I am thinking of setting up a shop in an outbuilding, and want to put a
panel in the outbuilding. Currently there is a single line running a
couple of outlets and a light in the building. For the shop, I want a 240v
for my table saw, and then 120v outlets for the drill press, and assorted
other tools, radiant heater, and dust collection system. I will be
replacing the house's 100amp panel with a 200amp, and will reinstall the
100amp in the out building. What gauge wire do I need to run between the
two panels? I will be putting it in a trench with a metal conduit. I am
guessing that a 2' deep conduit should be sufficient. Does that sound
Been retired too long to remember the gage size for 100A but #12
is only 20A.
I have my 200A coming from the pole to the house in 2" and though
I would want the 100A spur line to the shop in UF it needs to be
in pipe too. I would use 2" and lay 2 more 1" pipes beside it for
the future things that I know you will want later like phone,
intercom etc. (Three wire or 3 single wires, either. Some want a
bond wire also back to the main panel. I would.)
I see your thinking and maybe your are right but I have my 200A in
the 2" and the phone in the 1" and the cable in the other 1" all
in the same trench. been that way for at least the last 20 years
and so far have never had any interference at all. And they are
plastic. All lines come down the same power pole, travel 100 ft
underground and rise at the end of my house. Each utility
supplied and pulled their own wire in each conduct.
I like the idea of a metal conduit for three reasons. First is, if someone
comes along with a hand shovel, they should be protected from cutting the
wire for a few swings of the blade. Second is if there is a problem at
some point, I can just pull the wire through again, no re-trenching needed.
Third is providing RF shielding so I can also run a phone line and an
ethernet line. to the shop.
I do remember reading somewhere that it is possible to just lay wire without
a conduit, subject to depths and so forth...
Check your local municipality for depth of the trench requirement. If not
applicable, and if you have rock/gravel consistency mostly in the soil, lack
of settling, you'll probably be okay at 2 feet depth. I would bottom and
top the conduit with a couple inches of sand as a buffer from rocks, before
burying. Heavy equipment, trucks and automobiles driving over this trench
may compact excessively if in a shallow trench at 2 feet if the soil is
generally soft. Run deeper in that case. Some municipalities in my region
require 6 feet in depth as an example.
You never indicated a circuit breaker to feed from the 200 amp panel to the
100 amp panel, or, pulling both phases from the 200 amp panel bus directly
Local building code should provide the information you seek on wire gauge
(AWG). The wire material (aluminum or copper) and distance you route this
wire between the panel and termination, determines the wire gauge.
Most local municipalities base their electrical code upon the NEC. The
applicable year of NEC publication that pertains depends on the legislation
The NEC was flaky on what the requirement is for the grounding conductor
between two panels when there is a large distance separation. Bear in mind
you're running both phases, not just one phase between the two panels.
Hire a licensed electrician.
If you live in rural area, talk to your neighbors and see how they did
things as well. You may see mistakes they made so you can avoid them. A
standard trencher may work if the soil is soft, and lacks any substantial
rocks. Otherwise, use a drivable rock saw. Looks like a small tractor with
a big wheel with cutting teeth/spikes on the back. Also check where your
plumbing and current electrical lines are buried before digging.
I will check. The area is known for the crops of rocks that are in
everyones gardens. Generally I would consider it hard, so it would make
sense to cushion it with some sand. Thanks for the suggestion.
I would run it through a circuit breaker feed on the 200 amp panel. Just in
case I needed an easy shut off from the house.
Thanks, I will look it up before I get much further into the process.
Thanks. I will keep that in mind.
I will check before I dig, and I am running about 15 feet between buildings,
so a trencher may not even be able to get in the space.
Depends on how far the outbuilding is from the main panel. The concept here
is "voltage drop", the longer the conductor the thicker it needs to be to
handle the load. Code is no more than a 5% drop, better is towards 3% drop.
(Here's one -
http://www.csgnetwork.com/voltagedropcalc.html Note - 1/2 total circuit
means one way length). For 100 amps you are going to use Aluminum due to
cost of copper with a minimum of 1/0 regardless of length.
Use plastic conduit. Conduit gets buried at least 18" down or you can skip
the conduit and use direct burial cable at least 24".
Conduit is sized by taking the size of the total cables and having that
volume be no more than 40% of the interior conduit volume. Plus no more
than two 90 degree bends. Oversize the conduit as pulling conductors in a
conduit just big enough can be a pain.
Including the drop down one wall and climb back up the other wall, the full
distance is under 35'. Where can I determine voltage drops and the like?
I was planning conduit of some sort, and thinking of metal to provide
shielding so that I can run a second, phone and ethernet, in the same
trench. I know there needs to be some separation, and I will look that up
If the shop is considered residential use, #4 THWN conductors in 1-1/4"
conduit buried 2' deep will meet the NEC requirements.
If the shop is to be used commercially, the minimum size wire would be #3
for a 100 amp subfeed. In either case, a #8 ground conductor should be
installed along with the feeder conductors.
These are NEC recommendations, building codes in your area may be more
Per NEC table 310.16 a copper #4 THWN is only rated for 85 amps and this is
a 100 amp panel. And if this is commercial use (assume you mean "continuous
load") then the cable should be rated for 120 amps and #3 is only rated for
And if the outbuilding is 300 feet away both those conductors are grossly
Carolyn - Since all the answers are slightly different, make sure you
confirm everything you plan to do with your local electrical inspector.
Even if you don't plan to get a permit you can still visit and tell them
what you are planning to do so everything is done to code.
This will not be continuous load, however I may sell the odd piece. From
what I know, code is the minimum requirement. More is usually better - so
if #3 is only rated for 100 amps, then I will up it to give me some margin.
If I run the wire back and forth across the property, then I could us up
300'. If I do a straight run, it will be hard to use more than 20', not
including the drop into the ground and the climb back out at the other end.
That I will do. I wanted to get an idea if I was heading in the right
direction, so that I can get a rough idea of the cost of the project. All
of the answers helped. Thanks!
It is a residential property, the shop is primarily for my 'hobby'
interests, however I may sell an item or three. Any idea where I head to
determine the 'legal' difference between residential and commercial? In
particular here in Eastern Ontario.
Before I do more that get a rough plan / concept for the project I will
confirm all the code requirements.
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