I decided to do the electrical work to power the pump. Previously I hired an electrician to wire the main panel, because I thought I was in a hurry and I didn't think I knew enough to do it right. I watched & learned, and this time I was ready to do it myself.
You might think I wanted to save money, but that doesn't make much sense. I'm much slower than a pro, and only a small part of the price is the labor. Mostly, I like doing electrical work, so it was an opportunity for play.
Here's the setup:
- The main panel w/ power meter is at the edge of the property.
- A 150' long, 3' deep trench leads from the main panel to a convenient spot near the pump.
- The 240V 15A pump sits in a large, underground concrete tank.
- A "transducer" also lives in the tank, to measure the water level.
- A small computer watches the inputs from the transducer, and turns the pump on as appropriate.
The plan was to run 2" conduit from the main panel to a 6" x 6" pressure treated post near the pump. The post would also hold the septic control/alarm panel. Then there are two 3/4" conduit runs from the control panel to the pump chamber - one for the pump, and one for the transducer. I don't really know why they get two separate conduits. Maybe to avoid electromagnetic interference.
I started with the idea that the pump needed a 240V 15A circuit, which normally requires 14 gauge copper wire. Long conductors should be a little thicker, because voltage drops over distance. So I was thinking 12 gauge wire. 12 gauge is very common, so that's good news.
A 240V circuit has two hot conductors, and possibly a ground. There's no neutral wire. If you have two hots and a neutral, it's a 240V/120V circuit. Ovens and dryers are usually 240V/120V. Ovens often have a 120V clock and a 120V receptacle for your coffee maker or whatever. Dryers have a 120V motor, so the gas-heated model can use the same motor and not require special wiring. By comparison, electric hot water heaters are usually 240V only.
I wanted to include a convenience receptacle on the post as well, so I was going to pull a second set of conductors (hot + neutral) for that purpose.
I was considering the option of putting a subpanel on the post, and taking short-run circuits off that for the pump & receptacle. It seemed more useful in the long term to have a subpanel, but also more complicated and expensive. I wasn't sure which to choose, so I asked on the DIY Stackexchange site. Their answers were helpful but inconclusive. They did suggest oversizing the pump circuit even more, to 10ga, to further protect the pump motor.
Thinking about that problem, I realized a great benefit of the subpanel approach: it leaves plenty of headroom for one large load. Consider:
Option A: two 15A circuits, 100' long, on 12 gauge conductors. One circuit carries a 10A load. According to my 2011 NEC handbook (215.2(A)(4) and Chapter 9, Table 8), the voltage drop would be 2 x 100' x 1.21 ohm/kft x 10A / 1000 = 2.4V.
Option B: one 30A subpanel, 100' long feeders, on 8 gauge conductors. One branch circuit carries a 10A load. Voltage drop is 2 x 100' x 0.764 ohm/kft x 10A / 1000 = 1.5V. That's because the 10A load gets to take advantage of larger conductor that it shares with the rest of the subpanel. When the subpanel's other circuits are idle, which is most of the time, there's a lot of headroom available.
Using a subpanel would be good for the pump motor, so I decided to go that way.
So the plan is 8 gauge feeder conductors through 150' of 20" conduit from the main panel to the subpanel. A 240V 15A breaker to the pump circuit with 12 gauge wire. A 120V 20A circuit with 12 gauge wire to a convenience receptacle on the same post.
Next: shopping!
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