Short answerPick the smallest pipe that keeps friction under about 2 ft per 100 ft and velocity under 5 ft/s at your flow. For 1 to 4 gpm over a few hundred feet, 1 in polyethylene usually does it.
Key takeaways
- Friction falls with nearly the fifth power of pipe diameter: one size up often cuts it by two thirds.
- Aim for under about 2 ft of loss per 100 ft and under 5 ft/s velocity.
- For 1 to 4 gpm over a few hundred feet, 1 in polyethylene usually does the job.
How the tables are computed
Every figure below is the Hazen-Williams equation in US units: friction loss per 100 ft = 0.2083 x (100 / C)^1.852 x gpm^1.852 / d^4.8655, where d is the inside diameter in inches. C is a roughness coefficient: 140 for polyethylene, 150 for PVC, both standard design values for smooth plastic pipe in service. Velocity is 0.4085 x gpm / d^2 feet per second.
Polyethylene water pipe (SIDR) is sold by its inside diameter, so the nominal size is the ID. Schedule 40 PVC is sold by its outside diameter, so its ID is a little bigger than nominal (1 in Schedule 40 is 1.049 in inside).
Polyethylene (PE) pipe, feet of head lost per 100 ft
| gpm | 1/2 in | 3/4 in | 1 in | 1-1/4 in | 1-1/2 in | 2 in |
|---|---|---|---|---|---|---|
| 0.5 | 0.90 | 0.13 | 0.03 | 0.01 | 0.00 | 0.00 |
| 1 | 3.26 | 0.45 | 0.11 | 0.04 | 0.02 | 0.00 |
| 1.5 | 6.90 | 0.96 | 0.24 | 0.08 | 0.03 | 0.01 |
| 2 | 11.8 | 1.63 | 0.40 | 0.14 | 0.06 | 0.01 |
| 3 | 24.9 | 3.46 | 0.85 | 0.29 | 0.12 | 0.03 |
| 4 | 42.4 | 5.90 | 1.46 | 0.49 | 0.20 | 0.05 |
| 5 | 64.2 | 8.92 | 2.20 | 0.74 | 0.31 | 0.08 |
| 6 | 89.9 | 12.5 | 3.08 | 1.04 | 0.43 | 0.11 |
| 8 | 100+ | 21.3 | 5.26 | 1.77 | 0.73 | 0.18 |
| 10 | 100+ | 32.2 | 7.94 | 2.68 | 1.10 | 0.27 |
| 15 | 100+ | 68.2 | 16.8 | 5.68 | 2.34 | 0.58 |
| 20 | 100+ | 100+ | 28.7 | 9.68 | 3.99 | 0.98 |
Schedule 40 PVC, feet of head lost per 100 ft
| gpm | 1/2 in | 3/4 in | 1 in | 1-1/4 in | 1-1/2 in | 2 in |
|---|---|---|---|---|---|---|
| 0.5 | 0.27 | 0.07 | 0.02 | 0.01 | 0.00 | 0.00 |
| 1 | 0.99 | 0.25 | 0.08 | 0.02 | 0.01 | 0.00 |
| 1.5 | 2.10 | 0.53 | 0.17 | 0.04 | 0.02 | 0.01 |
| 2 | 3.58 | 0.91 | 0.28 | 0.07 | 0.03 | 0.01 |
| 3 | 7.58 | 1.93 | 0.60 | 0.16 | 0.07 | 0.02 |
| 4 | 12.9 | 3.29 | 1.02 | 0.27 | 0.13 | 0.04 |
| 5 | 19.5 | 4.97 | 1.53 | 0.40 | 0.19 | 0.06 |
| 6 | 27.4 | 6.96 | 2.15 | 0.57 | 0.27 | 0.08 |
| 8 | 46.6 | 11.9 | 3.66 | 0.96 | 0.46 | 0.14 |
| 10 | 70.5 | 17.9 | 5.54 | 1.46 | 0.69 | 0.20 |
| 15 | 100+ | 38.0 | 11.7 | 3.09 | 1.46 | 0.43 |
| 20 | 100+ | 64.7 | 20.0 | 5.27 | 2.49 | 0.74 |
Picking a size
For most solar stock wells pumping 1 to 4 gpm, 1 in poly does the job over a few hundred feet and 1-1/4 in covers runs of a thousand feet or more. Drop pipe inside the well is often matched to the pump outlet; the VEVOR pumps we list have 1/2 in (12V model) and 1 in (24V and 48V models) outlets.
- Find your flow in gpm from the sizing calculator.
- Read across to the first size that loses less than about 2 ft per 100 ft and keeps velocity under 5 ft/s.
- Multiply by the run length in hundreds of feet and add 10% for fittings. If that is more than a tenth of your lift, go up a size: pipe is cheap next to panels.
Questions
- Is 3/4 in pipe big enough for a solar well pump?
- At 1 to 2 gpm over a short run, often yes: 3/4 in poly loses about 1.0 ft per 100 ft at 1.5 gpm. At 4 gpm it loses about 5.9 ft per 100 ft, and over 500 ft that is about 30 ft of head. Check the table at your flow.
Put your own gallons, lift and pipe in and see every step of the working.