Two constraints, and the one people forget
A feeder has to satisfy two independent requirements. It has to carry the current without overheating, which is ampacity, and it has to deliver the voltage at the far end that the equipment expects, which is voltage drop. Short feeders are decided by ampacity and the drop calculation is a formality. Long feeders are decided by voltage drop, often by two or three conductor sizes, and running the ampacity table alone produces a feeder that is perfectly safe and functionally useless.
The output above tells you which one governs your case. Where voltage drop wins by two sizes, that is the number to build to. Where ampacity wins, the drop figure is still worth knowing, because a feeder at 4 percent leaves only a little for the branch circuits beyond it and the equipment at the end of a branch circuit sees the sum of both.
That accumulation is the reason a 3 percent target on the feeder is conventional. The commonly quoted design goal is 3 percent on the branch circuit and 5 percent total from the service to the point of use, which leaves roughly 2 percent for the feeder if the branch circuits are to have their full allowance. Those figures are efficiency recommendations rather than enforceable limits in the model code, though some jurisdictions have adopted them as requirements and some specifications impose tighter ones.
Load, demand and what the number in the field means
The load figure this page wants is a calculated load, not a list of nameplate ratings added together. A workshop with a table saw, a dust collector, a compressor, a welder and lighting has a connected load far above anything it will ever draw, because the saw and the welder are never running at once and the compressor cycles. Adding nameplates gives a feeder sized for a scenario that will not happen.
| Subpanel | Feeder commonly seen | What decides it |
|---|---|---|
| Detached garage, lights and receptacles | 30-60 A | Usually the general lighting and receptacle allowance |
| Home workshop with 240 V machines | 60-100 A | The largest machine plus everything running alongside it |
| Garage with an EV charger | 60-100 A | The charger is continuous and dominates the calculation |
| Finished accessory building | 100 A and up | Treated much like a small dwelling calculation |
The demand factor field defaults to 1.00 because the usual workflow is to do the demand calculation properly by the method the adopted edition prescribes, and then enter the result here. Applying a demand factor in this field assumes you know which one applies to your occupancy and that you are permitted to use it. The continuous-load field handles the other direction: loads running three hours or more get an additional 25 percent by convention, and that portion is entered separately because it usually applies to only part of the load.
Distance is the cost driver
Voltage drop is proportional to length and inversely proportional to conductor area, so doubling the run and holding the drop constant means doubling the copper. The economics of a detached structure are dominated by that relationship, and there are exactly three levers.
The first is a bigger conductor, which is linear in cost and eventually absurd. The second is a shorter route, which is often available and rarely taken because the obvious trench line is the one people dig. The third is higher voltage, which is the powerful one: delivering the same power at 240 volts rather than 120 halves the current and, because the drop is measured against a supply twice as large, cuts the percentage drop by roughly four. That is the whole reason detached shops get a 240 volt feeder rather than a heroic 120 volt one.
What this arithmetic cannot see is the trench. Burial depth, the raceway or cable type permitted in the ground, whether the feeder passes under a driveway, and the local requirements for a feeder crossing to a separate structure all move the cost more than the conductor does, and several of them are jurisdiction-specific.
The parts that are not arithmetic at all
A subpanel is not a smaller panel. The arrangement of the neutral and the grounding conductor at a subpanel differs from a service, and getting it wrong produces a system that works perfectly and puts current on things that are meant to be at earth potential. Whether a disconnect is required, how many conductors the feeder needs, what grounding electrode arrangement a separate structure requires, and how the panel is protected are all decided by the adopted code edition rather than by any calculation on this page. None of that is described here, deliberately.
Electrical work on a building generally needs a permit and an inspection, and many jurisdictions restrict who is allowed to do it at all. An error in this part of a house burns houses down and kills people. This page sizes and estimates. It does not replace the judgement of a licensed electrician, and it gives no instruction for opening equipment or making connections. Every threshold on this page — fill percentages, volume allowances, ampacity, the continuous-load factor, derating for ambient temperature and for the number of current-carrying conductors — is shown as an input with a common working default. The figure that governs your installation is the one in the code edition your jurisdiction has adopted, with whatever local amendments came with it, as your Authority Having Jurisdiction reads it. Check the defaults before you rely on them. Bring these numbers to the person doing the work; they are a starting point for a design, not a design.
Related: voltage drop calculator, wire size calculator, EV charger circuit calculator, conduit fill calculator, and the extension cord size calculator for the temporary power that runs the build.
Questions people ask
What size wire do I need for a 100 amp subpanel 150 feet away?
Ampacity and voltage drop give different answers and the larger one wins. On ampacity alone a 100 amp feeder is commonly served by conductors in the 3 AWG copper or 1/0 aluminium region, depending on the insulation and the terminal temperature ratings, and that figure comes from the table in your adopted edition rather than from this page. On voltage drop, 100 amps over 150 feet at 240 volts needs about 53,750 circular mils for 3 percent in copper, which 3 AWG at 52,620 just misses, so 2 AWG, or about 88,300 in aluminium, which is 1/0. Enter your actual numbers above rather than working from this example, because a load of 78 amps rather than a full 100 changes the answer by a size.
Can I run a subpanel to a detached garage on a single cable?
The number of conductors a feeder to a separate structure needs, and how the neutral and grounding conductor are handled at each end, has changed across code editions and is one of the areas where installations from different decades look different for good reason. Modern practice for a detached structure generally runs a separate equipment grounding conductor alongside the circuit conductors, with the neutral and ground kept apart at the subpanel, and the structure typically needs its own grounding electrode arrangement. What applies to your job depends on the edition your jurisdiction adopted. This is a question for the permit application, not for a rule of thumb.
Should I oversize the feeder for future loads?
Usually yes, and it is one of the few places where over-buying is straightforwardly rational. The conductor is a small fraction of the installed cost of a buried feeder — the trench, the labour, the restoration of whatever was above it and the permit are the expense — and the only way to upsize later is to do all of that again. Going one or two sizes up on the initial pull is cheap insurance against a future welder, a car charger or a heat pump. The constraint is that the terminals at both ends have to accept the conductor, and lug capacity on smaller panels runs out sooner than people expect.
Why does my subpanel voltage sag when a machine starts?
Because starting current is several times running current, and voltage drop scales with current. A motor pulling five times its running amps for a second drops five times the volts for that second, so a feeder sitting comfortably at 2 percent under load can dip to 10 percent during a start. Lights on the same feeder flicker, contactors chatter, and a second motor already running may stall. The fix is a larger feeder, a shorter one, or reducing the inrush at the machine with a soft starter. If the sag is severe enough to be visible on every start, the feeder is undersized for the duty rather than for the load.
Does the feeder breaker have to match the subpanel rating?
The feeder overcurrent device protects the feeder conductors, and the panel has a rating that the device must not exceed. Those two constraints frequently produce a breaker smaller than the panel — a 100 amp rated panel fed by a 60 amp breaker is completely ordinary, and it means the panel provides space and bus capacity while the feeder sets the actual limit. Fitting a panel larger than the feeder needs is a common and sensible move, because breaker spaces run out long before feeder capacity does. The reverse, protecting a feeder with a device sized for the panel rather than for the conductors, is the dangerous error.