Wire Size Calculator
Enter the current, the length of the run and the voltage drop you can accept. The calculator finds the smallest wire size that meets the voltage drop and the current rating used here.
How to use this Wire Size Calculator
- Enter the current the circuit will carry.
- Enter the one-way length of the wire run in feet.
- Enter the supply voltage and the voltage drop you can accept.
- Choose copper or aluminum, the circuit type and whether the load is continuous.
- Press Calculate. The result is the larger of the sizes needed for voltage drop and for the current rating.
How to choose a wire size
A wire must meet two tests. It must lose little voltage, and it must carry the current safely.
- Minimum circular mils = factor x K x current x length / (voltage x allowed drop)
- Current to size for = current x 1.25 for a continuous load
- Wire size = the larger of the size that meets the circular mils and the size whose rating covers the current
Worked example
A 20 A circuit at 120 V runs 100 feet one way, and you allow a 3% drop. For copper the minimum is 2 x 12.9 x 20 x 100 / (120 x 0.03) = 14,333 circular mils. The smallest size at or above that is 8 AWG (16,510). On current alone, 14 AWG has a rating of 20 A in the table used here, which just covers a 20 A load, although many rules limit small wire to a lower breaker size. So voltage drop decides, and the answer is 8 AWG. The drop with 8 AWG is 2.60%.
On a short run the current rating usually decides instead. For a 50 ft run at the same current the drop alone would allow a smaller wire, but the table rating of the wire still sets the minimum. For a continuous load, such as an EV charger that runs for hours, the calculator sizes the current at 125%. Wire size rules also limit small conductors to lower breaker sizes in many cases, so confirm the breaker and wire with the code and an electrician.
Assumptions & limits
- The current ratings are commonly published figures for 75 C insulation. Your insulation, ambient temperature, number of conductors in a conduit and local rules can change them. Check the National Electrical Code and your local rules, and use a licensed electrician for real installations.
- Copper and aluminum constants of 12.9 and 21.2 are common planning values.
- Small-conductor breaker limits, termination ratings and derating are not modelled.
- The calculator covers a steady load on two wires or three-phase. It is not a design for a service or a feeder.
- It is an educational tool, not a substitute for a licensed electrician.
Frequently asked questions
How do I choose the right wire size?
Check both the voltage drop and the current rating. The wire must meet both, so use the larger of the two sizes.
Why does length matter?
A longer wire has more resistance, so more voltage is lost along it.
What is a continuous load?
A load that runs for three hours or more. Many rules size the circuit current at 125 percent for those loads.
Can I use aluminum wire?
Yes, but you need a larger size than copper for the same current, and special connectors may be needed.
What does AWG mean?
American Wire Gauge. A smaller number means a thicker wire.
Is this enough to wire my home?
No. Always check the code and use a licensed electrician for real installations.
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Last updated: November 2026