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Location: Wisconsin

I am having the electric company transition my wiring from overhead to underground, upgrade my service (it might only be 60amp right now) to 200amp, having another company replace my panel with a new one (200amp), and installing a pedestal to make it solar ready if I ever choose to go that route.

The pricing on that portion of it seemed pretty fair but they also estimated the power to the detached garage and it's way too high in my opinion. Much higher than I would have imagined. They were estimating $25 per foot for (hand?)trenching and running wire (total of $3375) and then a few hundred more for some basic light switch installs, etc. This seemed rather expensive to me.

The house is roughly 135 feet away from the garage. I will only add a couple outlets (or an outlet strip), light switch, and some lighting. I expect this to basically be a 20amp circuit.

According to this voltage calculator, it would recommend this for copper:

1 conductors per phase utilizing a #6 Copper conductor will limit the voltage drop to 1.99% or less when supplying 20.0 amps for 135 feet on a 120 volt system.

and this for aluminum

1 conductors per phase utilizing a #4 Aluminum conductor will limit the voltage drop to 2.06% or less when supplying 20.0 amps for 135 feet on a 120 volt system.

I'm assuming those are both THHN, whatever that means. I'm not sure I am looking at the proper wires from HomeDepot but I assume the difference will be negligible and it would cost me a few hundred dollars for the wire itself. Then to rent a trencher it would only cost about $100. Then to do the wiring it would mostly just cost me time.

Is this something I could do myself? Am I missing some aspect of this that would make it harder than it really is? What wiring should I be looking at? Does it make sense to use conduit here or can I get away with in-ground wiring?

  • Can you post a link to what you're looking at on HD's site? – ThreePhaseEel Sep 20 '17 at 3:12
  • @ThreePhaseEel Nothing specific at all. I was just checking general prices. Not really sure what to get yet – Joe Phillips Sep 20 '17 at 3:39
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    Construction trades are expensive when electricians do them. You know you only need 6" depth if you use Rigid conduit if it's not going under a vehicle pathway. – Harper Sep 20 '17 at 4:19
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    What exactly was the electrician's spec for the work that costs $25/ft? Are they running 220, and locating a sub-panel in the garage? That's what I would do! Even if only planned on using a single circuit for a light and plug today. That's future proofed. It also has better resale value. – Tyson Sep 20 '17 at 11:18
  • @Tyson It just specified a 20amp circuit. No subpanel mentioned – Joe Phillips Sep 20 '17 at 15:32
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Thing about a trencher is it's real easy to hit other utility lines and suddenly you know why they wanted $25/ft. One way to work around that is use Rigid conduit. This only needs to be 6" below surface (top of pipe 6" below surface) where vehicles do not drive over. Expensive pipe, but easy/safe to trench.

6Cu/4Al for a 20A circuit. Seriously?

A little bit of voltage drop never hurt anything, as long as it stays little. Realistically you won't run 20A on a 20A circuit, the legal maximum for continuous use is 16A. Let's look at these.

First what if you just used 12AWG? At 5A (actual draw), your drop is 1.96%. At 10A actual, your drop is 3.91% which is absolutely fine and won't hurt a thing. At 16A, 6.26% - you might see an incandescent dim very slightly when you turn a 1500W heater on, but again, not a big deal. You have worse with a power tool on an ordinary 16AWG extension cord.

If we bump to 10AWG -- 5A 1.19% -- 10A 2.37% -- 16A: 3.78% That seems like plenty to me. I'm not even going to look at 8AWG. You get the idea: these wire upgrades are pretty optional.

But if you're so worried about voltage drop that you'd spend almost $2/foot for wire, let's look at it another way.

Transformers!

I routinely see 5kVA (5000 watt) 240-480/120-240 transformers on Craigslist for about $100. Bring 240V from the house. When voltage doubles, voltage drop quarters, so this works very efficiently. Feed the transformer primary (jumpered for 240V). Jumper the secondary for 120V and feed a tiny "main" 120V panel. If we aim to draw 16A@120V, that's only 8A@240V. Voltage drop calc says: 2.42% drop with 14AWG wire. 1.57% drop with 12AWG. Very acceptable!

Suppose we later decide we want alot more, and want to "max" that 5 KVA transformer. Fine, just push the 12AWG to its 20A limit, and have 3.91% voltage drop, again quite livable.

So the transformer plan is one you can keep in your back pocket in case the voltage drop on your 12AWG wire starts to annoy, or you just want more power. Simply install a subpanel, maybe a 6-8 circuit type (2 for building shutoff, 1 for your primary load) and wire it so it could either be a subpanel or a main panel. Once it's more than a simple circuit, you will need a grounding rod system at the garage.

Speaking of $2/foot, stop buying at Home Depot. Period. Done. Home Depot is cheap on the things they know you'll price-check them on, like rolls of #14 Romex. But by and large, they charge you "gotcha" pricing, because they know you have no idea where to find building supplies otherwise. HD's "by the foot" price is simply outrageous. They want $5 for 120mm square boxes I pay $2 for. Five for $2 for couplers I pay 16 cents apiece for. It's completely insane. Their selection is also terrible and they don't stock key parts of wiring systems like surface conduit. Find a locally owned proper electrical supply house, or several, and cross-price them on the wire and parts you need. Get a shop whose prices you like and get loyal to them.

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    I recently used 2 10kva transformers to go 475' for a 240 panel. Great deal on used xformers online. By boosting the voltage and transforming back down I was able to save a huge amount on wire and install work because of the smaller wire size. This example is a bit excessive for OP needs but smaller x formers get really cheap. – Ed Beal Sep 20 '17 at 13:44
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You need THWN

Since underground conduit is assumed to be full of water, you need wet-location wire, such as THWN. Fortunately, garden-variety building wire you get for conduit jobs is dual-rated THHN/THWN 99.9% of the time, so finding THWN is just as easy as finding THHN.

Also, use a bare copper ground wire -- you'll need to use 6AWG for this if you use the 6AWG copper or 4AWG aluminum hot wires because of 250.122(B) which requires the ground to be upsized when upsizing wires to control for voltage drop, but you'll still be able to save on conduit fill area that way. The phase and neutral wires can be copper or aluminum as they're running from breaker lug to breaker lug most likely anyway, and breaker lugs have no trouble with aluminum wire provided they're torqued correctly.

Put in the fattest conduit you can get

This is a job for the fattest schedule 80 PVC you can lay your hands on -- it'll make future upgrades easier, and also make your pulling job easier especially since you have 135' and two 90deg sweeps to contend with. Also, make sure to get Schedule 80 -- it's more resistant to accidents than Schedule 40 is, which is important for the terminations.

You'll need a disconnect at the garage (and perhaps a ground rod too)

You'll need some sort of disconnecting means at the garage. A spa disconnect that's been retrofitted for straight-up 120V use works, or you can even use a 20A light switch, but the best plan is to put a decent sized main breaker subpanel in a NEMA 3R enclosure in there (a 20 slot, 100 or 125A unit with a backfed 20A main breaker would be my bare minimum), making sure the bonding screw gets pulled (unless you go with Harper's transformer plan) and the neutral and ground feeder wires are landed to the correct busses in the subpanel. This does mean you'll need to run some more of that bare 6AWG copper from the subpanel grounding bus to a driven ground rod as the exception to 250.32(A) no longer applies, but that's cheap (and good) insurance even on an underground run (I'd consider it mandatory for an overhead run).

Last but not least: TORQUE MATTERS.

One thing you'll want to make sure of with this job is that your connections are torqued to manufacturer specifications. "Gudntite" can easily be too tight, leading to connection failures down the road with setscrew lugs, especially with aluminum wire as it's softer and easier to damage than the stiffer copper stuff. Also, this is a requirement as of the 2017 NEC, see 110.14(D) for details.

In practice, this means you'll need an inch-pound torque screwdriver for torquing down the connections properly -- make sure it can go down to 10 in-lbs for receptacles and switches, and up to 50 in-lbs for torquing down the breaker and lug setscrews. Better that than the electrical version of being Greg Biffle trying to lose lugnuts mid-race, don't you think?

  • Interesting note about the torque – Joe Phillips Sep 20 '17 at 17:06

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