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We're installing a septic system on otherwise undeveloped land. Eventually there will be a house near the septic system, but that could easily be years away. The electric service entrance is on a post on the edge of my property.

The septic system has a 240V 15A pump. We're putting 140' of 2" conduit in a trench to provide power to a post with the pump's control box.

I'd also like to put a 120V recep on the post, because it would be very convenient to have one there.

I could put breakers in the main service panel, one 240V for the pump and one 120V for the recep, and run those through the conduit. Or I could put a subpanel on the post, and run just one circuit from the main panel.

2 separate circuits seems simpler, but it would be nice to keep the space open in the main breaker panel.

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FYI, I went with the subpanel. I'm glad I did, because the septic control/alarm panel wants its own circuit, too. Total = 3 circuits. Also, the subpanel approach leaves some headroom for the pump's startup load. –  Jay Bazuzi Jun 17 '11 at 3:40
    
And the inspector signed off on my work. Yay! –  Jay Bazuzi Jun 23 '11 at 5:01
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2 Answers

up vote 5 down vote accepted

Jay, you have a couple of requirements here. First is to supply 240VAC to your pump. Since 140 feet is a long run and the pump is rated at 15 amps, it would be wise to oversize the wire gauge to 10/3 UF or DB twisted AWG. This will avoid excessive voltage drop, thus protecting your pump. You must still have it fused at the main box at the recommended amp size breaker. Do not put a higher rated circuit breaker than the pump requires. This would allow pump to overheat before reaching the amps necessary to trip the breaker. The same theory applies to the 120VAC GFIC outlet. Oversize the conductors, but do not overfuse at the panel.

A weather tight subpanel would be a good way to go, if you can absorb the extra cost. It will allow you to have breakers closer to your pump and outlets. This really would only be for the convenience to not having to walk a distance to reset them if you are working in the pump area. As Niall already said, you will only save one run of 12/2 or 10/2 and one breaker location in the main with a subpanel, so there is really no financial advantage.

Be sure to pull an extra fish line and leave it in the conduit for future use or maintenance, and don't forget your caution ribbon just below ground grade when you backfill your trench. Good Luck.

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Since you're digging the trench anyway is it worth running another empty conduit for phone/cable TV/etc at the same time or will the phone company insist on burying their own trench? –  mgb Jun 3 '11 at 15:24
    
@mgb- in Jay's case, the conduit run is probably not going to future house location, so not applicable. However, if not crossing a public way, an extra telcom conduit with pull lines would be fine if laid per utility specs. You will need a pole(riser)on your side of the street however. In most areas phone and cable will not bury cable unless it is required by local ordinance. They will supply the wire, but the expense to bury it is usually the homeowners. –  shirlock homes Jun 3 '11 at 17:23
    
It occurs to me that a subpanel means all the branch circuits share same copper. That means that most of the time the pump will be protected by the extra copper. –  Jay Bazuzi Jun 3 '11 at 22:47
    
@mgb and @shirlock homes: actually, it is very close to the future home location and the incoming phone line is on the same post as the power meter and I intend to run the wire in the same trench as the power, in its own conduit. I am also burying a 3rd conduit, just in case. I don't know if it will ever be useful, but it's way cheaper than digging a new trench. –  Jay Bazuzi Jun 17 '11 at 3:47
    
My septic installer just backfilled the trench but didn't put in a caution tape. Damn. I will hand-dig one in before the ground gets hard. –  Jay Bazuzi Jun 23 '11 at 5:03
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I believe you need to have the subpanel controlled from a double-pole breaker in the main panel, so you would only be saving one slot by having a subpanel.

Looking at the relative costs, I think you'll save money running two circuits:

  • 10/3 (assuming a 30A subpanel) costs more per linear foot than 14/3 and 14/2 combined. (You might need thicker gauge for the run length you have, but the relative cost should be similar).
  • cost of a subpanel vs. none.
  • cost of the extra breaker in the main panel.

Unless you're certain to need more power at the pump site, I'd say to put in the two circuits.

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