and 12 hours of use. Then they found out coleman had gone bankrupt, and that pramac (who had bought coleman at bankruptcy) was still making the same generators (painted different color) and had the parts including same gen head available - pramac wanted $600 for replacement gen head, same mfgr (chinese) and would not offer any warranty credit on the bad one.

i'd stay with a well known name brand (honda, makita, porter cable, milwaukee, northern star (northern star btw still builds em here in the US and i'm real happy with my northern star and it's been abused, unintentionally, but abused nonetheless).

i could live comfortably in winter with 3500 watts of power, but to be on the safe side I've got a 6000w makita for backup in winter, my first gen, for summer use when i need central air, is a 13kw gen.

one, whatever gen, get one with as large a fuel tank as possible - 3-5 gallons doesn't last long and it gets old going out to fill the tank.

as far as backfeeding the house panel thru a breaker in your shed, i'd strongly suggest, for your generator's safety as well as lineman working on the power lines in the area, some sort of lock out - you can get transfer switches, they're in the $300 - 500 range or more, depending on generator output

i went with an generator lockout kit (Generator InterLock Kit) - basically a mechanical device that a) you would dedicate a dbl breaker to your generator, but it prevents you from throwing current into the panel box unless you've first thrown the main breaker to "off" position.

if you back feed into the power grid you could hurt a lineman and/if power comes back on, you could melt your generator down from current from power grid trying to feed your gen

i built my own as at the time, the interlock folks didn't have one for my panel box and wanted too much for a custom one

here's a shot of the one i made - trust me, accidents happen, and you do want some sort of lock out, either a transfer switch or interlock kit. There's also some sort of plate your electrician can install between your house meter and the house incoming line that you simply plug your gen into it and it automattically locks out the grid - but don't know a lot about them

the red plate is simply a reminder for my brother-in-law, in case i'm on the road and he needs to turn on gen for my wife
hope that helps

Last edited by larryccf; 09-17-09 at 07:48 AM.
Old 09-17-09, 07:26 PM
Thread Starter
Join Date: Feb 2009
Location: New Hampshire
Posts: 10
Inexpensive - not cheap

So I went with the ETQ 7250 watt from Home Depot. (Promo code UPD150 dropped my price $150 with free shipping) At 70db noise level and 6.48 gallon gas tank I'm hoping this will do the job. Also got a Connecticut Electric 30 Amp 10 circuit transfer switch (kit). The house is 20 Amp - I believe, at least that's what the techs that came to the house said. So I should be OK with the 30 Amp switch or should I have gotten the 20?

Next problem, my house was built in the late 80's and utilities are undergound in my nieghborhood. My box is about as far away from any door or window it can be and the included 20ft power cable isn't going to get me connected with the generator outside, so I want to install a remote power inlet box.

My shed is about 110ft behind the house. I dont know how loud 70db is but I was thinking... I want to eventualy put power to the shed so why not store the generator in there. Already had Dig Safe out here for that project anyway. So why not run a line from it to the house for my remote power inlet? I'd have to run 2 lines, one from the house to the shed for regular power, and one from the shed to the house for the Gen power. (right?)

This ensures the CO2 factor is away from my family, and hopefully deadens more noise - the Gen will be stored in the shed but taken out to the back of it to be run (and most likely chained to the shed after all the news stories about the December Ice Storm last year).

So my question is.... what gauge wire do I want to use to run from the Gen to the transfer box? Is 110ft too far?
Old 09-18-09, 05:43 AM
bob1908's Avatar
Join Date: Apr 2009
Location: land of lincoln
Posts: 129
Smile wire sizing

if it would have been under 80' you could use #10 gage
but it is over 100 i would use @ lease #8 gage but more like #6 gage

when i wired my house i used some well wire it was 4 conductor 8 gage and i had enough to double it so for each leg i have 2 8 gage wire. my gen was about 70' in north carolina and now it is about 30' in IL and i still use it doubled use this link to help you

American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies

Last edited by bob1908; 09-18-09 at 06:04 AM. Reason: adding info
Old 09-18-09, 10:44 AM
Pendragon's Avatar
Join Date: Jul 2002
Location: USA
Posts: 1,839
> Harmon Pellet insert

Don't know what this is or how much power it takes.

I'm guessing from what was in the post that your hot water is supplied by the oil burning furnace, so it's electrical needs are slim.

>Electric Dryer - to be swapped out for gas when we get

Most electric dryers take about 5000 watts, get that thing swapped out for a gas one.

Sounds like you bought a 7200 (is that running, or surge?).
If that's running watts, you should be able to run pretty much anything in the house at the same time (lights, tv, microwave, dishwasher, etc) short of a large electrical load such as an electric dryer or heat pump/electric furnace.

You can get kits to convert the portables to run on propane/nat gas, just remember if you go to nat gas you will lose about 20% of the rated output (no such loss with propane).

Remember to run the generator under a load about once a month.
Old 09-19-09, 08:51 AM
Thread Starter
Join Date: Feb 2009
Location: New Hampshire
Posts: 10
7250 Watts

Sounds like you bought a 7200 (is that running, or surge?).

7250 watts running 8600 Surge.

So you think I can connect my 10 circuit transfer switch to me breaker box and pretty much run the house? Thay would be great!!

Just picked up the transfer switch for $200. That and Generator $820. Now just some line to run from the house to the shed and a remote inlet box...

And lots of manual labor.

Is it true I need my trench from the house to the shed 3 feet deep??? at 100 feet away...I'm going to be digging till it snows!
Old 09-19-09, 08:29 PM
Pendragon's Avatar
Join Date: Jul 2002
Location: USA
Posts: 1,839
You are going to be limited by what your transfer panel will allow. Since it only has 10 circuits, you are only going to be able to power whatever is on those 10 circuits.

EASY whole house interfaces are available, but they are NOT cheap. I paid about $800 for mine 5 years ago, and it plugs in between the meter and the meter can. Switching is automatic whenever it senses generator power. There are SOME utilities, especially up north, that rent these for a small fee. It's worth checking in to.
This is the one I have, 50 amp versions as well as ones for auto-start gensets are available from the manufacturer:
Generlink Auto Transfer Switch — 30 Amp, Model# MA23-N | Transfer Switches | Northern Tool + Equipment

My 6500/13000 (a Porter-Cable commercial model with a 13hp Honda, $1800 new) will run everything in my home, all electronics, lights, fridge, upright freezer, small window a/c's, washing machine, microwave, commercial coffee maker, etc. Now, I did do some planning, and most all my lights are CFL's so they take substantially less power than incandescents. I have gas stove and gas dryer (water heater is next). The only thing it won't run is the 4 ton heat pump, too much start up amps causes the transfer panel to trip but it will run the blower motor. With a window a/c or two, it will keep the house reasonably cool enough to sleep in.

When running on gasoline 24/7 after Ivan, it took a little less than 10g a day. I stopped it about every 12 hrs and it never took a whole 5g can to refill. Gotta love those Honda engines.
On a 250g propane tank, I don't even worry about it, but would still check it every 24hrs or so for oil level, etc.

The water heater is electric (for now), but the genny will run it as well if I don't try to run a whole lot other high load items at the same time. So I can run the water heater till it's done (30-45m), then turn it off for a day or two. Or just turn it on a bit before someone wants to take a shower.

The big whole house units mainly provide the power to START and run your central heat/air, as well as being automatic, having scheduled run testing and being hard piped to a fuel source. They ARE nice, but I don't think they are as nice as what they generally cost to have professionally installed unless you have a LOT of power issues.
Setting up the portable for whole house use has the advantage of if you ever want to use the genset somewhere else such as a jobsite, a relatives house, etc, you can just unhook it and go.

As for the trenching, that depends on your local code. But 3' for direct bury cable seems to be fairly standard. You might could go 2' if you can put it in conduit, but again, that's up to your local electrical code. Time to go rent a trenching machine, you WILL thank yourself.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes