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Generator for a well pump

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TheWantonElectrician

Member
Location
Madison, WI
Occupation
Master Electrician (Residential and small-scale commercial)
I have a customer that's asking me to install a generator hook up so they can use their portable generator to keep their well pump running during an outage. I'd like to focus the conversation on the size of the generator they'd need. I spoke to a rep from the company that installed their well pump and they gave me the specs that the pump has a 3HP motor, has a 17A running load and "something like 35 starting amps". So, should I assume they'll need a generator that can produce around 8,400 starting watts (240V x 35A)? Would that also necessitate using a 50A plug and receptacle?
 
tom baker

tom baker

First Chief Moderator & NEC Expert
Staff member
Location
Bremerton, Washington
Occupation
Master Electrician
The inlet you install will match the size of the oulet on the generator.
Generac may have a size calculator for motors, or try one of the submersible mfgs.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
TheWantonElectrician said:
I have a customer that's asking me to install a generator hook up so they can use their portable generator to keep their well pump running during an outage. I'd like to focus the conversation on the size of the generator they'd need. I spoke to a rep from the company that installed their well pump and they gave me the specs that the pump has a 3HP motor, has a 17A running load and "something like 35 starting amps". So, should I assume they'll need a generator that can produce around 8,400 starting watts (240V x 35A)? Would that also necessitate using a 50A plug and receptacle?
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30 amp should be fine. That starting current is only for a second or less, on utility power anyway, though it maybe just a little longer depending on abilities of the generator. The thing is the prime mover needs to be able to handle the surge of loading at startup without slowing down too much, more so than the alternator itself needs over sized for this brief loading event.
 
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Birken Vogt

Senior Member
Location
Grass Valley, Ca
"Starting watts" on a Chinese portable is just a lie to justify their model number being bigger. In fact they can often start motors with a much larger LRA than starting watts because the rotor is not really locked. But everything is a gamble these days.
 
D

Dzboyce

Senior Member
Location
Royal City, WA
Occupation
Washington 03 Electrician & plumber
For temporary use on 5 hp and under 230v/1 ph sub pumps I use my Lincoln welders. I have three ofvthose. Up to 20hp, I use a Cat XQ-20 generator. Single phase 230v/ 3 phase 230/460 volt. Won't start a 20 hp across the line, but it will with a drive and a slow ramp up. Also have a 60Kw, and another Cat 225KW generator. It grunts a little, but will start and run a 150 hp motor. I've run that one 24/7 90 days straight. Only stopping to service it. Same thing with the 20kw
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
Birken Vogt said:
"Starting watts" on a Chinese portable is just a lie to justify their model number being bigger. In fact they can often start motors with a much larger LRA than starting watts because the rotor is not really locked. But everything is a gamble these days.
Click to expand...
On the other end of the extremes, during an extended outage after an ice storm I had a farmer hook up what was probably a 18 -20 kW tractor PTO driven generator directly to an unload auger from his grain bin because he needed corn for the cattle. They do what they have to do at times and I don't blame him as I would have attempted to do this as well if I were in his shoes. When he flipped the output breaker of the generator on it snapped the universal joint on the PTO shaft.

When I came he wanted to know what/why that happened. I looked at his motor connections and realized he had a line to line short circuit with what he ended up doing. The tractor he was using also was capable of putting out a lot more power than the generator he was driving. Had he used a tractor with similar output to the generator maybe it stalls the tractor.

Your mentioned Chinese portable likely has enough of a prime mover to be able to put out that starting current without stalling the prime mover or even dragging it down far enough it just isn't going to get the job done, the driven alternator likely could start even larger motor if it had enough capability from the prime mover.
 
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Birken Vogt

Senior Member
Location
Grass Valley, Ca
I did have one call out on a Chinese portable that would not start a huge inertia blower. But it could have run it just fine. The engine barely burped. It just totally collapsed the regulator (Honda style "half moon") and AC output basically went to zero. The load had to be disconnected then reconnected to get it to try again but it would never do more than make it rotate slowly for a few revolutions. A Multiquip diesel started and ran it just fine.

However, well pumps and AC compressors get up to speed much faster, within a fraction of a second. Never seen a portable fail to start one, within reason.
 
S

shortcircuit2

Senior Member
Location
South of Bawstin
TheWantonElectrician said:
I have a customer that's asking me to install a generator hook up so they can use their portable generator to keep their well pump running during an outage. I'd like to focus the conversation on the size of the generator they'd need. I spoke to a rep from the company that installed their well pump and they gave me the specs that the pump has a 3HP motor, has a 17A running load and "something like 35 starting amps". So, should I assume they'll need a generator that can produce around 8,400 starting watts (240V x 35A)? Would that also necessitate using a 50A plug and receptacle?
Click to expand...
I have had good luck with 6500-watt generators with surge capacity for the well startup surge. Stay away from "inverter type" as these hit a wall at their rated wattage without surge and will not be good for well pump motors.

Generator to outside located (per 702.12(B) 2023 NEC) 30-amp inlet to house panel with panel cover interlock kit to back-feed the whole house.

Not switching the neutral thru transfer? Make sure customer buys a "floating neutral" generator...or one that is convertible.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
Birken Vogt said:
I do not see what problem it could ever cause. Assuming it runs back to an interlock or transfer switch at/near the main service.
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Likely doesn't cause much problem in a lot of instances, but if you don't switch the neutral in the transfer equipment then you are essentially doing the same thing as bonding the neutral a second time at a sub panel which allows neutral current to flow on any metallic paths between the "sub panel" and the supply end.

Any ground fault protection whether GFPE or GFCI will be subject to unwanted tripping because of current flow outside of the intended path.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
kwired said:
Likely doesn't cause much problem in a lot of instances, but if you don't switch the neutral in the transfer equipment then you are essentially doing the same thing as bonding the neutral a second time at a sub panel which allows neutral current to flow on any metallic paths between the "sub panel" and the supply end.

Any ground fault protection whether GFPE or GFCI will be subject to unwanted tripping because of current flow outside of the intended path.
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Only the GFPE or GFCI on the generator.
 
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Birken Vogt

Senior Member
Location
Grass Valley, Ca
kwired said:
Likely doesn't cause much problem in a lot of instances, but if you don't switch the neutral in the transfer equipment then you are essentially doing the same thing as bonding the neutral a second time at a sub panel which allows neutral current to flow on any metallic paths between the "sub panel" and the supply end.

Any ground fault protection whether GFPE or GFCI will be subject to unwanted tripping because of current flow outside of the intended path.
Click to expand...
The current will be flowing on the 10 awg white wire and/or the 10 awg green wire in the generator cord and not much else. Once it reaches the MBJ, current flow in the rest of the house will be normal.
 
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