Wire Size Calculator
AWG copper wire sizing by amp load and run length, factoring voltage drop.
- Minimum AWG (copper)
- 8
- Required circular mils
- 14,333
- Actual voltage drop
- 3.13 V (2.6%)
- Within target
- Yes
Ampacity 50 A
Two rules govern wire size
Every wire size decision balances two rules. Rule one: the wire must carry the amps without overheating. That is the ampacity table in NEC Chapter 3, and it is what most electricians think of first. For copper THHN in a 75°C rated terminal: 14 AWG carries 15 A, 12 AWG carries 20 A, 10 AWG carries 30 A, 8 AWG carries 50 A, and so on up.
Rule two: the wire must not drop more voltage than the load can tolerate. Voltage drop is not enforced by code the way ampacity is (NEC 210.19 informational note only), but 3 percent drop on branch circuits and 5 percent total on feeders is the accepted design target. On long runs, voltage drop drives the size up above what ampacity alone would require.
The voltage drop formula
Voltage drop for single-phase circuits: Vd = (2 x K x I x L) / CM, where K is 12.9 for copper (21.2 for aluminum), I is amps, L is one-way distance in feet, and CM is circular mils of the conductor.
Rearranging to solve for wire size: CM = (2 x K x I x L) / Vd. For a 20 A load at 120 V with 3 percent allowable drop over 100 feet: CM = (2 x 12.9 x 20 x 100) / 3.6 = 14,333 circular mils. That falls between 12 AWG (6,530 CM) and 10 AWG (10,380 CM), so the next size up is 8 AWG (16,510 CM).
This is why a 20 amp circuit at the house main panel can be run in 12 AWG, but the same 20 amps at a shop 150 feet down the property requires 10 AWG or 8 AWG. Ampacity is fine; voltage drop is not.
Copper vs aluminum
Copper is the default for branch circuits and small feeders. Aluminum saves cost on large feeders (typically 2 AWG and larger) and is common for service entrance and large sub-panels. Aluminum has 61 percent the conductivity of copper, so aluminum requires larger wire for the same ampacity.
The rough conversion: aluminum is two sizes larger than copper for the same ampacity. Where you would use 4 AWG copper, use 2 AWG aluminum. Aluminum terminations require anti-oxidant compound and must be torqued to specification, or the joint will fail from thermal cycling.
Common shop and outbuilding runs
50 A sub-panel to a detached shop 100 feet from the main panel at 240 V: voltage drop is the driver. Required CM at 3 percent: (2 x 12.9 x 50 x 100) / 7.2 = 17,917 CM. 8 AWG copper (16,510 CM) is under; step to 6 AWG copper (26,240 CM) with an ampacity of 65 A.
60 A sub-panel to a shop 200 feet away at 240 V: (2 x 12.9 x 60 x 200) / 7.2 = 43,000 CM. That is between 4 AWG (41,740) and 3 AWG (52,620). Use 3 AWG copper or 1 AWG aluminum.
Long agricultural runs often justify going to 240 V or 480 V just to shrink the wire cost. Doubling the voltage cuts the current in half at the same load, which quarters the voltage drop for a given wire size.
For electrical contractors
Wire sizing questions are search-driven and pre-qualify a customer who already has a project in mind. Peak Local Leads builds and ranks local electrical sites for one contractor per Texas market. Free week of calls, flat monthly after.
Frequently asked.
- What size wire for 200 amp service?
- The standard sizing is 4/0 AWG copper or 250 kcmil aluminum for a 200 A residential service at typical service drop lengths. Always verify against the local utility spec.
- What size wire for 20 amps at 100 feet?
- 12 AWG copper handles 20 A ampacity but drops more than 3 percent at 100 feet. Upsize to 10 AWG copper for a comfortable 1.9 percent drop.
- How do I calculate voltage drop?
- Single phase: Vd = (2 x K x I x L) / CM. K = 12.9 for copper. I is amps, L is one-way distance in feet, CM is circular mils.
- Is voltage drop required by NEC?
- It is not a strict code requirement; NEC 210.19 and 215.2 include voltage drop only as an informational note recommending 3 percent on branch circuits and 5 percent total. It is a design best practice, not an inspection failure.
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