sub panel in detached barn


Old 02-23-05, 11:11 AM
Thread Starter
Join Date: Dec 2004
Location: Taylorsville, MD
Posts: 27
sub panel in detached barn

I'm in the planning stage of running electric to a barn behind my house. The distance from panel to panel will be about 150 feet max. I will bury 2 inch conduit approximatley 24" in the ground and pull either a single #8-3 w/grnd copper wire or four seperate #8 THHN copper wires from panel to panel (2 hot, 1 neutral, 1 ground). I will have a 40 amp breaker in the main panel and two 20 amp breakers in the subpanel. The barn will only have lighting and outlets (total right now 14), nothing major. Since there will be metal to metal between the buildings (phone, intercom) i will ground to the main panel and not install a ground rod at the barn.

my questions are:

1. does everything mentioned look ok?

2. will the 2 20 amp breakers at the sub panel be standard 120v?

3. anything i've missed or overlooked?


Mark...Taylorsville, MD
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Old 02-23-05, 11:39 AM
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
You cannot run a cable inside the conduit. You need to run individual wires inside the conduit.

At 150 feet, voltage drop is an issue. To use a 40 amp main breaker you will want larger wire, at least 6 gauge.

I believe that you want a ground rod installed at the barn.

I would go with a more than 40 amps, and a larger panel at the barn. I would also consider more than two circuits at the barn. I would keep the lighting separate from the receptacles, and have at least two circuits for the receptacles, probably four circuits total (two lighting and two receptacles, although maybe one lighting if it's a small barn). With a larger panel you would then have capacity for more circuits later on, including possibly a 240 volt circuit.
Old 02-23-05, 11:53 AM
Thread Starter
Join Date: Dec 2004
Location: Taylorsville, MD
Posts: 27
Thanks for the reply Bob...

single wires would be fine, and probably easier to pull i would imagine...

correct me if i'm wrong...i'm real fuzzy with the 120/240 conversion thing...but running two hot wires makes it a 240v using the voltage drop calculator here Stan's show that 150 ft at 40amps and a #8 wire is a 3.33% drop? instead of a single 40 amp at the main panel, should i do two 20 amps?

I can install the ground rod at the barn and still use the ground between panels?

the barn is only i've got a total of 14 drops (10 lights and 4 outlets)...and a 20 amp circuit will allow 10 drops, for a total of 20, allowing me 4 extra drops at a later date, if needed. This is only for animal use and storage.
Old 02-23-05, 12:03 PM
Join Date: Jan 2002
Location: Chicago
Posts: 1,915
Run a #6, you will be glad in the long run. The cost difference should not be that large.

You need to have both a ground wire, and a ground rod at the barn. The subpanel will need to have separate ground and neutral bus bars. The ground between the panels and the ground rod serve different purposes.

Since your loads will be mostly 120V the neutral will have the be the same size as the hots.

There may be more requirements due to the animal storage part, I don't have any experience with barns. There are none in my area.
Old 02-23-05, 12:16 PM
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
Make sure when you pull the wires that you use lubricant. It must be made for wires, don't use anything else. Also. don't glue the conduit with wires intact, the glue eats the insulation.

Yes, you have 240 volts available at the destination, but unless you balance your loads evenly, you could in theory have a 40 amp 120 volt load. Try putting 40 amps at 120 volts into the calculator you liked to, and see what it says.

When planning on receptacle circuits, think about what you will be plugging in. If, for example, you will be using large floor fans to circulate the air when it's hot, you may need more than one circuit if you have more than one fan.

By the way, there is no limit to the number of receptacles that you can have on a circuit, for a residential application. You mention 10, which is a rule of thumb only. As I said above, it's the anticipated load that is important. For a lighting circuit, you simply go by the wattage for each light. (And that means the maximum wattage, not necessarily what you intend to install.)

I misread your original post. I thought you meant 14 receptacles in addition to the lights. If you only plan on four receptacles and then lights, you could easily get away with two circuits, but it all depends on what you want to run.

I would still plan on having extra capacity, which you do have already. Even if you maxed out two 120 volt circuits you would still have 40 amps at 120 volts available for two more circuits.

My point about extra capacity is that it would be a waste of money to find yourself ripping out everything a year from now to install new equipment for more capacity, when you can make allowances now. Yes, larger wire and a larger panel are more expensive, but not that much more.

The ground wire to the main panel and the ground rod serve different purposes.

Do not run telephone, intercom or any other wires in the same conduit as the electrical wire. Install a separate conduit for these low voltage wires, and keep it at least one foot from the electrical conduit.
Old 02-23-05, 12:36 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
If two 20-amp circuit is all you'd ever need, then you could ignore most of our advice and do almost anything you wanted. However, most of us don't trust that two 20-amp circuits will satisfy all your future needs forever.

You'll want a double-pole breaker in the main, and run three insulated conductors plus ground.

If you put in two 20-amp circuits to begin with in the barn, put them in on different legs of the power (i.e., one breaker above the other, not side by side).

Bury your conduit at least 18" deep.

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