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We are building a 2-story detached garage. The upstairs will be my wife's studio. She is a painter and quilter. The lower level will be a standard garage for tools, etc. I do not plan to weld but who knows about the future.

I am thinking a 60 amp subpanel. The distance would be 70 feet or less from main panel to sub panel. I plan to run PVC pipe underground. What depth does that have to be in Kentucky? Also would using burial cable inside the PVC be added protection? Number of conductors and size? We will have a hotel type air conditioner heater that calls for 208V-230V

  • You can get away with #6 copper for a 60A panel, but you'll find that running larger aluminum wire (like 1/0) is cheaper and might allow you to upgrade the panel later. It does have some terminal connection considerations, though. I'll leave it to the pro sparkies we have around for a complete answer. – isherwood Oct 16 '18 at 20:29
  • Is digging down a couple feet for a trench an option for you, or will you hit bedrock well before then? – ThreePhaseEel Oct 16 '18 at 23:08
  • Digging down will not be a problem at all – Scott Ansert Oct 19 '18 at 18:47
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PVC is inexpensive conduit, which can be good, but it requires an expensive trench (whether or not you are in Kentucky) when running power.

More expensive Rigid or IMC steel conduit may let you have a less expensive trench, and also serves as the Grounding (not Grounded) conductor. You need 4 conductors for typical 120/240 USA/Canada service - Two Hots, one Grounded (Neutral), one Equipment Grounding Conductor (which can be the conduit for steel conduit. Otherwise, it's 4 actual wires.)

The minimum cover requirement (general) is 6" for Rigid or IMC, and 18" for NMC (that is, PVC or other non-metallic conduit.) That is from the surface to the closest part of the conduit (not the bottom of the trench, the top of the conduit.)

Depth can be reduced by adding concrete on top. Depth increases (generally) for running under driveways, though for "one or two family dwelling driveways used only for dweling related purposes" it remains at 18" for NMC (but it increases to 18" if using rigid or IMC.)

Using cable (rather than individual conductors) inside conduit is madness, generally speaking, as it's very hard to pull and greatly increases the size of conduit you need for the fill calculations. If you want to "add protection" use only schedule 80, no schedule 40, and carefully backfill with sand or finely screened material around the conduit rather than pushing dirt/rocks back on top of the conduit. Or contemplate concrete cover, and save on the trench depth.

Add a separate communications conduit (possibly empty at present, but so handy if you ever run network wire or fiber) while you have a trench open, and be sure to place "buried electric line below" tape in the top 6" of the trench.

As for wire size, it depends on what you would actually like to allow for - if only 60A service, 3 gauge aluminum would be adequate and far cheaper than copper - 2, 1, or 1/0 would allow more amperage if desired at some point in the future (or now, since it's cheaper in the long run to simply buy a bigger panel once, than to buy a smaller panel and replace it with a bigger panel later.) The amperage draw of your air conditioner may be a deciding factor, and you have not provided that information - if it uses potentially 30 amps and you want a typical "shop service" of 50-60 amps available without having to worry about whether the air conditioner is running, you might want to plan for 100 amps to be safe. If it's only 15 amps, that might not be as much of a concern, or you could go for 75 amps (but it's an odd size - a 100 amp panel may be cheaper, due to volume of sales.)

For the average home electrical worker, I think it's safest to run all the wire, then hire an electrician for a short stint actually making the aluminum connections - the newer alloys are not the scary stuff the old 10/12Ga aluminum was, but it's still a bit of a process connecting them properly, and experience in doing that is worth paying a pro to do, in my opinion. If not, read up carefully on the proper procedure and follow it, including joint goop, stainless steel brushes, and using a torque wrench on the connecting bolts.

2 inch schedule 80 provides plenty of room for 4 1/0 conductors. You can downsize the grounding conductor and possibly get down to an acceptable fill for 1-1/2", but it will be a miserable pull, so I'd stick with 2 inch and a comfortable pull. If going with 2 or 3 AWG, 1-1/4 inch would do, but 1-1/2 inch (or 2 inch) would be easier. Your grounding conductor only needs to be 6 or 8 AWG aluminum for 100 or 60 amp service (i.e. 3x 1/0 and 1x 6AWG is perfectly acceptable for 100 amps.)

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I'd go with Schedule 80 PVC here

Since digging down (trenching) to the 22-24" needed for PVC conduit is not an issue for you, I would use 2" Schedule 80 PVC conduit (not plumbing pipe!) with prefabricated wide sweeps at each end to turn the run aboveground.

This provides plenty of space for an upgrade to 100 or 125A down the road, even if you are only putting in 60A now, as well as providing mechanical damage protection. A 1" schedule 80 PVC secondary conduit for telecoms is also a good idea while you have the trench open (conduit is cheap compared to having to rent the trencher again).

Use individual wires, NOT cable here

Pulling cables through conduits is a recipe for frustration and swears, in addition to their egregrious use of conduit fill compared to individual wires. Given that you're going with 60A for now, I'd use 6AWG copper THHN/THWN-2 for the hots and neutral with an 8 or 6AWG copper grounding wire (insulated or bare).

Going larger (with 1AWG or 1/0AWG aluminum hots and neutrals and a 6AWG copper grounding conductor) is possible now, but shouldn't be needed as most PTHPs (Packaged Terminal Heat Pumps, or what you are referring to when you say "hotel style air conditioner/heater" -- there's no reason to get a PTAC/electric heat unit over a PTHP in this day and age) made today aren't going to chew up more than 16A at 230VAC, maximum, as long as you keep the electric standby heat reasonably sized (under 4kW).

You'll need grounding electrodes at the structure

In addition to running a full four-wire feeder, you'll need to fit a pair of ground rods (driven 8' deep and at least 6' apart) and a 6AWG grounding electrode conductor connecting them to each other and to the subpanel for the structure. You'll also need to make sure that the bonding (green) screw or bonding strap in the subpanel has been removed (or is not present in the first place), and you'll need separate neutral and ground bars in the subpanel as well.

Don't skimp on the panel

Most "60A" panels are rather small -- I would use a 125A, 24- or 30-space, main breaker panel here to provide adequate room for future expansion. The oversized subpanel main breaker can be left in, as it simply is serving as a feeder disconnecting means for the structure. Last but not least, you'll need to make sure that the panel and breaker lugs are torqued to specification with an inch-pound torque wrench or screwdriver (this is a new requirement in the 2017 NEC).

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