Free Wire size calculator
Enter your load current, run length, system voltage and allowed drop, and this calculator returns the minimum wire size (AWG) by the NEC circular-mil method — for copper or aluminum, single- or three-phase — with the actual voltage drop at that size, updated live, as you type.
On this page12 sections
Sizes for voltage drop only — also check ampacity and breaker rating against the NEC and a licensed electrician.
Results are estimates. Consult a professional.
How the wire size calculator works
Long wire runs lose voltage to the resistance of the conductor, and a conductor that is too thin lets that loss grow until equipment underperforms. This calculator works backwards from a voltage-drop limit: you tell it the load current, the one-way distance, the system voltage, and the drop you are willing to accept, and it returns the smallest standard AWG conductor that stays inside that limit — then shows the actual drop you would get at that size.
The circular-mil sizing formula explained
Each input pushes the required conductor size in a direction you can reason about before you ever run the numbers. The bigger the cross-section a run demands, the thicker the wire — and a lower AWG number means a thicker wire.
Current and distance — they multiply
Current and one-way length sit together on top of the fraction, so they scale the requirement directly: double the current or double the distance and you double the circular mils you need. The factor of 2 (or √3 for three-phase) accounts for the full circuit path, not just the run out to the load.
Allowed drop — the budget you set
The allowed voltage drop sits on the bottom of the fraction, so a tighter budget demands a bigger conductor. Halving the allowed percentage roughly doubles the required circular mils. This is the lever you control: a 3% limit is stricter — and needs more copper — than a 5% limit.
The material constant K
K captures how resistive the metal is per foot per circular mil. Copper sits at 12.9 and aluminum at 21.2, so aluminum needs about 64% more cross-section to carry the same current the same distance with the same drop.
AWG to circular-mil reference table
These are the standard copper and aluminum building-wire sizes the calculator chooses from, with their cross-sections in circular mils. A lower AWG number is a physically thicker wire; "1/0" through "4/0" (spoken "one-aught" to "four-aught") are larger than 1 AWG.
| Wire size (AWG) | Circular mils |
|---|---|
| 14 AWG | 4,110 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
| 4 AWG | 41,740 |
| 2 AWG | 66,360 |
| 1 AWG | 83,690 |
| 1/0 AWG | 105,600 |
| 2/0 AWG | 133,100 |
| 3/0 AWG | 167,800 |
| 4/0 AWG | 211,600 |
Cross-sections per NEC Chapter 9, Table 8. The calculator picks the first size whose circular mils meet or exceed the requirement.
A worked example using the wire size calculator
Dana is running a single-phase 120 V branch circuit to a workshop subpanel 100 feet away that will draw about 20 amps. She wants to keep voltage drop within the NEC-recommended 3% using copper conductor, and needs to know the minimum wire size before pulling cable.
Step 1 — Find the voltage-drop budget
V_drop_allowed = 120 V × 3% = 3.6 V. That is the most the run may lose and still satisfy the 3% target.
Step 2 — Compute the required circular mils
CM = (2 × 12.9 × 100 × 20) ÷ 3.6 = 51,600 ÷ 3.6 = 14,333 circular mils. The factor is 2 because this is single-phase, and K = 12.9 for copper.
Step 3 — Round up to a standard size
10 AWG (10,380 cmil) is too small; the next size up, 8 AWG (16,510 cmil), clears the requirement. So 8 AWG copper is the minimum for voltage drop.
The NEC 3% voltage-drop guidance
The National Electrical Code does not make voltage drop a hard rule for most circuits — it appears as a recommendation in informational notes rather than a mandatory limit. The widely used figures are 3% on a branch circuit and 5% total across feeder plus branch combined.
- 3% on the branch circuit — the run from the last panel out to the load (NEC 210.19(A) informational note).
- 5% total — feeder plus branch combined, from the service to the load (NEC 215.2(A) informational note).
- Sensitive or motor loads — some equipment and local rules call for tighter limits; check the equipment listing.
Why it matters: voltage at the load is what actually powers equipment. Excessive drop dims lighting, cuts motor torque and starting current, and wastes energy as heat in the conductor. Sizing to 3% keeps equipment within its rated voltage and leaves headroom for the feeder's share of the budget.
Copper vs aluminum conductors
Aluminum is cheaper and lighter than copper but more resistive, so it needs a larger cross-section for the same job. In the formula this shows up as the K constant: 12.9 for copper versus 21.2 for aluminum — about 64% higher resistance per circular mil.
| Property | Copper | Aluminum |
|---|---|---|
| K (Ω·cmil/ft) | 12.9 | 21.2 |
| Relative size for same drop | Baseline | ≈ 64% more cross-section |
| Typical sizing rule of thumb | — | Roughly two AWG sizes larger |
| Cost / weight | Higher | Lower |
For the same current, distance and drop, aluminum lands on a noticeably larger wire size than copper.
Aluminum is common for large feeders and service entrances where the cost saving is significant, while copper dominates branch circuits. Aluminum terminations need listed connectors and anti-oxidant compound; follow the equipment and code requirements for the metal you choose.
Voltage drop is only half the job — ampacity comes first
This calculator sizes a conductor for voltage drop only. Before that, every conductor must also meet its ampacity — the maximum current it can carry without overheating — and be protected by a correctly rated breaker or fuse. Ampacity is a separate NEC requirement (Table 310.16), not something this tool checks.
The two requirements work together: pick the larger of the ampacity-required size and the voltage-drop-required size. On short runs ampacity usually governs; on long runs voltage drop often forces a bigger conductor than ampacity alone would. Both must be satisfied, along with derating for temperature and conductor bundling, and the overcurrent device must match the conductor.
Safety, accuracy and limits
The arithmetic is exact. For the current, distance, voltage and drop you enter, the calculator computes the precise circular-mil requirement and selects the smallest standard AWG that meets it, then reports the true drop at that size to full precision.
Real installations carry factors the formula does not: ambient temperature changes conductor resistance, large alternating-current conductors add reactance the DC method ignores, and bundled or high-temperature runs require derating. The circular-mil method is the standard approximation for typical 60 Hz building circuits and matches the voltage-drop calculator's results, but it is an estimate, not a code sign-off.
Frequently asked questions about the free Wire size calculator
About this wire size calculator
This wire size calculator runs entirely in your browser using the NEC Chapter 9 circular-mil method. Enter the load current, one-way distance, source voltage and the voltage drop you'll allow, pick copper or aluminum and single- or three-phase, and it returns the smallest standard AWG that stays within your limit plus the actual drop at that size — computed locally and updated as you type. It sizes for voltage drop only; ampacity and breaker rating are separate NEC requirements.
It pairs with the voltage drop calculator (its inverse) and Ohm’s law. Browse more electronics calculators or the full calculator library.