Wire Size Calculator estimates conductor size from amps, voltage, run length, phase, copper or aluminum, voltage drop limit, insulation rating, ambient heat, and ampacity derating.
How It’s Calculated
The calculator runs two independent sizing checks and returns whichever one demands the larger conductor. The first check sizes the wire to keep voltage drop under your target percentage. The second sizes it to meet the conductor’s allowable ampacity after every applicable derating is applied.
A wire that passes ampacity can still fail voltage drop on a long run, and a wire that passes voltage drop can still fail ampacity on a short, heavily loaded one — the tool checks both and never lets either one slide.
Voltage drop sizing
The NEC doesn’t hand you a formula for this part — 210.19(A) and 215.2(A) only carry informational notes recommending a 3% limit on branch circuits and 5% total for branch plus feeder combined.
To turn a percentage target into an actual conductor size, the calculator applies the standard resistivity-based voltage drop formula used throughout the trade:
$$CM = \frac{2 \times K \times I \times L}{V_{drop}}$$ for single-phase and DC circuits, and $$CM = \frac{\sqrt{3} \times K \times I \times L}{V_{drop}}$$ for three-phase circuits.
- $CM$ — required conductor area, in circular mils
- $K$ — the conductor’s resistivity constant: 12.9 for copper, 21.2 for aluminum (ohm-circular-mils per foot)
- $I$ — load current, in amps
- $L$ — one-way conductor length, in feet
- $V_{drop}$ — allowable drop in volts, calculated as system voltage times your allowed drop percentage divided by 100
The single-phase/DC formula’s factor of 2 accounts for the round trip current makes through the circuit — out on one conductor, back on the other.
Three-phase substitutes $\sqrt{3}$ because line current and line-to-line voltage relate through that factor rather than a simple doubling. Once $CM$ is known, the calculator walks up its gauge table and picks the first conductor whose circular-mil area meets or exceeds it.
Ampacity sizing
The base ampacity numbers come from NEC Table 310.16 — allowable ampacities for conductors in raceway, cable, or earth, at a 30°C ambient with no more than three current-carrying conductors bundled together. Everything else applied on top of that base value is a derating:
$$I_{required} = I_{load} \times LF$$
where $LF$ is the load-duty factor you select (1.0 by default). The wire then has to clear an adjusted ampacity:
$$I_{adjusted} = \min\left(I_{base} \times CF_{temp} \times CF_{count},\ I_{termination},\ I_{small}\right)$$
- $I_{base}$ — the Table 310.16 ampacity at your chosen insulation temperature rating (60°C, 75°C, or 90°C)
- $CF_{temp}$ — the ambient temperature correction factor from NEC Table 310.15(B)(1)
- $CF_{count}$ — an adjustment factor (1.0 by default) for more than three current-carrying conductors sharing a raceway, per NEC Table 310.15(C)(1)
- $I_{termination}$ — a ceiling equal to the ampacity at your selected termination temperature rating, since NEC 110.14(C) generally limits how hot a terminal is rated to run even if the conductor insulation is rated higher
- $I_{small}$ — the NEC 240.4(D) small-conductor override: 15A for 14 AWG copper, 20A for 12 AWG copper, 30A for 10 AWG copper, 15A for 12 AWG aluminum, 25A for 10 AWG aluminum, regardless of what the table math produces
The calculator walks the same gauge table looking for the first (smallest) conductor whose adjusted ampacity meets or exceeds $I_{required}$. Whichever of the two searches — voltage drop or ampacity — lands on the larger conductor is the one reported.
Worked Example
A 120V single-phase branch circuit carries a 30A load 100 feet from panel to load, on copper conductors with 75°C insulation and 75°C terminations, 30°C ambient, no bundling, target voltage drop of 3%.
Allowable drop in volts: $V_{drop} = 120 \times 0.03 = 3.6\text{ V}$.
Required circular mils: $$CM = \frac{2 \times 12.9 \times 30 \times 100}{3.6} = 21{,}500\text{ cmil}$$ 8 AWG (16,510 cmil) falls short of that. 6 AWG (26,240 cmil) clears it, so voltage drop alone puts the floor at 6 AWG.
Now the ampacity side. Required ampacity is $30 \times 1.0 = 30\text{A}$. At 30°C ambient the temperature correction factor is 1.00, and with no bundling the count factor is also 1.00. 10 AWG copper’s Table 310.16 base ampacity at 75°C is 35A, but the 240.4(D) small-conductor override caps 10 AWG copper at 30A — which still exactly meets the 30A requirement. So ampacity alone only calls for 10 AWG.
The calculator reports the larger of the two: 6 AWG copper. The long run is the binding constraint here, not the current.
| Metric | Value |
|---|---|
| Actual voltage drop | 2.95 V (2.46%) |
| Voltage at the load | 117.05 V |
| Remaining drop margin | 0.65 V |
| Circular-mil headroom over the 21,500 cmil minimum | 22.05% |
| One-way conductor resistance | 0.0492 Ω |
| Conductor (I²R) power loss | 88.50 W |
| Loss per foot of total conductor run | 0.44 W/ft |
| Available adjusted ampacity | 65.00 A |
| Ampacity headroom / utilization | 35.00 A (46.15% used) |
Worth noting what the next size down would have done: 8 AWG on this same run produces a 4.69V drop — 3.91% — which blows past the 3% target even though 8 AWG has plenty of ampacity to spare. That’s the voltage drop check doing its job independently of the ampacity check.
What the Result Means
The reported gauge is always the larger of the voltage-drop minimum and the ampacity minimum, and the tool tells you which one drove the decision. The circular-mil headroom figure shows how much extra cross-section that gauge gives you beyond the bare minimum — a jump between standard gauges is rarely exact, so some headroom is normal and not a sign the size is wrong.
Ampacity utilization is a hard boundary, not a comfort metric. NEC Table 310.16 ampacity is enforceable — a conductor loaded past its adjusted ampacity is a code violation, full stop.
A voltage drop above the target percentage is a different kind of result: NEC 210.19(A)’s recommendation to keep branch circuit drop under 3% is a performance guideline, not a hard safety limit the way ampacity is, which is why the calculator flags anything over 5% as a warning rather than a hard failure.
If the termination rating is set to 90°C, the calculator flags that separately. A conductor rated 90°C doesn’t mean every breaker, lug, or piece of connected equipment is rated to run that hot — most standard terminations are only listed for 60°C or 75°C, so a 90°C result is only valid if you’ve confirmed every termination in that circuit is specifically listed for it.
If neither check can be satisfied by any conductor up to 2000 kcmil, the calculator reports the requirement as exceeding limits. At that point the fix isn’t a bigger single conductor — it’s parallel conductors, a higher system voltage, a shorter run, or a load reduction, and that’s a design decision for a qualified professional rather than something a single-conductor calculator can resolve.
What Changes the Result
- Run length — required circular mils scale directly with length, so doubling the distance roughly doubles the voltage-drop-driven conductor size.
- Allowable voltage drop percentage — tightening the target from, say, 3% to 2% increases the required circular mils by the same proportion, since $V_{drop}$ sits in the denominator.
- Conductor material — copper’s K of 12.9 versus aluminum’s 21.2 means aluminum needs roughly 64% more circular mils to hit the same voltage drop target for the same current and length, on top of aluminum’s lower ampacity at every gauge. The gauge table also has no aluminum ampacity entry below 12 AWG, reflecting that 14 AWG aluminum branch-circuit conductors aren’t part of this model.
- Ambient temperature — pushes the correction factor away from 1.00 in either direction. Go above 30°C and ampacity drops; go below and it rises. Each insulation rating also has a ceiling this model supports: 60°C insulation stops correcting past 55°C ambient, 75°C insulation past 70°C, and 90°C insulation past 80°C — beyond those points the calculation halts rather than guessing.
- Conductor count — bundling more current-carrying conductors into the same raceway applies an additional derating factor on top of the temperature correction, reflecting reduced heat dissipation.
- Insulation rating versus termination rating — the adjusted ampacity is capped at whichever of the two is lower, so pairing 90°C-rated wire with a 75°C-rated termination gets you 75°C-level ampacity in practice.
- Load duty — the load-factor setting scales the required ampacity above the plain running current, which is why the same 30A load can call for different wire depending on how that setting is set.
Copper ampacity reference (NEC Table 310.16, 30°C ambient, ≤3 current-carrying conductors, before correction)
| AWG / kcmil | Circular mils | 60°C | 75°C | 90°C |
|---|---|---|---|---|
| 14 AWG | 4,110 | 15 A | 20 A | 25 A |
| 12 AWG | 6,530 | 20 A | 25 A | 30 A |
| 10 AWG | 10,380 | 30 A | 35 A | 40 A |
| 8 AWG | 16,510 | 40 A | 50 A | 55 A |
| 6 AWG | 26,240 | 55 A | 65 A | 75 A |
| 4 AWG | 41,740 | 70 A | 85 A | 95 A |
| 2 AWG | 66,360 | 95 A | 115 A | 130 A |
| 1/0 AWG | 105,600 | 125 A | 150 A | 170 A |
| 2/0 AWG | 133,100 | 145 A | 175 A | 195 A |
| 4/0 AWG | 211,600 | 195 A | 230 A | 260 A |
Ambient temperature correction factors (NEC Table 310.15(B)(1))
| Ambient temperature | 60°C factor | 75°C factor | 90°C factor |
|---|---|---|---|
| ≤10°C | 1.29 | 1.20 | 1.15 |
| 11–15°C | 1.22 | 1.15 | 1.12 |
| 16–20°C | 1.15 | 1.11 | 1.08 |
| 21–25°C | 1.08 | 1.05 | 1.04 |
| 26–30°C | 1.00 | 1.00 | 1.00 |
| 31–35°C | 0.91 | 0.94 | 0.96 |
| 36–40°C | 0.82 | 0.88 | 0.91 |
| 41–45°C | 0.71 | 0.82 | 0.87 |
| 46–50°C | 0.58 | 0.75 | 0.82 |
| 51–55°C | 0.41 | 0.67 | 0.76 |
| 56–60°C | — | 0.58 | 0.71 |
| 61–65°C | — | 0.47 | 0.65 |
| 66–70°C | — | 0.33 | 0.58 |
| 71–75°C | — | — | 0.50 |
| 76–80°C | — | — | 0.41 |
Frequently Asked Questions
What size wire do I need for a 30 amp circuit?
Ampacity alone puts 10 AWG copper right at the 30A small-conductor limit under NEC 240.4(D), but that’s only half the answer. Run that same 30A load 100 feet at 120V with a 3% drop target and the voltage-drop calculation pushes the requirement up to 6 AWG, so the honest answer depends on distance as much as current.
How far can I run 12 AWG wire before voltage drop becomes a problem?
Using 12 AWG copper’s 6,530 circular mils in the drop formula, a 120V circuit at 20A with a 3% target hits its voltage-drop limit at roughly 45 feet one-way. Past that distance, even though 12 AWG still has ampacity to spare, the drop percentage climbs above the target and the calculator moves you up a gauge.
Does aluminum wire need to be a larger gauge than copper for the same job?
Yes. Aluminum’s resistivity constant (21.2) is about 64% higher than copper’s (12.9), so hitting the same voltage drop target on the same run needs proportionally more circular mils. Aluminum’s ampacity table also runs lower at every matching gauge, and this model doesn’t include a 14 AWG aluminum ampacity value at all.
Is the 3% voltage drop target actually a code requirement?
No — it comes from informational notes attached to NEC 210.19(A) and 215.2(A), which recommend 3% for branch circuits and 5% total combined with feeders. Those are performance recommendations, not enforceable limits, which is why exceeding 5% triggers a warning in the results rather than blocking the calculation the way an ampacity failure would.
What’s the difference between the insulation rating and the termination rating?
The insulation rating (60°C, 75°C, or 90°C) sets which column of the NEC Table 310.16 ampacity table applies to the conductor itself. The termination rating caps that number at whatever the connected equipment’s terminals are actually listed for, since most breakers and lugs are only rated to 60°C or 75°C even when paired with 90°C-rated wire — the lower of the two always governs.
Why does a hot attic or outdoor run need a bigger wire than the same circuit indoors?
Ampacity tables are built around a 30°C ambient baseline. Above that, a correction factor below 1.00 reduces the usable ampacity of every gauge — at 45°C, for example, a 75°C-rated conductor only retains 82% of its table ampacity — so the same load may need a larger conductor purely because of the surrounding temperature, independent of current or distance.
Does bundling multiple wires in the same conduit change the size I need?
Yes. More than three current-carrying conductors sharing a raceway get an additional derating factor under NEC Table 310.15(C)(1), on top of any ambient temperature correction, because bundled conductors dissipate heat less effectively than an isolated one.
This calculator is for planning and estimation purposes only. Final wire sizing, breaker selection, and installation must be verified against your local electrical code and performed or inspected by a licensed electrician.