Wire Size Calculator
Find the wire gauge that safely carries the load and keeps voltage drop within limit, from the current, one-way distance, voltage, conductor metal, and phase. Calculates in your browser.
Recommended wire size
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Show the math
Two sizes are found and the larger one is recommended:
Ampacity: the smallest wire that can safely carry the load, from NEC Table 310.16 (copper or aluminum, 30 °C ambient, up to three current-carrying conductors). Loads of 100 A or less use the 60 °C column, as NEC 110.14(C) requires for most terminals; larger loads use 75 °C. 14, 12 and 10 AWG copper are also capped at 15, 20 and 30 A (NEC 240.4(D)).
Voltage drop: the smallest wire whose drop stays within your limit:
drop = (factor × K × amps × distance) ÷ circular mils
factor = 2 (1-phase) or √3 (3-phase); K = 12.9 Cu, 21.2 Al
This is a planning estimate. Continuous loads (3 hours or more) need wire and breakers rated for 125% of the load, and bundling, high ambient temperature, or conduit fill reduce ampacity. Follow the NEC edition your area adopts and your local code, and use a licensed electrician. All math runs locally on your device.
What this does
A wire size calculator finds the gauge that can safely carry a circuit's current and keeps voltage drop within limit, from the current, distance, voltage, conductor metal, and phase.
How to use it
- Enter the current and voltage.
- Enter the one-way distance.
- Choose copper or aluminum.
- Read the recommended gauge.
How it works
The tool finds two sizes: the smallest wire rated for the load current (NEC Table 310.16, 60 °C column up to 100 A, 75 °C above, with the 15/20/30 A limits on 14/12/10 AWG copper), and the smallest wire that keeps voltage drop within your limit, commonly 3% on a branch circuit. It recommends the larger of the two.
Vdrop = 2 × I × R × distance
Understanding your result
The recommended gauge carries the load and meets your drop limit; the result says which of the two set the size. Ampacity usually decides short, heavy runs and voltage drop decides long ones. It is a planning guide: continuous loads need 125% sizing, bundling and heat reduce ampacity, and the breaker rating and your local electrical code have the final say on what is legal and safe.
Example
20 A over 100 ft of copper at 120 V needs 8 AWG to stay within 3% drop, while 50 A over 10 ft at 240 V needs 6 AWG for ampacity.
Sources & methodology
- NFPA 70, National Electrical Code (2017), Table 310.15(B)(16) ampacity (Table 310.16 from 2020), 240.4(D) small conductors, 110.14(C) terminations, Chapter 9 Table 8 conductor properties
- Cerrowire, Ampacity Chart, Manufacturer reproduction of the NEC ampacity table
Last updated .
Frequently asked questions
How much voltage drop is acceptable?
A common guideline is 3% on a branch circuit and up to 5% total including the feeder. The tool steps up the gauge until your limit is met.
Copper or aluminum?
Aluminum has higher resistance, so it needs a larger gauge for the same drop. The tool uses the right resistivity for each.
Does it check ampacity?
Yes. It checks the NEC Table 310.16 ampacity for the load (60 °C column up to 100 A, 75 °C above) and the small-conductor limits, then recommends the larger of the ampacity size and the voltage-drop size. Adjust for continuous loads, bundling, and heat, and follow your local code.
Do you store my numbers?
No. Everything is calculated locally in your browser.
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