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I wanted to run my plans by qualified electricians before running electric to my new barn. I have a 270' run from my house to the new barn. I am running 1 1/4" sched 40 2' down the entire distance. I plan on running 3 strands of #8 thwn plus ground off a 40 amp double pole breaker in my home 200 amp service panel out to a subpanel in the barn which I haven't bought yet. I have already sunk a ground rod 8' down within a few feet of where the subpanel will go. I will remove the bond between ground and neutral in the subpanel and ground it to the ground rod. I have very little load requirements in the barn: some lights, a few outlets and ability to run a table saw which draws about 15 amps so 40 amps should be more than adequate and it gets pricey to run anything larger.

I was thinking of using a 100 amp subpanel and putting in a 40 amp double pole main breaker. That should give me plenty of room to grow. Does all of this make sense? Do you have any other recommendations?

  • Wow! Thanks to everyone who responded. I got way more info than I expected. – Michael Caisse Nov 4 '18 at 0:53
  • Regarding the requirement for 2 ground rods? I only have one on my current 200 Amp service and I've seen a lot of disputes about this online. Also, I tied the wire in the concrete of my barn floor (800 sf) to my ground rod so I think I have a strong ground but have no idea how to check it. I love the idea of upping the voltage and using smaller wire but I think I'm not qualified to do it and its a bit risky for me. Also, aluminum wire makes a lot of sense but I already ran much of the conduit and some of it is actually less that 1.25" (Long story:-( ) – Michael Caisse Nov 4 '18 at 1:07
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Leave the subpanel breaker as-is

For the barn subpanel, leave its breaker at 100A (or whatever the factory provides). You don't need a breaker there, you need a shut-off switch. Using a "main-breaker panel" is just cheaper and neater than having a totally separate shut-off switch... nobody cares if the shutoff switch says "100A" on the handle :)

Breakers protect wires and equipment, and the 40A breaker in the house panel more than suffices to protect the 100A rated panel. If your logic is "oh, I want the breaker to trip locally first", timing breakers like that just doesn't work. It's always the far one that blows.

(On the other hand if you used large enough wire to justify a 150A supply breaker, then the 100A breaker would protect the 100A rated panel. You might reasonably do that if the 150A feeder was going onward to supply other subpanels at other barns).

Consider running bigger pipe

It's easy to get in a hurry to pinch pennies, but there are two places not to do that. First is on pipe. Price it all ways, but the cost increment to larger conduit is often not that much, and buys you a lot of flexibility. (also consider the "aluminum factor").

Consider aluminum wire

Ironically, one place people fail to pinch pennies is going with copper wire as a default, or worse, as a moral panic. There's a lot of FUD running around about aluminum, and that had some truth when it comes to small branch circuit wiring (the wires to your outlets and lights). Look closer, the bad actor was actually copper lugs/terminals that did not play nice with aluminum wire. Aluminum lugs play well with copper though -- which is why your subpanel's main lugs will be aluminum. Aluminum remains the wire of choice for feeder (though only the power company is still allowed to use the scandalized AA-1350 alloy, you'd use the new AA-8000 alloy).

At 40A circuit size (#8 copper or #6 aluminum), there still isn't much of a cost motivation to go aluminum. Starting the next size up, #6Cu or #4Al, aluminum makes increasing sense. So if you are thinking of a size bump for voltage drop, price aluminum - especially for such a long distance.

On feeders, copper is not an upgrade. Go with Al if it makes cost sense.

You may not need to bump for voltage drop

For a certain circuit size (40A), there is a minimum wire size you must use (8 AWG) or the wire will overheat. This is good for any length of wire, though at excessive lengths, the resistance of the wire stacks up, and you start running into serious voltage drop. The voltage drop is proportional to current actually being drawn in that moment - lights alone won't provoke much drop, but start the table saw and you'll get a bunch.

Voltage drop can be computed, and there are voltage drop calculators on the web to make that easy. The classic blunder, made even by many experts here, is to calculate voltage drop based on the number written on the handle of the circuit breaker (40A). That is wrong. Voltage drop should be computed based on your actual, expected loads, while thinking about how sensitive they actually are to voltage drop.

For instance if you have a delicate data center and a huge air conditioner motor on the same feeder, you'd need to get out the sharp pencil and really run the numbers. For LED lighting (again with auto-ranging power supplies) and a "12A" table saw, maybe not so much.

That said, if you were using 80% of circuit capacity (32A) most of us would proscribe a wire size bump at about 110 feet, so in your case, 2-1/2 bumps (5 wire sizes) to #3 Cu (really #1 Al, unless you own a copper mine). Needless to say, your conduit pipe needs to be large enough to accommodate that wire, back to my advice to upsize conduit.

On one hand, you won't be loading this circuit to 32A. On the other hand it is still quite a distance. It's leeeegal to use #8 Cu wire and just lump the voltage drop, but I think I would price #4 Al (=#6Cu) just the same. Cheaper to invest now than have to tear out and re-pull such a long run.

Also, 120V circuits suffer twice as badly from voltage drop. So if you will mainly have one load, and it's 120V, think about additional bumps still. Make sure you are saying "120V" in the voltage drop calculator. For instance a 120V table saw will have four times the voltage drop of a 240V saw of same power. If there's also a dust collector, and it's on the same leg, that will make things worse still. If it's on the opposite leg, that will help things somewhat, by moving some of the return current to the other leg.

If all else fails: transformers and 480V

One thing I love to do in these cases is get a couple of appropriately sized transformers (10KVA in your case) and connect them tail to tail, 480V or 600V primary to primary. You backfeed the house transformer with plain 240V (no neutral). It sends 480V or 600V down the 2 wires to the barn, for 1/4 to 1/6 the transmission losses for the same power (1/16 to 1/36 for 120V loads). The second transformer drops it and splits it to 120/240V.

The transformers make it a separately derived service, which means the local panel here becomes a main panel which must have neutral and ground bonded. The ground rod becomes your one ground. You don't need to pull a ground or neutral from the house, 2 wires does the trick. The wires can be smaller - in your case 12 AWG would suffice.

This is wizard-tier stuff, and you'd want to very carefully design it so there's basically no chance of anyone getting near the 480/600 primary voltage. That stuff bites very hard.

Misc.

You actually need two ground rods, unless you do a complex test to confirm the efficacy of the one ground rod.

As you know, you need a ground wire also. The ground wire can be smaller than the conductors, but take note -- Its size must be based on the conductor size not the circuit ampacity. Don't go "A 40A circuit can use a #10 ground wire", that is wrong - if you upsize wire for voltage drop, you must upsize the ground wire also; a table from conductor size to ground size tells you what you need.

If you're even thinking of using an alternate power source in the barn, such as generator or solar, install a Siemens main-lug subpanel with the "shutoff switch" breaker in the #1 position. That will let you use their $30 factory-spec generator interlock later. FYI, a generator at the barn cannot backfeed the house unless you put a second set of feeders in the pipe.

You can have up to 3 circuits in the pipe before you have to start derating wires. If you have upsized wires for voltage drop, you can go to 4 circuits. Since it runs to an outbuilding, each circuit would need to have a reason that it couldn't just be supplied out of the subpanel, e.g. Switched circuit for lights or pump, gen backfeed, etc.

Never forget the dust collector. When there's a table saw, there's a dust collector. Don't forget to include it in the figures. As mentioned, putting it on the opposite 120V pole will actually ease voltage drop, by splitting the load across all 3 wires instead of just 2.

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You may want to consider direct burial wire as opposed to the use of the conduit. Conduit is more susceptible to getting water in it and exposing the wires to being wet all the time. The proper type of direct bury wire has a thick continuous jacket that actually protects the internal conductors from water better than the conduit and you eliminate the struggle to push wires through long conduit sections.

Also consider using #6 wire for the long run (270 feet) as it will provide much less voltage drop at 40A than the #8 that you propose.

Using a subpanel rated at 100A should not be a problem at all as long as you use the proper main shut off breaker. Obviously the breaker back at the originating box at the house will be the key protection point for overloads and shorts that could occur and should be sized at the 40A rating just as you propose.

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    Not seeing the win. Yes, conduit is 100% full of water 100% of the time (at least that's our assumption). That's why you use THWN rated wire. It's designed for that. And it's not like direct burial cable isn't wet - it's wet 100% of the time too. It's just that UF cable jacket has the additional burden of dealing with physical penetration from compaction and stones. The upside of buried cable is it's much harder to steal. – Harper - Reinstate Monica Nov 3 '18 at 17:52
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You will need a 4th wire in the conduit

Since you are running 240V to the subpanel, and what I presume is PVC conduit based on the mention of "schedule 40", you will need separate neutral and ground wires in the feeder, which will mean a 4th THWN in the feeder run. This is in addition to the ground rod system, as the ground wire returns wayward utility electricity back to the utility, while the ground rod returns wayward natural electricity back to nature.

(The alternative to this would be to use a transformer or two to establish a new main bonding point, but for a run this short at these current levels, the economics of using a transformer are rather poor compared to simply running the fourth wire.)

You don't need to downsize the subpanel main breaker

The main breaker in a subpanel like this is simply a disconnecting means, so leaving the stock 100A main in place is A-OK. In fact, many panel lines do not support a 40A breaker in the main breaker position, even. While you're panel shopping, though, one thing you should not do is shortchange yourself on panel spaces. A 24-space or 30-space panel is highly recommended here in order to provide room for future expansion, as it's far cheaper to buy more spaces now than to replace a full panel down the road.

TORQUE ALL LUGS TO SPEC

There is one more thing that you will need to do, and that is to use an inch-pound torque wrench or torque screwdriver to torque all panel and breaker lugs to manufacturer specified torques. This is required by 110.14(D) in the 2017 NEC, and also improves the reliability of your wiring job.

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