Skip to content
Calcapedia
Menu
Trades

Wire Size Calculator

Get a planning estimate of the copper or aluminum AWG size for your amps, volts, distance and allowed voltage drop.

Level: Grades 9–10

Printable worksheet

Wire Size Calculator

Inputs
Conductor
Circuit type

From the source to the load, one way.

3% is a common planning target, not a legal limit.

Result

Suggested size (planning estimate)

8 AWG copper

For 30 A over 150 ft at 240 V, the planning estimate is 8 AWG copper. Voltage drop sets the size here. 10 AWG could carry the current, but it would lose 3.75%.

Smallest size for the current alone
10 AWG
Voltage drop
2.36%
Voltage drop in volts
5.65 V
Voltage at the load
234.3 V
Table limit used for this size
50 A

See the step-by-step solution ↓

copper sizes for this run (75 °C column, 30 °C air, up to 3 conductors)
copper sizes for this run (75 °C column, 30 °C air, up to 3 conductors)
AWGTable ampacity (A)Limit used (A)Drop
1420159.47%
1225205.96%
1035303.75%
850502.36%
665651.48%
485850.93%
31001000.74%
21151150.59%
11301300.46%
1/01501500.37%
2/01751750.29%
3/02002000.23%
4/02302300.18%

Step-by-step solution

  1. Find the smallest size the table allows for 30 A, using the 75 °C column and the limits for small wires

    sized for current=10 AWG

  2. Work out the resistance of 8 AWG wire over the run. Current travels out and back, so double the one-way length

    2×45.72×0.002061=0.1884 Ω

  3. Multiply by the current to get the voltage lost in the wire

    0.1884×30=5.653 V

  4. Compare that loss with your source voltage to see the percent drop

    5.653240=2.36%

  5. Voltage drop sets the size here. 10 AWG could carry the current, but it would lose 3.75%.

    planning estimate=8 AWG copper

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Get a first estimate of what size wire a job needs. Enter the current, the voltage, the distance and how much voltage you can afford to lose. You'll get a suggested copper or aluminum size in AWG (American Wire Gauge).

This is a planning estimate, not an approved design. It doesn't show that a job meets any code. Local code and a licensed electrician decide what actually gets installed.

Where it's used

Why this math matters outside the classroom:

  • Detached garage planning. A homeowner planning a 30 A, 240 V feed over 150 feet sees an estimate of 8 AWG copper before calling an electrician.
  • RV and solar runs. A DIY planner sees how a 12 V circuit needs much larger wire than a 240 V one over the same distance.
  • Material pricing. An estimator compares copper and aluminum sizes for a 100-foot feeder before asking an electrician for a quote.
  • Physics class. A student sees how doubling a run's length doubles the voltage drop, from 1.2% to 2.4%.

How to use it

  1. Choose copper or aluminum and the type of circuit.
  2. Enter the load current and the source voltage.
  3. Enter the one-way distance from the source to the load.
  4. Enter the voltage drop you'll allow, then read the suggested size and the table.

What the inputs mean

Load current
Use the full load. Loads that run for hours may need extra margin, so ask your electrician how to treat them.
One-way distance
Enter the one-way distance. The calculator accounts for the return path itself.
Allowed voltage drop
Voltage drop is the voltage lost along the wire. Too much can make motors run hot and lights run dim. A target of 3% is a common rule of thumb.

Formula and method

DC and single-phase drop = 2 × L × R × I
three-phase drop = √3 × L × R × I
drop % = drop ÷ V × 100
size = smallest wire that passes both checks

L is the one-way length, R is the wire's resistance per unit of length, I is the current and V is the source voltage. Thicker wire has lower resistance, so it loses less voltage.

The calculator runs two checks. First, the wire must carry the current without overheating. For that, it uses the 75 °C column of NEC Table 310.16, which is the US electrical code's main ampacity table. Ampacity means how many amps a wire can safely carry.

That column assumes no more than three current-carrying wires together and air at 30 °C (86 °F). Sizes 14, 12 and 10 AWG also have lower limits for circuit protection, and the calculator uses those. Second, the voltage drop must stay within your limit.

Worked example

You need to feed a 30 A load at 240 V, single-phase, 150 feet away, with copper wire and a 3% limit.

  1. The table limits point to 10 AWG for 30 A.
  2. At 150 feet, 10 AWG would lose 3.75%, which is over your limit.
  3. The next size up, 8 AWG, loses 2.36%, which is within your limit.

Result: The planning estimate is 8 AWG copper. Local code and a licensed electrician decide the final size.

Understanding the result

If the size is larger than the one needed for current alone, voltage drop is the reason. That's common on long runs and at low voltages such as 12 V.

The table under the result shows every size for your run. That helps when you're comparing cost against a lower drop.

Assumptions and limitations

  • Ampacity comes from the 75 °C column of NEC Table 310.16, as reproduced in published charts. We did not read the NFPA text itself.
  • Resistance is the DC value of a solid round wire at 20 °C. Hot or stranded wire resists a little more, so real drop can be higher.
  • Inductance and power factor are left out of AC circuits.
  • This is never a statement that a design meets code. Local rules and code editions differ.
  • It doesn't adjust for hot locations, bundled wires, insulation type, terminal ratings, conduit fill or long-running loads. Any of these can require a larger wire.
  • It doesn't choose breakers, and it isn't for motors, welders or air conditioners, which have their own rules.
  • Electrical work can be dangerous. Have a licensed electrician design and inspect it.

Frequently asked questions

Is the size here code-compliant?

We can't say that, and you shouldn't assume it. It's a planning estimate. Local code and a licensed electrician decide.

Why does 14 AWG show 20 A but use 15 A?

The table lists 20 A in this column, but a separate rule for small wires limits circuit protection. So the calculator plans with the lower 15 A figure.

Why is aluminum larger than copper?

Aluminum resists current more than copper does. So it needs a thicker wire to carry the same current.

Which distance do I enter?

Enter the one-way distance. For a load 150 feet away, enter 150 and not 300.

Sources

How this calculator is checked

Method: NEC Table 310.16 75 °C values as reproduced by Turn2Engineering (cites NFPA 70 2026) and Border States (1996 table), which agree for sizes 12 AWG to 4/0; 14 AWG from Turn2Engineering. Small-conductor limits from an NEC 240.4(D) study guide. The NFPA text was not read. Drop model is shared with voltage-drop. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-31. Found a problem? Tell us.

Embed this calculator

Put this Wire Size Calculator on your own website or blog for free. Paste this code where you want it to appear: