Wire Size for a 300 Foot Run to a Shed, Barn or Pool House

The short answer: the wire size for a 300 foot run is decided by voltage drop, not by how much current the conductor can carry. A conductor that is perfectly legal for the load at 50 feet will leave you with dim lights and a hot motor at 300 feet. Expect to go up two or three sizes, and expect that to be the biggest line on the quote.

This is the single most common surprise for people who want power in a shed, a barn, a workshop or a pool house at the far end of the property. The building is simple. The trench is simple. Then the electrician says 4 AWG and the price triples.

Here is what is actually going on, with the numbers.

Wire size is set by voltage drop, not by ampacity

Every conductor has an ampacity, which is the current it can carry without overheating. For a 30 amp circuit at 240 volts, 10 AWG copper is enough. That is what the ampacity tables say, and over a short run that is the end of the discussion.

But current pushing through a long wire loses voltage on the way. The wire has resistance, the resistance turns a bit of your voltage into heat, and what arrives at the far end is less than what left the panel. Over 20 feet nobody notices. Over 300 feet it becomes the whole problem.

The National Electrical Code does not forbid a large voltage drop. It recommends staying within 3 percent on a branch circuit and 5 percent overall, and it says so in informational notes to 210.19 and 215.2. You can read the code text yourself: NFPA offers free online access to the National Electrical Code. Informational notes are advice, not rules, so an inspector will rarely fail you on voltage drop alone. Your equipment will fail you instead.

What a 300 foot run actually costs you, with the numbers

Take a real case. A detached workshop, 300 feet from the house, fed at 240 volts, with a 30 amp load. Copper conductors. The target is 3 percent, which on 240 volts is 7.2 volts.

The formula for a single phase run is this. Circular mils equals two, times K, times the current, times the one way length, divided by the volts you are willing to lose. K is 12.9 for copper.

2 x 12.9 x 30 x 300 divided by 7.2 gives 32,250 circular mils.

6 AWG copper is 26,240 circular mils, so it does not make it. 4 AWG copper is 41,740, so that is your answer. And 4 AWG is three trade sizes above the 10 AWG that ampacity alone would have allowed.

If you had used 6 AWG anyway, the drop would be 8.85 volts, or 3.7 percent. Not a disaster, but your motor starts harder and runs hotter every single time.

Aluminum wire for a long run is worth a serious look

Aluminum carries current less well than copper, so you need a bigger conductor. For the same run the calculation uses K of 21.2 instead of 12.9, which gives 53,000 circular mils and lands you on 2 AWG aluminum.

That sounds worse. On a 300 foot run it usually is not. Aluminum costs a fraction of copper per foot, and at that length the material is most of the bill. Two sizes bigger in aluminum is regularly cheaper than the copper equivalent, and for a feeder to an outbuilding it is completely normal practice.

The catch is the terminations. Aluminum needs listed connectors, the right antioxidant compound and correct torque, and it needs to be done properly. This is not the place to save money on labour.

Feed a long run at 240 volts, not 120

If you only need lights and a few outlets, the temptation is to run a single 120 volt circuit. Do not.

Voltage drop as a percentage is calculated against the voltage you started with. Halve the voltage and you double the percentage for the same current. Run the same 30 amps at 120 volts over 300 feet and you need 2 AWG copper instead of 4 AWG, purely because of that.

Run 240 volts to the building and split it into 120 volt circuits at a small panel on the far end. You use less copper, you have both voltages available, and you have room to add a circuit later without digging the trench again.

Meter at the road? You are paying for the heat

On a lot of rural properties the meter sits on a pole at the road and the house is a few hundred feet back. Everything past that meter is yours: your conductors, your trench, and your losses.

That last word is the one people miss. The energy a long run turns into heat has already gone through the meter, so it lands on your bill every month. Warming up a pasture, at retail rates.

How much depends entirely on the current, and it does so on a curve, not a straight line. Losses go up with the square of the amps. Here is the same 300 foot copper run at 40 amps.

6 AWG loses 11.8 volts and burns 472 watts. 4 AWG loses 7.4 volts and burns 297 watts. 2 AWG loses 4.7 volts and burns 187 watts. 1/0 loses 2.9 volts and burns 117 watts.

At light load none of this is worth talking about. Two amps in the evening wastes about a watt. But a heat pump, a well pump, a welder or a car on a charger will hold that run at 30 or 40 amps for hours at a time, and then the difference between 6 AWG and 2 AWG is a few hundred watts, continuously, forever.

This is also the honest argument for 240 volts over 120. To move the same power at half the voltage you need double the current, and double the current is four times the loss. Not double. Four times.

So the bigger conductor is not only about lights that stop flickering. Over the life of the run it pays part of itself back. Ask your electrician to price the next size up as well, and compare that once against what the losses cost you every year.

Detached building rules people forget

It needs its own disconnect. A building fed from another building needs a means of disconnecting all the ungrounded conductors, at that building, in a readily accessible spot. NEC 225.31 and 225.32. In practice that is the small panel you were going to install anyway.

It needs its own grounding electrode. A separate structure needs a grounding electrode system of its own, usually a ground rod or two. NEC 250.32. Ground rods go in pairs at least six feet apart unless you can prove a single rod measures under 25 ohms, which almost none of them do.

Four wires, not three. Two hots, a neutral and a separate equipment grounding conductor. The old practice of re-bonding the neutral to ground in the outbuilding has not been allowed for new installations since the 2008 code. Neutral and ground stay separated out there.

The trench has rules too. NEC 300.5 sets minimum burial depths. Direct buried cable goes 24 inches deep. In a nonmetallic raceway such as PVC it is 18 inches. A residential 120 volt branch circuit rated 20 amps or less and protected by a GFCI can go at 12 inches. Measure to the top of the conduit, not the bottom of the trench.

And before the shovel goes in the ground, call 811. It is free, it is a few days, and it is the difference between a trench and a very expensive afternoon.

Voltage drop calculator: run your own numbers

Knowing the answer before the quote arrives changes the conversation completely. You stop wondering whether you are being upsold and start asking useful questions, such as whether aluminum would work here, or whether the panel at the far end could be fed at 240 volts instead.

What I check on site for a long run

If I walked this job, these are the things I would want to see, in this order.

The actual routed length, not the straight line on the map. Trenches go around trees, septic fields and driveways, and the wire follows the trench. A 300 foot straight line is often a 380 foot run.

The real load, honestly counted. Not the load today, the load in three years when there is a compressor, a welder or a heater out there. Adding capacity later means a new trench.

The termination points. Enough room in the panel, correct lugs for the conductor material, and a proper equipment ground path all the way back.

The ground rods, actually driven and actually connected. This is the single most common thing I find missing at outbuildings. The panel is there, the disconnect is there, and the electrode is a rod lying in the grass beside the wall.

The neutral and ground bars in the far panel, separated. If someone has bonded them out there, current returns through the ground path and through anything metal that connects the two buildings.

This article is general information about the National Electrical Code and is not an inspection, an approval or advice for one specific installation. Code adoption differs by state and by local jurisdiction. Always confirm with your local authority having jurisdiction and a licensed electrician.