Franklin Electric's minimum generator rating is 2 kW for a 1/2 HP 3-wire submersible, 3 kW for 3/4 HP, 4 kW for 1 HP and 5 kW for 1.5 HP. Franklin's separate table for 2-wire motors (no control box) is 50% higher: 3, 4.5, 6 and 7.5 kW. Constant-pressure pumps and Grundfos SQ pumps need less.
What we did: We put Franklin Electric's standard, 2-wire and constant-pressure minimums and Grundfos SQ figures in one table, and worked out which pumps a 3,500-watt generator can start.
⚠️ Safety first. A generator connected to house wiring must go through a transfer switch. Franklin’s manual says transfer switches “must be used any time a generator is used as standby or back up on power lines”. Run portable generators outdoors only, at least 20 feet from the house (CPSC). Wiring a well pump circuit is work for a licensed electrician or pump installer.
Find your pump’s numbers
| What to find | Where to look |
|---|---|
| Horsepower | Control box label or install record (the motor is down the well) |
| 2-wire or 3-wire | Separate control box = 3-wire. None = 2-wire. |
| Voltage | Control box label, or the breaker: double-pole means 240 V. In Franklin’s 4-inch single-phase line, only the 1/2 HP motor comes in 115 V; 3/4 HP and up are 230 V. |
| Constant-pressure drive? | A drive box (e.g. Franklin SubDrive or MonoDrive) instead of a pressure switch |
A 230 V pump needs 240 V. Many portables give 240 V from one receptacle only; check before buying a cord or inlet.
Minimum generator size
Minimums for the pump alone.
| Pump / motor | 3-wire (with control box) | 2-wire (no control box) | Source |
|---|---|---|---|
| Franklin 1/2 HP | 2,000 W | 3,000 W | Franklin AIM, “Generator Minimum Rating” (3-wire and 2-wire tables) |
| Franklin 3/4 HP | 3,000 W | 4,500 W | Franklin AIM |
| Franklin 1 HP | 4,000 W | 6,000 W | Franklin AIM |
| Franklin 1.5 HP | 5,000 W | 7,500 W | Franklin AIM |
| Franklin 2 HP | 7,500 W | — (no 2 HP row in the 2-wire table) | Franklin AIM |
| Franklin MonoDrive 1/2 · 3/4 · 1 HP (constant pressure) | 2,000 · 3,000 · 3,500 W | Franklin MonoDrive manual | |
| Franklin SubDrive20 · SubDrive30 (SubDrive Connect, constant pressure) | 6,000 · 7,000 W | Franklin SubDrive Connect manual | |
| Franklin SubDrive15 (constant pressure; not listed in the current SubDrive Connect manual) | 3,500 W | Franklin AIM, 2023 edition (superseded) | |
| Grundfos SQ/SQE, motor 0.7 · 1.15 · 1.55 · 1.85 kW | Recommended 1,500 · 2,500 · 3,200 · 3,500 W (minimum 1,200 · 1,900 · 2,600 · 2,800 W) | Grundfos SQ/SQE manual |
- Use rated (running) watts, not peak. Grundfos: generator 50% above the pump’s input power (P1).
- Add freezer, furnace blower, lights if they run at the same time.
- Derate for altitude or natural gas, per the generator maker (Franklin).
- Franklin 2-wire in kVA: 3.75 / 5.7 / 7.5 / 9.375 (1/2 to 1.5 HP). The generator must hold 65% of nameplate voltage during start.
Will a 3,500-watt generator run my well pump?
| Pump | Minimum | 3,500 W rated output |
|---|---|---|
| 1/2 HP, 3-wire | 2,000 W | Meets the minimum, with 1,500 W left for other loads |
| 1/2 HP, 2-wire | 3,000 W | Meets the minimum, with little left over |
| 3/4 HP, 3-wire | 3,000 W | Meets the minimum, with little left over |
| 3/4 HP, 2-wire | 4,500 W | Below the minimum |
| 1 HP, either type | 4,000 / 6,000 W | Below the minimum |
| MonoDrive 1 HP (or the older SubDrive15) | 3,500 W | Exactly at the minimum, nothing left |
If 3,500 W is the peak rating, the answer changes.
Three mistakes the pump manuals warn about
- Adding a soft starter to a small generator. Franklin: a minimum-sized generator already “acts as a soft start to the motor”, and adding a reduced voltage starter “causes the potential for severe motor damage and failure due to low voltage”. Smaller generator? Use a constant-pressure drive or a built-in soft start (Grundfos SQ: 2-second run-up).
- Running out of fuel with the pump on. Franklin: start the generator first; stop the motor first. A coast-down or empty tank can damage the thrust bearing. Pump breaker off before refueling; back on after restart.
- Ignoring frequency. At 1–2 Hz below 60 Hz the pump won’t meet its curve; 1–2 Hz above may trip the overloads (Franklin). Low pressure? Check Hz with a meter.
Connect it safely
Our drawing of one common arrangement, not a wiring plan. Distance from CPSC; transfer switch requirement from Franklin Electric AIM (2026, p.6). Have a licensed electrician design the connection.
- Never plug into a household outlet.
- Outdoors, 20 feet away (CPSC): never in a garage, basement, crawlspace or shed; exhaust away from doors and windows. Battery or battery-backup CO alarms on every level; prefer CO shut-off.
- Hire an electrician for the inlet, transfer switch or interlock (main panel work).
Unknown HP: well pump size calculator. Pump cycling every few seconds? Fix it first: pressure switch settings.
FAQ
What size generator do I need to run a 1 HP well pump? 4,000 W rated output for 3-wire, 6,000 W for 2-wire (Franklin). Pump only; add other loads.
Will a 3500 watt generator run a well pump? Yes for 1/2 HP, 3/4 HP 3-wire or MonoDrive 1 HP, if 3,500 W is rated output. No for 3/4 HP 2-wire or any standard 1 HP.
Do I need a 240 volt generator for my well pump? Usually. Only Franklin’s 1/2 HP 4-inch motor comes in 115 V. A double-pole pump breaker means 240 V.
Sources
- Franklin Electric, AIM Manual 60 Hz Submersible Systems, 2026 edition (M1311 06-26) — “Engine Driven Generators” / “Generator Operation” p.6; “Generator Minimum Rating” (3-wire or 3-phase Motors; 2-wire Motors) p.7; “2-Wire Motor Solid State Controls” p.11; “3-Wire Control Boxes” p.12; 4-inch/1-Phase Motor Specifications p.39
- Franklin Electric, SubDrive/MonoDrive Constant Pressure Controller Installation Manual M1560 (225417101 Rev 7, 05-13) — “Generator Sizing for SubDrive/MonoDrive” p.22
- Franklin Electric, SubDrive Connect Owner’s Manual 226203103 (©2024) — “Generator Sizing” p.10
- Franklin Electric AIM Manual, 2023 edition (M1311 10-22; superseded by the 2026 edition) — Table 73 “VFD Generator Sizing / SubDrive” p.90, used only for SubDrive15 (3,500 W), which the current SubDrive Connect manual no longer lists
- Grundfos, SQ/SQE Installation and operating instructions, 96160909 (11.2024), section 2.2 “Generator operation” and section 10 “Technical data
- U.S. Consumer Product Safety Commission, “Winter Storm May Knock Out Power; CPSC Warns of Deadly Carbon Monoxide and Fire Risks” (2026-02-19)
All sources checked 2026-10-05.
Method: we put Franklin Electric’s 3-wire, 2-wire and constant-pressure generator minimums and Grundfos’s SQ/SQE figures in one table, converted kW to watts, and compared each minimum with 3,500 W of rated output to see which pumps that size can start. Page references are in the sources above. We have not tested generators.
Editorial note: This page was researched and fact-checked by the County Road Home team against the manufacturer documents and public sources listed above. AI tools assisted with drafting; every number was checked by an editor before publishing. We are not licensed well contractors or electricians. For electrical work, use a licensed professional. Found an error? Email hello@countyroadhome.com. Last checked: 2026-10-05.