A homeowner wants power in the detached garage at the back of the lot — a sub-panel for a couple of circuits, a welder, some lights. The garage is 150 feet from the house. This is one of the most common "real" jobs an electrician sizes, and it is a perfect teaching case, because it forces three separate decisions to agree: how much current the conductor can carry, how much voltage it loses over distance, and how the grounding has to be run to a separate building. Get any one wrong and the job is either unsafe, sluggish, or illegal. Let us size it the way you would on paper before you order wire.

Start with the load, not the wire

You cannot size a feeder until you know what it feeds. The proper way is a load calculation — adding up the connected loads with the demand factors the code allows — but the practical first move is to settle on the sub-panel's rating. Say the homeowner's welder and a small shop point you at a 60-amp sub-panel. That 60 amps is the design current the feeder must serve, and it is the number every later decision keys off.

Now the trap: a beginner sizes the conductor straight to 60 amps off the ampacity table, orders the wire, and moves on. Over 150 feet, that conductor will carry the current just fine and starve the welder anyway. Distance is the variable that turns a simple sizing into a real one.

Decision one: ampacity

First, the heat question. Using Table 310.16 and the termination logic of 110.14(C), you find the smallest conductor whose ampacity meets the load. For a 60-amp feeder on 75°C-rated terminals, you read down the 75°C column to a conductor rated at 60 amps or more — landing, in copper, around the 6 AWG range, depending on the material and the exact conditions. If you stopped here, the wire would not overheat. It would pass the ampacity test cleanly.

But ampacity only protects the conductor from cooking. It says nothing about whether the load gets enough voltage. That is the second, independent test.

Decision two: voltage drop over the distance

This is where 150 feet changes the answer. Voltage drop for a single-phase feeder is approximately:

VD = (2 × K × I × L) / CM

with K about 12.9 for copper, I the load current, L the one-way length in feet, and CM the conductor's circular-mil area. Notice that L is 150 here, and it sits right in the numerator — long runs drive the drop up linearly. Run the 6 AWG copper that passed ampacity through this equation at 60 amps over 150 feet and the percentage drop climbs uncomfortably toward and past the 3-percent guideline that good practice — and many specs — call for. The conductor is heat-safe but performance-poor.

The cure is the only lever that helps: increase the cross-sectional area. Step up to 4 AWG, or even larger, and the bigger CM in the denominator pulls the drop back under the target. You are no longer sizing for ampacity at this point; you are sizing for distance. On long feeders it is routine for voltage drop, not ampacity, to be the constraint that picks the wire — and the further the run, the more often that is true. The 3-percent figure itself lives in an NEC informational note, which is a recommendation rather than a blanket mandate, but a spec or local amendment can make it binding, and either way an undersized long feeder is bad work.

Decision three: the fourth wire

Here is the rule that catches people who have only ever wired within one structure. When you feed a separate building or structure, the grounded (neutral) and grounding (equipment ground) functions must be kept separate at the sub-panel, and you run a feeder with a full equipment grounding conductor alongside the circuit conductors — a four-wire feeder for a 120/240-volt sub-panel: two hots, a neutral, and a ground. The neutral and ground bars in the detached panel are not bonded together; the neutral floats, and the ground bar bonds to the enclosure and any grounding electrode at the structure.

This is the modern requirement under Article 250's rules for separate buildings (250.32), and it reversed the older practice some veterans still remember, where a re-bonded neutral and a local ground rod were once permitted under limited conditions. The detached structure also needs its own grounding electrode. The point for sizing: that equipment grounding conductor has to be sized too, from the grounding-conductor sizing rules, and it rides in the same trench or raceway. Forgetting the fourth wire — or bonding the neutral at the wrong end — is one of the most common and most serious mistakes on this exact job.

Pulling it together

So the 150-foot garage feeder is the product of three agreeing decisions. Ampacity sets a floor — say 6 AWG copper for 60 amps. Voltage drop over 150 feet pushes you up from that floor, often to 4 AWG or beyond, because the distance loses too much voltage at the smaller size. And the separate-structure rules dictate a four-wire feeder with an isolated neutral and a properly sized equipment ground, plus a grounding electrode at the garage. The conductor you actually order is the largest size any one of those three tests demands — usually the voltage-drop size, on a run this long.

What you must not do is let any single test win alone. Size only to ampacity and the welder runs rough. Size generously but bond the neutral at the garage and you have created a parallel ground path and a real hazard. The job is correct only when all three are satisfied at once.

The honest caveat

This walkthrough is the shape of the calculation, not a substitute for it. The real job needs a proper load calculation, the exact ampacity and grounding-conductor values from the current tables, the burial-depth and raceway rules for the run, local amendments, and — for anything you are not certain of — a licensed electrician's or engineer's review. Feeders to separate structures involve life-safety grounding; verify everything against the edition of the NEC your jurisdiction has adopted.

Three calculators that have to agree

The reason this job is satisfying to do well is that it is three calculations talking to each other, and the reason it goes wrong is doing them on a scrap of cardboard in the truck. Voltly puts them side by side: the ampacity tool gives you the heat-safe floor from 310.16, the voltage-drop tool — free in the app — shows you what 150 feet does to a 60-amp load and how far you have to upsize, and you can name and save the whole thing as a job so the garage feeder's numbers are there when you order wire or come back to wire the panel. The NEC reference, with the grounding articles cited, sits in the same offline app for when you need to check the four-wire rule on site. If you size feeders for a living, Voltly is built to make these three agree before you cut anything.