Electrical – wire greenhouse subpanel

electricalelectrical distribution

250ft from main panel in house to greenhouse out back. Greenhouse will have propane heater and one 120v outlet.

Today I dug 100 ft trench to the house 1ft deep. I put 1" and 3/4" conduit in the trench along with a 12AWG THWN solid copper wire (tracer). The 1" conduit will have a MDPE gas line, the 3/4" conduit will have THWN wire. I know MDPE can be direct burial but I like the idea of having it in 1" conduit just in-case.

I'm running 4 THWN conductors 10AWG. At the house, 10-3 NM-B will splice to the THWN in the LB as it exits the house. In the greenhouse I have a small outdoor rated 70A subpanel; 2 circuits.

Questions:

1) Do I need a grounding rod at the sub panel? I know it should NOT get tied to neutral.

2) I will install 30A GFCI breaker in main panel, 1 pole or 2?

3) Can I install two 15A breakers in sub panel, each would be 120V?

4) The breakers in sub panel can be non-GFCI, correct?

thanks.

Edit – Update

  1. Trench is 18" now, my back is out also!

  2. I will make sure the LB I use is labeled with volume calc.

  3. I changed to a 30A 2 pole non-GFCI in main panel; then put the GFCI's in the sub panel. I'm hoping this would cut down on nuisance GFCI trips.

  4. HomeDep0t won't take back my 10AWG custom cut THHN wire, I really want to switch to 8AWG for voltage drop. But the subpanel circuits will have very light loads so if I can't find a buyer for the 10AWG, then that's what I'll use.

  5. Can someone review the connections in pic? This is Square D 70A Homeline Outdoor Load Center. I added the wiring diagram to the pic.

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Best Answer

First off -- I would redo the trenching job as it's not deep enough. 18" is the Code specified minimum for electrical and locations vary from 18" minimum cover to 24" minimum cover for gas -- going deeper yet is an option though. Also: running the gas line inside a conduit sleeve could confuse the next bloke who has to work on this.

As to the co-location of gas and electrical -- it does not appear there are any Code mandated separation requirements for this. If separation is desired, though, I would use a minimum of 36" horizontal separation (i.e. backhoe bucket width) -- any less, and I might as well have put them in the same trench anyway!

I would put a grounding rod at the subpanel -- Code requires this for outbuildings. I would then use a 2 pole 30A regular breaker for the feeder breaker and then 15A GFCI breakers in the external subpanel -- using a 2 pole 30A GFCI to protect the feeder sounds OK, until you realize that a long run of wet wire is going to add significant leakage current and could trip the GFCI on its own even with the wire intact due to excess capacitive coupling to the ground wire.

Oh, and I would double check my LB to make sure it's volume marked i.e legal to make the splice in -- it should be, but it's best to be safe than sorry.

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