wire greenhouse subpanel

Question

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.

Answer 1

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.

Answer 2

Aside from the issues about the trench in comments...

Voltage drop will be pretty wicked if loaded to capacity, at 250 feet one-way run on 10Ga copper.

http://www.electrician2.com/calculators/vd_calculator.html

1. You need a grounding system at the sub-panel. If there's a concrete foundation and it was properly set up for a UFER ground, you would not need a rod; if you cannot verify the ground resistance of one rod, you might need two rods (at which point code magically stops requiring measuring the ground resistance - it's cheaper to use two rods than to get the resistance measured, in general, so it's the common approach.)
2. 2 pole breaker is standard for a 4 wire subpanel feed - gives you 240 V, or 2x120V

3. You can install 4 15A single pole breakers and load them to capacity (two 15s at 120 = 1 15 at 240, so 30 at 240 will run 4 15s at 120, if they are distributed to both legs as usual.)

4. (really 3A) You can also install more breakers, if the LOAD on the breakers is not more than 30A total. You don't need to design based on the full load of the breaker if the circuit will only have 1000W on it - you design for the actual load.

5. (really 4) GFCI is covered by the feed, yes.