From: John De Armond
Subject: Re: Shed wiring
Date: Sun, 16 Sep 2007 04:06:00 -0400
On Sat, 15 Sep 2007 15:17:34 -0400, Mike Ruskai
>1) Is 6 gauge wire sufficient for 40A over about 125 feet, a bit more
>than half of which is in buried conduit? What about 50A?
It's code-compliant but I'd go up a wire size or two on a run that long so that you
won't have any problem with lights dimming or electronics being affected if you turn
on a large load (heater, air compressor, etc.) The marginal cost is small.
>2) How wide should the conduit be? This is the only wire that will
>be inside, so beyond fitting the wire, the only concern is heat
I believe that the code is still max 55% fill. In this situation, by the time you
get the standard size of conduit large enough to make hand pulling practical over
that long a distance, you won't have to worry about fill level. Lacking a power
windlass and other professional pulling tools, you're going to want at least 2 pull
boxes over that run. Pulling anything longer than that by hand is quite difficult.
You'll find it a LOT easier to pull and probably cheaper to use 4 strands of THHN
stranded conductor rather than a cable. Cables usually have PVC outer coatings which
generate high friction against PVC conduit even in the presence of cable pulling
lube. In contrast, the THHN nylon outer covering is quite slick and usually doesn't
need lube. It's much more abrasion-resistant too. Important both during the pull
and afterward if the conduit cracks for some reason or if abrasives (sand, dirt,
small rock) somehow get inside the conduit.
>3) How deep should the conduit be buried?
I don't know what the code is anymore (you should be able to find code specs online
if that matters to you) but I always go below the frost line. If you don't go below
the frost line, heaves can push a rock into the conduit and over the years will wear
through. Assuming that you're going to rent a trencher, run it as deep as the
machine can easily go. That should get you below the frost line AND will get you
deep enough that when the next guy comes along and buries another run at the minimum
depth, he won't go deep enough to cut your conduit.
OK, your questions answered, I now ask, is there a severe budget crunch on this
project? If not, why not go up a few more wire sizes and wire for the sub-panel's
full capacity? You may not now see a need for 125 amp service but you never know
what the future may hold, neither for your nor the next owner.
I also echo the advice elsewhere in this thread to drop in another conduit for
internet/phone/low voltage whatever wiring. WiFi's neat and all that but wired
ethernet still moves the bits the fastest. You don't have to pull any wire in right
now but I would install a pull rope for future use. Even if you never use the
conduit, it's nice to have it there just in case.
While I'm at it, I also recommend dropping a hunk of 1/2" or 3/4" direct burial PVC
tubing for water service into the trench. Again, you might not need water in the
shed now but who knows what the future brings?
Couple of years ago I helped a friend build a metal building shop about 100 ft from
his house. He argued with me about running water out there and finally gave in and
laid in a water pipe. Swore he'd never use it. Then a funny thing happened. I gave
him an old ice machine from my restaurant. He works outside and was spending a
fortune filling his drink cooler every day with C-store ice. The ice machine needed
electricity and potable water, of course. Suddenly that water line in the shop
looked mighty fine! It's been tough but so far I've managed not to say "I told you
so". At least not to his face :-)