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I will be installing a 60amp sub panel in my hoop house greenhouse that I just built and am not sure what exactly I need to buy to run 140 ft direct burial from the main service panel(the power pole itself) to the sub-panel in my greenhouse. I'm not sure what cable(s) I need to get exactly, I believe I need 6AWG but I don't know what I need to ask for exactly at Home Depot. Do I get something with 2 hot, 1 neutral, and 1 ground cable and run all of that underground to my sub-panel do I need copper or aluminum? I'm a little overwhelmed with all of the different types of cables.

Also, it sounds like I need to install a grounding rod at the greenhouse as well, so I just run a copper wire from the grounding rod straight to the sub panel as well as the cable from the main service panel. I guess they would both just be installed inside the sub-panel on the same grounding bar?

Thanks for your help

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  • What sort of loads will you be powering in the greenhouse? – Harper - Reinstate Monica May 19 '20 at 1:09
  • What make and model is the panel at the greenhouse? – ThreePhaseEel May 19 '20 at 1:11
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    Trenches are expensive. Conduit is cheap. Direct burial is therefore almost always a Bad Idea .vs. using conduit. You may also find that conduit and wire CAN cost less from a "real electrical supplier" than from HD. – Ecnerwal May 19 '20 at 1:15
  • i already have the trench dug. I used a backhoe to dig a pond and went ahead and dug the trench too. I haven't bought the panel for the greenhouse yet. I wanted to figure out what wire I needed exactly. I'm a little overwhelmed with what to ask for at Home Depot. The greenhouse will power some lights, a water pump, and some fans. – Cdubdub May 19 '20 at 1:24
  • Oh, and what make and model is the main panel at the pole for that matter? – ThreePhaseEel May 19 '20 at 1:24
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Fat conduit and fat panels are your friends here

Given that you've already gone through all that work to dig a 140' trench, why would you want to have to do that again if you ever need more than 60A at your greenhouse, as you would if you had to replace a direct buried cable? PVC conduit is not much more expensive, allows you to use a slightly shallower trench depth (21-22" instead of 25-26"), and provides you with the ability to have much more expansion room in the future, as a 1.5" PVC conduit can easily accommodate a 125A feeder.

Inside that conduit, 3 6AWG stranded copper THHNs (hot, hot, neutral) with an 8AWG or 6AWG bare copper grounding wire (to return wayward utility electricity to the utility) are the simplest way to ship 60A out to your greenhouse. You'll of course need the correct 60A two pole breaker for your pole's panel, as well as two 8' grounding rods driven 8' or more apart at the greenhouse and connected to the subpanel at the greenhouse with 6AWG bare copper to form a grounding electrode system so that wayward natural electricity has a path back to nature.

Furthermore, when you're going subpanel shopping, there's absolutely no reason to skimp on the subpanel, as again, saving a few bucks now is not worth the hassle a full panel causes down the road. You'll want to get a 24-space or 30-space, 100A or 125A, main breaker panel for the greenhouse and fit a matching accessory ground bar or two to it as well as removing the neutral-to-ground bonding screw or strap from the panel. This way, neutral and ground are separated at the subpanel, as they should be, so that normally flowing power doesn't try to return on the grounding system and cause problems in the process.

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  • Awesome, Thank you. Will I not need to get 4AWG wire for the voltage drop at 140ft though? So this wire would be perfect to pull through conduit huh? lowes.com/pd/… – Cdubdub May 19 '20 at 15:26
  • Uh, no. If you are upsizing the wire, use aluminum wire (even larger aluminum wire - it will cost you far less than smaller copper. There's a breakpoint around 6 where not much aluminum being available and the "cost of the insulation" .vs. "cost of the wire inside it" make copper still sensible. 2AWG Aluminum will set you back less than half the price of 4 AWG copper. – Ecnerwal May 19 '20 at 16:22
  • @Cdubdub there's no reason to upsize for voltage drop control for such a relatively short run, anyway – ThreePhaseEel May 19 '20 at 22:28
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Hi Cdubdub: You have most of it right. There are tons of information here regarding sub-panel installation. In a nutshell, you'll need 6 gauge wire (if THHN in conduit) for the hots and neutral and I think you can get by with an 8 gauge for the ground (not sure about that) . You'll also need to install ground rods and connect them to the grounding busbar in the sub-panel. Do not bond (connect) the neutral and ground in the sub-panel. The neutral must be isolated (floated) from the ground.

Do a few searches here on sub-panels and you'll get a wealth of information.

EDIT: Ecnerwal makes a great point. Put that in conduit...much safer in the long run and not expensive.

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