Voltage Drop Calculator: Wire Size Too

How do you calculate voltage drop?

The AmpSizer voltage drop result for 100 ft of 12 AWG copper carrying 20 A on a 120 V circuit is 6.6% — 7.9 volts lost, leaving 112.1 V at the outlet. That misses the 3% target, and the smallest conductor that passes is 8 AWG.

The arithmetic is one line: 2 × amps × ohms per 1,000 ft × one-way feet ÷ 1,000. The two is there because current goes out on one conductor and back on the other, so the copper is twice the distance you measured. A balanced three-phase circuit uses 1.732 instead of two.

Three percent on a branch circuit and 5% overall are informational notes in the NEC, not enforceable limits — but a motor or a heater at the end of a soft feed draws harder to make up the difference, and that is what shortens its life.

Two rules decide every conductor

A wire has to carry the current without cooking, and it has to deliver the voltage at the far end. The first rule is a table lookup; the second is arithmetic on the length of the run. Whichever asks for more copper wins, and that is the number this site returns.

Voltage drop on a single-phase or DC circuit — the current goes out and comes back, so the length counts twice:

volts dropped = 2 × amps × (ohms per 1,000 ft) × one-way feet ÷ 1,000

On a balanced three-phase circuit the two becomes the square root of three, 1.732. Everything else on this site is that formula, Table 310.16, or a fill percentage.

Copper ampacity at 75 °C, straight from Table 310.16

20 A14 AWG · 2.08 mm²
25 A12 AWG · 3.31 mm²
35 A10 AWG · 5.261 mm²
50 A8 AWG · 8.367 mm²
65 A6 AWG · 13.3 mm²
85 A4 AWG · 21.15 mm²
115 A2 AWG · 33.62 mm²
150 A1/0 AWG · 53.49 mm²

Every calculator here runs on three things: a table of what a conductor can carry, a table of what it weighs in resistance, and one line of arithmetic. Change the metal and the resistance changes. Change the distance and the drop changes. Nothing else moves.

Why two rules and not one

Ampacity is about heat. A conductor carrying current warms up, and the table gives the current at which its insulation reaches the rated temperature. It has nothing to do with how far the wire goes.

Voltage drop is about distance. Every foot of copper has resistance, and resistance turns some of your voltage into heat along the way. It has nothing to do with whether the conductor is safe.

A wire has to pass both, and which one decides changes with the length of the run. Almost every tool on the internet answers one of them and leaves you to notice the other.

Where the long runs go wrong

A branch circuit inside a house is almost never decided by voltage drop; the distances are too short. The runs that get it wrong are the ones that leave the building: a detached garage, a well, a barn, a gate opener, a pond pump, a shed with a saw in it.

At 120 V the drop is twice as costly in percentage terms as it is at 240 V for the same load in watts. That is why running 240 V out to a detached building and stepping down at the far end is often cheaper in copper than running 120 V the whole way — and why the longest-run figure in the results is worth more attention than the wire size itself.

What the rest of the site covers

31 calculators follow the same chain: what the wire can carry after derating, which breaker protects it, how many conductors fit in the pipe, what fits in the box, and what an appliance nameplate is actually asking for.

The reference charts live with the guides, because a chart is something you come back to rather than something you fill in: the wire gauge chart with ampacity and resistance for every size, the breaker and wire pairing chart, and the conduit fill chart for every raceway. All three are generated from the same data files the calculators read.

Frequently asked questions

Is 4% voltage drop a code violation?

No. The 3% branch-circuit and 5% total figures live in informational notes to 210.19(A) and 215.2(A), and informational notes are not enforceable requirements. An inspector cannot fail a run for 4%. What 4% does mean is a real loss: a 240 V circuit delivers 230 V, a resistive heater gives about 8% less heat, and a motor pulls more current to make up the shortfall.

How far can I run 12 gauge wire on a 20 amp circuit?

About 45 ft at 120 V if you want to stay under 3%, and roughly 90 ft at 240 V because the same volts lost are a smaller share of a bigger number. Set the calculator to your own load and read the longest-run line: at 16 A rather than a full 20, the same 12 AWG stretches to about 57 ft on 120 V.

Do I measure the run one way or there and back?

One way. Measure from the panel to the load along the path the cable actually takes — up the wall, across the joists, back down — and let the formula double it. Doubling the distance yourself and letting the formula double it again is the most common way people end up two sizes larger than they need.

Does voltage drop change what breaker I use?

No. The breaker protects the conductor, so it is sized from the load and capped by the conductor. Upsizing a wire for voltage drop does not change the device — but it does change the ground: 250.122(B) makes you upsize the equipment grounding conductor in the same proportion, and that is the step people miss.

Which resistance figures does this use?

The direct-current resistance of stranded, uncoated conductors from NEC Chapter 9, Table 8 — 1.98 ohms per 1,000 ft for 12 AWG copper, 3.18 for 12 AWG aluminium. On long runs of larger conductors in steel conduit the alternating-current figures in Table 9 are a few percent higher, so treat this as the standard method rather than an impedance study.

Why is aluminium worse for voltage drop?

Because aluminium has about 61% of copper’s conductivity, so the same size drops roughly 1.6 times as much. That is why an aluminium feeder is normally two sizes larger than the copper one it replaces — 2/0 aluminium where 1/0 copper would do — and why aluminium is common on long feeders where the cost of the extra metal is still less than copper.

How we calculate this

Voltage drop is k × amps × R × one-way feet ÷ 1,000, where k is 2 for single-phase and DC circuits and 1.732 for balanced three-phase, and R is the conductor’s direct-current resistance in ohms per 1,000 ft from NEC Chapter 9, Table 8 — stranded and uncoated, which is what building wire is. Percentage is volts lost divided by nominal system voltage. Ampacity comes from Table 310.16 at the temperature column you pick, corrected for ambient by Table 310.15(B)(1) and for bundling by Table 310.15(C)(1). The device rating is the next standard size in 240.6(A), held down by the small-conductor ceiling in 240.4(D). Every figure is read from the same data files that draw the reference charts, so a chart and a calculator on this site cannot disagree. Nothing is stored and nothing is sent anywhere — it all runs in your browser.

Worked example

With Load current 20, One-way run 100, Conductor 12 AWG, Metal Copper, System voltage 120 V, Circuit Single phase, this page works out 6.6% voltage drop ≈ 7.9 V lost. 112.1 V left at the load · 12 AWG is 3.3 mm²

Longest run this size can do at 3%45 ft
Resistance used1.980 Ω per 1,000 ft
Ampacity of this size at 75 °C25 A

Sources:

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