# Generator Size



## Danny Womack (Feb 7, 2020)

Here is what I have: https://www.amazon.com/Reliance-Controls-Corporation-PB30-Generators/dp/B000BQT47S/ref=sr_1_4?keywords=Reliance+controls+generator+inlet&qid=1581078431&sr=8-4
https://www.amazon.com/ITE-200A-Murray-Siemens-Generator-Interlock/dp/B014BXH9QE/ref=sr_1_1?keywords=Ite-200a+Gould&qid=1581078789&sr=8-1
https://www.homedepot.com/p/Siemens-30-Amp-Double-Pole-Type-QP-Circuit-Breaker-Q230U/100002289
And approximately 40 feet of 8/3 wire connecting them.

I have a pool pump that I need to run if I lose power in the winter months - 230v x 9.3a = 2,139 running watts
Refrigerator approximately 800 running watts

What I don’t know is the running/starting wattage of my gas furnace blower (should I choose to use it because we have gas logs). I also plan to have a window unit for AC That would cool about 800 sq ft on standby in case of a summer outage. I also know that during summer months I could kill the pool pump and, depending on the running/starting wattage of the HVAC, possibly run that. Other than that, maybe a 225 watt tv and some random comfort related extras.

Having said all of that, I haven’t purchased a generator yet. What size portable generator will my current setup handle? Once I know the largest generator I can run, I’ll know what all I can possibly run during an outage.

Thanks in advance, Danny


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## iowagold (Jan 22, 2020)

well you are going to need a 240 volt gen.
at least to do the pool pump..
unless you get a second pool pump for the back up that is 120 ac..
so what is the pool heater?? electric or gas?
and house water heater? electric or gas?

yea do a site survey and it is best to have a meter system for the breaker panel..
that helps to see loads when you are on grid so you can label the breaker box when you see the loads...

lucky on my system here as I wired it all my self!!
over kill dedicated runs. lol
so at least I know what runs where!! and it is marked in the printed box cover.
square d has software for that on their site. or a template for MS word. fill in the blanks and print!!

for most homes the honda eu7000is is the best in class.
https://www.northerntool.com/shop/tools/product_200640183_200640183 
right now 02/07/2020 they have free shipping and $4449.00 price tag..
but they are quiet!!
and if you need tri fuel there is an optional kit now for $500.00 plus hoses to the house.

so the big question is are you able to install this your self?
or are you hiring it out?
if you are good with electrical these are not bad to put in!!
take your time!!

I used 6 gauge on my gen inlet feed to the interlock breaker panel... that gets me a better rating for power..
that is what worked for me. 50 foot was 110.00 retail I catch that on the 11% at menards and get it for 11 bucks less.
see the chart here
https://www.cerrowire.com/wp-content/uploads/2018/09/ampacity091418.pdf

always room for expansion!! run the wire once!! lol!!


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## Danny Womack (Feb 7, 2020)

The house is already set up for a generator. I suppose I could re-wire and/or change the inlet box or breaker if I needed to. I guess I’m just trying to find out, with the current setup, what the largest size generator I can run?


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## drmerdp (Apr 10, 2018)

Need some more info to make a proper call. 

What state do you live in? Whats your home heating situation, Natural Gas, Propane, heating oil or heat pump?

Do you have any intention on running an alternative fuel... Propane or Natural gas? 

Gas or electric stove/oven? 

Gas or electric Dryer?

Well pump or city water? 

Most Important, what’s your budget? I’d love to say 240v inverter gen, but $3-4K is a hard pill to swallow.


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## ToolLover (Jan 13, 2020)

So Danny,
You must be planning to back feed the house and use a lock out device.
I am not a fan of these devises, but to each his own.
I took a tally of the wattage and amperage you need to keep the items running.
@ 746 watts per HP.
2139 watts for the pool pump. 120 or 240 vac ? 
That wattage equates to nearly three hp or 8 to 9 amps @ 240 vac.
Most furnace blowers are 1/4 hp unless it is a huge furnace.
@120 vac that is 1.75 amps.
Then a 225 watt TV is another 2 amps.
So your needed power does not add to a lot of amps.
Then I see that IowaGold is recommending a $4400 generator.
You should check Generatorsdirect.com.
I got my 16 KW Generac for $3100 shipped. Add to that the cost of the transfer switch.
At present we are without power due to the winds in NC.
The Generac is purring and we are comfortable.
I installed a transfer switch and a sub panel to be selective on the things I wanted to run. Works good.
A whole house generator adds value to a home and allows the wife to be comfortable on an outage if I am not home as was the case this morning.
Ask your wife if she can power up the generator if you are not home, then decide.


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## Danny Womack (Feb 7, 2020)

Let me try asking this a different way. Sometimes things make sense in my mind but not when I type them out.

My wife and I bought a house that is already set up for a portable generator. It is set up with the inlet box, 8/3 wire, 30 amp 2 pole breaker, and interlock switch I mentioned in my original post. I’m simply looking to buy a portable generator.

Does the currently installed set up limit the size generator I can use? If so, what is the largest one it will handle?


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## ToolLover (Jan 13, 2020)

If you are sure that the feed wire is 8 Gauge THHN wire, then it is good for 40 amps, but the connector is only good for 30 amps.
That being said, 30 amps times 240 volts is 7200 watts for the maximum the connector box can safely be loaded to.
The general rule is not to load a device over 80%.
You should be the judge, but 80% of the connector is 5760 watts, that leads you to a 5500 watt generator.
The 80% rule assures that the devices connected to your home will not over heat and cause fire.
Now a note concerning the twistlock connector box you are considering.

In the early seventies we were required to run a safety check on all devices we were using.
Believe it or not the twistlock plugs both male and female were found to be dangerous.
Electrical shock was the culprit. 
If a hand tool was connected via twistlock then the cable could not be quickly pulled apart creating a potential electrocution situation if a device failed.
If you choose to use the male plug you have shown in your post, be sure to install an on site GFI circuit breaker to assure the plug is rendered inoperable.


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## drmerdp (Apr 10, 2018)

Danny Womack said:


> Let me try asking this a different way. Sometimes things make sense in my mind but not when I type them out.
> 
> My wife and I bought a house that is already set up for a portable generator. It is set up with the inlet box, 8/3 wire, 30 amp 2 pole breaker, and interlock switch I mentioned in my original post. I’m simply looking to buy a portable generator.
> 
> Does the currently installed set up limit the size generator I can use? If so, what is the largest one it will handle?


lol, that’s a very different question. There is no harm in a generator capable of the full 30amps. Both the generator and load center have over current protection and the greatest amperage draw is Inrush from induction loads(electric motors) which spikes very high for only a moment then settles to much lower numbers. But do you actually need a 7200 watt generator???

Bigger rated gen = more fuel usage. What if fuel is hard to come due to disaster, hurricane, earthquake, etc. The only thing you’ve told us is that you have a pool and want to be able to run the pump during an outage. 

I’d feel better knowing at the very least the answers to the questions I posed earlier before making a recommendation.


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## Danny Womack (Feb 7, 2020)

ToolLover said:


> If you are sure that the feed wire is 8 Gauge THHN wire, then it is good for 40 amps, but the connector is only good for 30 amps.
> That being said, 30 amps times 240 volts is 7200 watts for the maximum the connector box can safely be loaded to.
> The general rule is not to load a device over 80%.
> You should be the judge, but 80% of the connector is 5760 watts, that leads you to a 5500 watt generator.
> The 80% rule assures that the devices connected to your home will not over heat and cause fire.


ToolLover, I’m sure of what is already here but given what you’ve said, I could replace the input box with a 40amp (if one exists) and then swap the 30 amp 2 pole breaker with a 40 amp 2 pole?



drmerdp said:


> lol, that’s a very different question. There is no harm in a generator capable of the full 30amps. Both the generator and load center have over current protection and the greatest amperage draw is Inrush from induction loads(electric motors) which spikes very high for only a moment then settles to much lower numbers. But do you actually need a 7200 watt generator???
> 
> I’d feel better knowing at the very least the answers to the questions I posed earlier before making a recommendation.


Given what ToolLover said and I’m limited by a 5500 watt unit, would that be 5500 running watts? I’ve seen some estimates that say running wattage of an electric motor should be multiplied by 3 to get the wattage it would take to start the motor? My pump is 240 volts and 9.3 amps. That calculation tells me that it’ll take 6,700 watts to start the pool pump?

I live in Georgia. I’m not planning to run our electric stove, our electric dryer, and we have city water.
We have natural gas, central heat and air but I plan to use neither unless I have a lot of wattage/amps to use. We have natural gas logs.

As far as the pool goes, it’s only a concern in sub-freezing temps. Also I only need to run the pump long enough to drain the pool below the skimmer and recirculation lines. Once I do that, I open all the above ground valves, blow the remaining water out and leave the system off until power is restored. Running the pump to drain should take less than an hour. After that, the generator will keep us semi-comfortable.


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## drmerdp (Apr 10, 2018)

There is no reason to derate your generator circuit to 80%. And the fact that you have 8/3 shows the electrician wanted to keep voltage drop at the rated 30amps in consideration.

All of your important appliances are electric, why not consider that? 

Yes, induction motors require up to 5x their running wattage. But there is no head pressure on your water pump so it’s much less. 

If you choose to do a trifuel conversion in the future and run on natural gas, your generators output needs to be derated by 20% based on your operating load. So if you buy a 5000 watt continuous unit your effective continuous output should be considered around 4000 watts. This is a rule of thumb estimate. 

Generators rated output contain 2 numbers. Continuous rated wattage and surge rated wattage. Example 7000/9000 is 7000 Continuous or running. 9000 is surge. 

An slightly undersized generator starting an electric motor will cause a voltage drop, a peak in amps, and bump in heat, and a some wear and Tear on the motor. A way undersized generator will not start the motor. And could stall the generator and/or cause a voltage drop that could potentially harm and sensitive electronics that are also connected.

I’m assuming your central heat is a natural gas furnace. An eu2000 can run that. Window air conditioners are also pretty small loads.

I forgot to ask if you have an electric water heater, but you said you have an electric stove. If your water heater is electric then a 5000 watt generator is barely adequate and I’d be looking bigger. A 5000 watt generator would require much more attention to load management. 

Especially if you convert to natural gas, which I strongly recommend. It’s basically an endless supply of fuel, with zero carburetor maintenance.


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## Danny Womack (Feb 7, 2020)

drmerdp said:


> There is no reason to derate your generator circuit to 80%. And the fact that you have 8/3 shows the electrician wanted to keep voltage drop at the rated 30amps in consideration.
> 
> All of your important appliances are electric, why not consider that?
> 
> ...


We have a natural gas water heater as well as natural gas furnace.

So for the sake of discussion, I just looked at a generac 8000 and the 240 volt plug has a double pole 30 amp breaker built in. Wouldn’t that mean that even though the generator is capable of 8000 running watts it will only send 7200 to my panel, or am I thinking wrong about how it works?


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## ToolLover (Jan 13, 2020)

Danny, 60 amp boxes are available and you can install a 30 amp breaker, however to install a GFI breaker you will have to have four wires.
The electrician did not meet NEC if he only installed the outlet with three wires unless if he did it 30 years ago.
If you have only three the GFI will not work as it needs a separate ground for current flow to trip the breaker.
An important note was made that if you connect the generator to natural gas there will be a drop in power.
Not installing NG will bring on the problems of carburetor clogging if not maintained properly which is a pain.
You might consider a Harbor freight unit in the 6500 range as they are loud but cheap.
The 6500 unit will limit amperage to 27 amps but you can go bigger.
The larger unit will make up for the NG loss.
I had one and I sold it to a friend for $400.
He has had it for 15 years and it is still performing.
I have a ES 6500 Honda 27 amp generator and I connect it to NG and it performs beyond my expectations.
Note: When connecting to NG or Propane there has to be two regulators.
One on the output from the source and one on the generator.
Failure to properly regulate the gas flow will blow the unit regulator. 
The flow at the generator is where you set the frequency by speed. 
To be honest, the Honda is back fed and will run my house if needed.
There is another note I want to add: You might change the pump motor to a capacitor start motor. Starting amperage is reduced and you can add a second start relay capacitor.
I add them to AC units that tend to blink lights on start up.
I installed my first generator in 1992 on a seldom used cabin I built in West Jefferson, NC.
I had to convert the electrical to a 12 second delay start up due to lighting bursts. 
It is still preforming today.
Good luck!


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## iowagold (Jan 22, 2020)

I guess I would look at changing out the stove to natural gas..
or if you have room in the kitchen have both!!
I like to stove top cook with gas...
but the oven the electric works better for me...
lol
so I have the best of both worlds!! both!!
electric and gas ranges!!

that helps on the energy required during an outage..

I guess for me it is a no brainer...
I always go for the better gen set...
low cost is not the choice for me... been there and it failed to run when I needed it the most!!
I invest... so I think of the eu7000is as an investment...
and I value me so I think I am worth it!!
GRIN!!
LOL!! (laugh with me on this folks!!)

been there done that on the lower cost units...
the loud gens are annoying at best!! and after 5 hours or more run time...
yea in a camp ground or party a loud gen is like listning to a heavy metal band for 5 hours...
you could … but why??? lol!! or where are my ear plugs!!

you have to hear or not hear lol the inverter series honda units run!!
go to the local honda gen dealer and ask for a demo to hear one!!

and as far as fuel the eu7000is is injected!! no kidding!!
so it is way better on unleaded fuel per hour that any of the others out there in the same class of gen.

and if you move or do camping this is easy to take with you!!
it is on wheels!!
so make sure you use a cable chain when you use it!!
keep the honest , honest ...


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## iowagold (Jan 22, 2020)

oh yea did I miss this what are you heating the pool with for fuel??


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## drmerdp (Apr 10, 2018)

Danny Womack said:


> We have a natural gas water heater as well as natural gas furnace.
> 
> So for the sake of discussion, I just looked at a generac 8000 and the 240 volt plug has a double pole 30 amp breaker built in. Wouldn’t that mean that even though the generator is capable of 8000 running watts it will only send 7200 to my panel, or am I thinking wrong about how it works?


Since you have a natural gas Water heater, I’d say a >/=5000 watt unit is a good option for gasoline fuel efficiency. If you are tempted by natural gas, go a little bigger. 6000+ continuous. Pretty much any generator with a 389cc engine or larger. 

So that generac 8000...

Key Specs
Item#	75012
Brand	Generac
Manufacturer's Warranty	3 Yr Consumer Limited
Ship Weight	176.5 lbs
Surge Watts	8,125
Rated Watts	6,500
Start Type	Recoil
Engine	Generac
Engine Displacement (cc)	389

Rated at 6500 watts.. but surge of 8,125.

Your breaker on the generatorS l14-30 inlet is 30 amps/7200watts , the generators breaker on the l14-30 plug is 30 amps/7200watts.

Only the surge rating exceeds this, plus the generator can only produce clean power at 34amps 8125watts load for a momentary amount of time in order to sustain a [surge] in power such as a large electric motor.

*Here’s the thing, Neither of these breakers trip the instant 30 amps pass through them. They have a thermal action that requires the current to Cause heat and then trip. It would take 34amps for an extended amount of time to trip the breaker. 40amps might trip instantly though. 

For the record, on Natural Gas.
6500watts x .8 = 5200watts. No gasoline to contend with. Plus with basic load management you can Selectively use anything in your house including your oven OR range.

Now to throw a monkey wrench into everything.... Clean electricity... not just voltage and hertz... but Low THD, Total Harmonic Distortion. 5% or less. 

Any generator with a high quality alternator will advertise their THD number. If it doesn’t then you might be rolling the dice...Just sayin’.


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## ToolLover (Jan 13, 2020)

It seems that we are getting you into more problems than you might have expected.
Changing the stove, adding a NG generator and protecting your wiring! Getting complicated huh?
Well there is another change you will be confronted with if you do the above.
In order for the gas meter to provide the necessary volume you will have to get your gas provider to increase the size of the meter. 
Sorry about that, but we all have learned from experience.


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## drmerdp (Apr 10, 2018)

ToolLover said:


> It seems that we are getting you into more problems than you might have expected.
> Changing the stove, adding a NG generator and protecting your wiring! Getting complicated huh?
> Well there is another change you will be confronted with if you do the above.
> In order for the gas meter to provide the necessary volume you will have to get your gas provider to increase the size of the meter.
> Sorry about that, but we all have learned from experience.


Most natural gas meters are rated at 250CFH or 250,000 BTUH and there is wiggle room. So 1CFH is 1,000btu. A 389cc engine at MAX load is 120,000 btu. Water heater is 30,000 btu, most furnaces are 100,000btu. 

So he would be at or close to the mark, but granted A 389cc engine with a 120,000btu fuel consumption is at MAX load. Simple load management and you’ll have no problems.

This is just arming him with the knowledge to make an informed decision. 

Is the OPs gas meter near his electrical inlet? 

When I was figuring all this out in 2013, I wish someone was around to lay it all out on the table for me.


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## Melson (Dec 8, 2019)

120000 btu/h for 10 or 12 hp? 
I'm not at all well versed on this, but if I calculate 8kw, say, it comes out as roughly 27,000 btu/h. Is that right, or...?


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## drmerdp (Apr 10, 2018)

So the rule of thump is max load consumption and a 10,000 btu per 1 horsepower. 

The trick is to find the actual hp and not the embellished number that most companies advertise. 

Cubic centimeters is usually the most accurate. 
240 approx 8hp
270 approx 9hp
300 approx 10hp
340 approx 11hp
390 approx 12hp
420 approx 13hp


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## exmar (Jun 29, 2015)

Wow! Is the OP hiding, wishing he'd never brought up the question?? Rewire generator connection, remodel house, etc. etc.

Couple of questions to ask before you spend bunches of $$. Where are you located and how often and duration of outages? Was the pool (that seems the bad actor here) there when you bought the house, and presumably the previous owner used the 30Amp connection? 

The 30A connection is "generically" used for a 5500-6000 generator. Those, typically, have an additional 1,000W of surge capability, so 6500-7000W. The Pool "theoretical" inrush would be 2139X3=6417W so that would be within a 5500's ability. Also, "theoretical" and "recommended practice" come into play here. You may not see that kind of inrush. Also, the loads in this type of connection are brought on one at a time so you can start the pool, let it stabilize and then continue. Also, again, how often do you see outages? Here's a link to a Generac 5500 with 6875 surge capability. I'm not pushing Generac, lots of good units out there.
https://www.homedepot.com/p/Generac-5-500-Watt-Gasoline-Powered-Portable-Generator-5939/202527679

What you have seems to be set up for the occasional outage, Operate the disconnect and open all breakers in the load center, drag out the generator, start it, connect, close the two pole 30A breaker to feed the load center, then add loads per your outage plan.

Going to a Natural gas unit would require bringing or having a gas line run to where your connection is, quick disconnect, etc. ($$)

Did the previous owner get along with the 30A connection with the pool? Are you OK dragging out a gasoline unit when needed? I've been using those for over 20 years, using Stabil or equivalent fuel treatment with no issues. Your $$, your decisions.

Good luck,


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## Melson (Dec 8, 2019)

@drmerdp, I've been taught that 1 hp equals 746 watts, which equals 2544 btu/h.


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## Melson (Dec 8, 2019)

Are you saying that the rest is attributable to effencieny losses?


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## drmerdp (Apr 10, 2018)

Melson said:


> @drmerdp, I've been taught that 1 hp equals 746 watts, which equals 2544 btu/h.


I’m not familiar with that conversion, but there are many conversions that are applicable to different systems. That may be an hvac conversion for air conditioning.



Melson said:


> Are you saying that the rest is attributable to effencieny losses?


Engines waste most of their energy as heat through the exhaust so it’s most definitely inefficiency.


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## iowagold (Jan 22, 2020)

80% efficiency is a normal number
sometimes it is better or worse depending on the design on the gear.
back in the 40's 40% efficiency was a normal number.
after ww2 most stuff was 60%
now days with all the computer controls.
engines are at mid 80%
hvac is 80-98%
electronics is 40% to 90% depending on the design.


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## ToolLover (Jan 13, 2020)

So Danny, After being totally inundated with our wonderful experiences and free advice what have you decided to do?
Don't leave us in the dark to wonder if we have influenced you.


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## drmerdp (Apr 10, 2018)

ToolLover said:


> So Danny, After being totally inundated with our wonderful experiences and free advice what have you decided to do?
> Don't leave us in the dark to wonder if we have influenced you.


lol, yes do tell.


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## Danny Womack (Feb 7, 2020)

ToolLover said:


> So Danny, After being totally inundated with our wonderful experiences and free advice what have you decided to do?
> Don't leave us in the dark to wonder if we have influenced you.


I leave you guys alone for a day or two and look what happens. :tango_face_grin:

I’m going simple here. Our last outage was just over 24 hours. The one before in 2017 was for a couple of days. If the temps are sub freezing, I’ll simply use the Generac 6500 I purchased to pump water out of the pool, then use my blower to clear the lines. Add RV antifreeze then shut it down until the temps rise and the power is back on.

In the interim I’ll use the 6500 to run what I need to stay comfortable inside. I have all of my breakers labeled correctly now and my wattages listed. The only thing I don’t have is my AC condenser wattage. All of the stickers are missing.


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