Sizing Generator for Submersible Well Pump

[EdwardB]

I know there has been a long thread on this a few years ago, but I still have some confusion. I have a 5hp 240V single phase Franklin submersible well pump with a starter box. It is wird to a 30 amp breaker in my panel. I am trying to determine the minimum size generator to safely run this during outages. I emailed Franklin, and here was their response:

The 5 HP Franklin Electric motor that powers your pump, and I am assuming it is single phase draws 27.5 full load amps and requires a 60 amp breaker or if you have a dual element fuse it should be rated at 30 amps. A consideration when sizing a generator is that upon start up, for less than a second of time, the motor has a locked rotor amp draw of 121.0 that will trip some generators overload.

Here is a picture of the starter box: https://www.dropbox.com/s/tcj22kztihnc29r/WellStarter.JPG

Here is a link to a Franklin guide; page 5 references generator sizing: https://www.dropbox.com/s/9o4dsbzou5hrilc/FranklinSubmersibleMotorGuide2010.pdf

The suggested generator info in the brochure seems way more than the other information would imply. Any help would be appreciated.

 

[Dave M7040]

Perhaps this is a stupid question but why do you have such a large well pump. Most residential installations are fractional horsepower units, 1/2 or 3/4?
If you dont need this big a pump you could change the pump, buy a much smaller generator,and have $ left over.
Dave M7040

 

[EdwardB]

I also use the well for irrigation purposes.

 

[mbohuntr]

You can easily draw 300% of FLA when starting a motor. There is also a much higher inrush current that can trip a breaker. Industrial breakers use more sophisticated circuitry to trip at higher currents than that to protect from transient inrush currents in the hundreds or thousands of amps. They have adjustable trip settings on their faces to customize their protections.
We had to have an electrical engineer calculate the new settings for us to back feed a three phase transformer that kept tripping for that reason. When the normal power was restored, we reset the settings. I don't have any experience in generators, but it seems reasonable.

 

[EdwardB]

Where I am confused is that the breaker in the panel is just a standard 30amp breaker. It seems like it would trip if the well were really drawing that much current. The box info also confuses me. Maybe it has some "soft start" capacitors or the like?

 

[mbohuntr]

The generator has it's own internal breaker that will trip.

 

[mbohuntr]

A generator will have a second rating for "surge" amps. My 5k generator has a surge rating of something like 5800..... you must remember, When a device pulls more current than the gen can produce, the voltage will temporarily drop and the current shoots up before the generators engine can respond.

 

[daybreak1998]

Howdy,
At least you have taken the correct steps to figure things out. Researching the manufacturers recommendations is a important part. Yes, I would say that your control box has the added wizardry for soft start. When someone talks about a 5hp motor on a farm, it is a good size load for starting and running. Since you say its for irrigation, you then have tractors as well. Since this is all wired into your system, you will need to think about how it will all connect into your panel. Some questions below.
1. what is your service? type? (pole top, underground, separate meters)
2. distance from panel to this pump control box? Do you have a smaller panel in your pump house?
3. how many times do you lose power?
4. do you have any other wells on the farm?
5. besides powering the well, what other power things do you want to run? do you want to run it all at the same time? or run the 5hp pump, then shut that breaker off and run everything else?

self contained mounted genset system, open inside genset system, pto generator system?

 

[buickanddeere]

There are Franklin variable frequency drive boxes for three wire single phase and three phase well pumps. A storage tank is used to hold pressure during no and low flow. During mid, high to max water demand. The pump drive box varies the pump motor rpms to maintain a system pressure of your choice.
These VFD also provide an soft start capability.
My 3/4HP 240V well pump drags my 5500 run/8500W start Generac low enough to trip off some household electronic devices and dim the lights to reddish colour.
To start a 5HP single phase pump motor. You need a minimum of a 20Kw pto powered generator with a 30 pto HP tractor .
I don't know how the 30amp breaker isn't tripping unless there is a VFD already on the pump. A 5HP 240V motor in the code book needs a 70 amp breaker.

 

[EdwardB]

There are Franklin variable frequency drive boxes for three wire single phase and three phase well pumps. A storage tank is used to hold pressure during no and low flow. During mid, high to max water demand. The pump drive box varies the pump motor rpms to maintain a system pressure of your choice.
These VFD also provide an soft start capability.
My 3/4HP 240V well pump drags my 5500 run/8500W start Generac low enough to trip off some household electronic devices and dim the lights to reddish colour.
To start a 5HP single phase pump motor. You need a minimum of a 20Kw pto powered generator with a 30 pto HP tractor .
I don't know how the 30amp breaker isn't tripping unless there is a VFD already on the pump. A 5HP 240V motor in the code book needs a 70 amp breaker.

My system uses a pressure tank like you describe. Did you see the attached picture of my control box? I'm not sure that it is a VFD, but it must work some magic because it is only on a 30A breaker.