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Generator capacity

Old 09-11-2020, 04:27 PM
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Default Generator capacity

I am looking to buy a generator that will do my whole house with limits. I have propane for cooking and water heating on a demand system. Is my power the current sum of both legs of 220 service?
Old 09-11-2020, 04:37 PM
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The generator would need to be sized according to the CALCULATED LOAD of everything that would be energized automatically by the gen.

The NEC has 2 methods for sizing the load in a house and you would generally need to know the square footage of the house, what appliances, what motors are all going to be powered automatically when the generator kicks on.

Generally speaking when someone talks of a “whole house” generator they mean an automatic standby system and that would also include a transfer switch that energizes the entire electrical load of a house.

There are emergency back up panels and load shedding devices that can be used to decrease the size of generator required.

What do you mean by the sum of both legs of your panel? What do you mean by a whole house with limits?
Old 09-11-2020, 05:03 PM
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A lot of the capacity question is based on what you plan to run. We have a 25KW diesel standby whole house setup. I originally had a 24 KW propane unit, but it crapped out on us during hurricane Irma so I replaced it with the diesel. The smaller sized standbys are usually air cooled, above 18-20 KW, they are water cooled and are quieter. Most of the propane units run at 3600rpm and are louder, the diesels run at 1500-1800 rpms and mine is actually quieter than the old unit. Add up the amps for the appliances you plan to run and size the set accordingly. I added about 20 percent which is why I got the 25 KW.
Old 09-11-2020, 05:11 PM
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First decide if you want a permanently mounted generator that will kick in within seconds of loosing power without any input from you or “portable” generator you need to wheel out , plug in, start and constantly feed it fuel. Personally I would go with a permanent Generac or similar unit mounted to a pad since you have propane already -May need to increase tank size.

From your post most likely you will not be installing the unit or panel hookup yourself so I would call a qualified electrical contractor. They can walk you through the options and give you pricing options.
Old 09-11-2020, 05:17 PM
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Are you wanting to run HVAC? I can run my entire house minus the range and HVAC on a pair of Honda eu2000i’s. Can basically run what I wish in terms of electronics, lights, including electric hot water, plus one more heavy load device such as a coffee maker or microwave. If I kill hot water I can run one window unit as well.
Old 09-11-2020, 05:19 PM
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My question is how to determine the load my house has. I can check current on each leg of the service. My first check was 10 amps one leg and 7.5 on the other. I assume I am drawing a total of 17.5 amps in that condition which ohms law states is a little under 4,000 watts
Old 09-11-2020, 05:39 PM
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Originally Posted by gotfw View Post
My question is how to determine the load my house has. I can check current on each leg of the service. My first check was 10 amps one leg and 7.5 on the other. I assume I am drawing a total of 17.5 amps in that condition which ohms law states is a little under 4,000 watts
No. I assure you that when you checked that, there were appliances NOT running. The stuff you want to run during a power outage should be counted separately. Ex. 220v oven pulling 20 amps alone is over 4k watts. Now think about your lights, fans, fridge etc.


Just reread where you have propane for cooking. Are you on city water or a well? What size A/C unit? Microwave? I had a 5kw gen during Mathew that ran 2 fridges, small freezer, lights, fans. When the well pump kicked on the gennie would bog down as well as the microwave

Last edited by lencho30; 09-11-2020 at 05:46 PM.
Old 09-11-2020, 05:42 PM
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Originally Posted by gotfw View Post
My question is how to determine the load my house has. I can check current on each leg of the service. My first check was 10 amps one leg and 7.5 on the other. I assume I am drawing a total of 17.5 amps in that condition which ohms law states is a little under 4,000 watts
sounds like you know how to measure. But what were you measuring. What was on at the time? Pretty easy way to figure it out is to simply turn everything on you want to run while you are down at the same time, and measure it. After that factor in how many of those items have a startup surge (pretty much anything with a motor/compressor) and see how big that surge is. Then size the generator based on its rated sustained load, insuring it’s also capable of handling the surge loads.
Old 09-11-2020, 05:49 PM
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As a plumber I say be very carefully on what type of generator you choose. Today's on demand water heaters and other heating unit are very voltage sensitive. We had a bad storm here a year or two ago and a lot of people ran out and bought generators and that fried a lot of boilers and water heaters, I spent a month doing nothing but replacements. You need a true pure sine wave generator.
Old 09-11-2020, 06:04 PM
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My digital FPL meter tells me how many KW I am pulling at any moment. With the down stairs A/C running (3ton unit) lights tv and other crap the meter reads 4.2kw.

might be worthwhile looking at your meter.
Old 09-11-2020, 06:17 PM
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There is a meter that you leave on for a few days, preferably in summer, records your peaks and average use, etc.
I have a diesel 20Kw runs everything including two fridges, a deep freezer, boat on lift (10A) everything else and upstairs OR downstairs AC central.
It can actually run both but something else may come on (our oven or apt oven ) and trip it out so we don't run both centrals at once.
One thing to be aware of: If you run it during a hurricane, the bell end (electrical side) is breathing salty very wet air. Sets up rust which cannot be stopped.
The diesel end can breath salty air, but once that Bell end pulls that salt in, it's gonna die in a year or two.
No way to flush it out either, that i've found.

Old 09-11-2020, 07:37 PM
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If you go propane, make sure to get a Big enough tank to run 4-5 days. I had a 100 gallon tank for my propane genset and it ran out after 4 days. They use a lot of gas.
Old 09-11-2020, 08:13 PM
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Originally Posted by Hjudge49 View Post
If you go propane, make sure to get a Big enough tank to run 4-5 days. I had a 100 gallon tank for my propane genset and it ran out after 4 days. They use a lot of gas.

Look at the burn rate at idle. Most of the time the generator will be running with a light load. Sometimes it’s best to go smaller especially if you are on propane. 10kw vs 15kw May mean the difference in lasting a few extra days during an outage.
Old 09-12-2020, 03:17 AM
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Current draw from a compressor or motor start is higher than running current.

Load shed devices prevent high demand equipment from starting at the same time. Typically a house with multiple ac units will have the ac's connected to a load shed to stagger the starts.

Using your running current to size a genset may result in an undersized unit.


https://www.norwall.com/power-expert...by-Generators/

Last edited by ansel; 09-12-2020 at 03:38 AM. Reason: spelling
Old 09-12-2020, 06:30 AM
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Originally Posted by gotfw View Post
My question is how to determine the load my house has. I can check current on each leg of the service. My first check was 10 amps one leg and 7.5 on the other. I assume I am drawing a total of 17.5 amps in that condition which ohms law states is a little under 4,000 watts
That is not the correct way to size a generator.

there are factors such as start up current when motors turn on, and other things that MUST be taken into account.

what you did was spot check the load at some random moment in time. There’s nothing wrong with doing that, but that’s not the right way to size a generator.

Sizing is done by determining the calculated load which has nothing to do with putting an amp meter on the main feeder wires.
Old 09-12-2020, 01:35 PM
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I run a Generac 11KW for my whole house, except the dryer and stove. Being all electric here is Florida sucks. I have a load shed on the water heater. I have a 300 gal propane tank underground. I can go close to 6 days on that 300 gal tank, but most propane companies are out filling pretty darn quick. As long as they have access to your tank after the storm 4-5-6 days of propane is more than sufficient.

I can run a 2 1/2 AC, pool pump, pretty much all the circuits have been moved over the new panel. If you don't have NG, don't make the mistake of thinking you can run a 22KW on anything less than a 500 gal propane tank.
Old 09-12-2020, 03:42 PM
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Originally Posted by halfwaythere View Post
I run a Generac 11KW for my whole house, except the dryer and stove. Being all electric here is Florida sucks. I have a load shed on the water heater. I have a 300 gal propane tank underground. I can go close to 6 days on that 300 gal tank, but most propane companies are out filling pretty darn quick. As long as they have access to your tank after the storm 4-5-6 days of propane is more than sufficient.

I can run a 2 1/2 AC, pool pump, pretty much all the circuits have been moved over the new panel. If you don't have NG, don't make the mistake of thinking you can run a 22KW on anything less than a 500 gal propane tank.

Just went through this exercise on my home that is under construction. 2 ac units 9 ton total. Everything electric except stove burner top. Whole house to run everything is 38kw. 1000 gal buried tank. This was almost double what I thought it would be. Had a second contractor size generator and came back the same (a hair larger). We also have a well. Pool.
Old 09-12-2020, 04:15 PM
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Originally Posted by gotfw View Post
My question is how to determine the load my house has. I can check current on each leg of the service. My first check was 10 amps one leg and 7.5 on the other. I assume I am drawing a total of 17.5 amps in that condition which ohms law states is a little under 4,000 watts
Get an Electrician or local Electrical Company to bring a logger (I use Fluke) and install it in your panel to read how many Amps, you can leave it for hrs or days, the longer you leave it it will give you a more accurate study. Then you pick the right size of generator you would need.
Old 09-12-2020, 04:23 PM
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Originally Posted by waheero71 View Post
Get an Electrician or local Electrical Company to bring a logger (I use Fluke) and install it in your panel to read how many Amps, you can leave it for hrs or days, the longer you leave it it will give you a more accurate study. Then you pick the right size of generator you would need.
As an electrician and Generac dealer and installer; this is again, not the correct way to size a standby generator.
Old 09-12-2020, 04:35 PM
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Originally Posted by AsurfAholic View Post
As an electrician and Generac dealer and installer; this is again, not the correct way to size a standby generator.
This combined with a load calc, why not. By the way 90% of the generators we install are Generac 300kw and above. You care to elaborate?

Last edited by waheero71; 09-12-2020 at 05:17 PM.

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