Voltkit US · NEC 2023
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NEC 2023NEC 310.16 · 110.14(C)

Wire Size Calculator

Enter the load, voltage and one-way distance. The calculator finds the smallest conductor that carries the current and keeps voltage drop within your limit, then tells you which rule set the size.

Minimum conductor VD LIMITED
6AWG
By ampacity
8 AWG
By volt-drop
6 AWG
Drop
2.5%

NEC 110.14(C) · 310.16 · 240.4(D) · NEC 2023

Values checked against the printed NEC 2023 tables, using the same engine as the Voltkit app.

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  1. By ampacity (110.14(C))min(derated @ 90°C, 75°C column) ≥ 40 A8 AWG
  2. Usable ampacityat 8 AWG50 A
  3. By voltage drop≤ 3% over 150 ft6 AWG
  4. Governing (larger)voltage drop wins6 AWG
  5. Drop at governing size1-Phase, Copper2.46%

NEC 110.14(C) · 310.16 · 310.15(B)(1) & (C)(1) · 210.19(A) IN

Load & run

Phase
Voltage drop limit
Dwelling service or main feeder (310.12)
Continuous load (3 h or more)

Conductor

Material

Conditions

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How the wire size is chosen

Two checks run side by side and the larger wire wins. Examples 1 and 2 below are the same load at two distances, and the distance alone flips which check governs.

1. Ampacity

  • Start from the conductor's ampacity in Table 310.16 at its insulation rating.
  • Correct it for ambient temperature (Table 310.15(B)(1)) and for more than three current-carrying conductors together (Table 310.15(C)(1)).
  • Cap it at the termination column, per 110.14(C). Continuous loads must fit 125% of the load in that column.
  • Skip 14, 12 and 10 AWG when 240.4(D) wouldn't let them sit on a breaker big enough for the load.
  • For a dwelling service or main feeder with the 310.12 option on, the target is 83% of the rating instead of the load.

2. Voltage drop

The smallest size whose drop over the one-way length stays within your limit (3% for a branch circuit or 5% for the feeder and branch together, per the informational notes in 210.19(A) and 215.2(A)), using the approximate-K method from the Voltage Drop calculator.

Worked examples

All single-phase, 90 °C insulation on 75 °C terminals, 30 °C ambient.

  1. 40 A at 240 V, 150 ft, copper. Ampacity alone needs 8 AWG; keeping drop under 3% needs 6 AWG. Result: 6 AWG (voltage drop governs), dropping 2.46%.
  2. The same 40 A load only 40 ft away. Now 8 AWG (ampacity governs), at 1.04% drop.
  3. 100 A at 240 V, 250 ft, aluminum. Ampacity needs 1 AWG aluminum, voltage drop needs 3/0 AWG, so 3/0 AWG at 2.63%.

More distances, in copper and aluminum: What size wire for a 100 amp service 200 ft away?

Upsized the hots for voltage drop? On a feeder or branch circuit, 250.122(B) makes the equipment grounding conductor grow in proportion; the Grounding calculator works it out. Service conductors carry no EGC.

This page cites the code but doesn't reproduce NEC text or tables. Use your copy of the NEC 2023 for the exact wording.

Worked examples on this page are computed by the same engine as the Voltkit app. The page cites NEC 2023 sections but doesn't reproduce NFPA text or tables; use your copy of the code for the exact wording.

Worked examples

Frequently asked questions

Why does voltage drop sometimes pick a bigger wire than ampacity?

Ampacity is about heat: a conductor must carry the current without exceeding its temperature rating. Voltage drop is about distance: on a long run, a conductor that is cool enough can still lose too much voltage. The calculator finds the smallest size for each and uses the larger one, marked VD LIMITED when voltage drop governs.

What does the termination rating do?

NEC 110.14(C) caps the ampacity you can use at the temperature rating of the terminals. For circuits of 100 A or less, or 14 through 1 AWG, 110.14(C)(1)(a) uses the 60 °C column unless the equipment is listed and marked for 75 °C. Many breakers and lugs are marked 60/75 °C, but read the label rather than assume it. The conductor's 90 °C rating is still useful: you derate for heat or bundling from the 90 °C column, then check the result against the terminal column.

Why does a 25 A load get 10 AWG when 12 AWG copper is rated 25 A?

The small-conductor rule. NEC 240.4(D) caps the overcurrent device on small copper at 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG (15 A and 25 A for 12 and 10 AWG aluminum), whatever Table 310.16 says. 12 AWG copper has 25 A in the 75 °C column, but a 25 A load needs at least a 25 A breaker, which 12 AWG can't have, so the answer is 10 AWG. The calculator applies it (and says so under the result); for a continuous load the breaker has to be 125% of the load, and the rule is checked against that.

Can I use this for a dwelling service or main feeder?

Yes. Turn on "Dwelling service or main feeder (310.12)" and enter the rating as the load. For a 120/240 V single-phase service of 100 through 400 A, or a feeder carrying the entire load of a dwelling unit, 310.12 lets the conductors have an ampacity of 83% of the rating: a 200 A service needs 166 A, which is 2/0 AWG copper or 4/0 AWG aluminum, instead of 3/0 AWG copper at 100%. Ambient and bundling corrections still apply, and voltage drop is checked at the full rating. A feeder to a detached garage or shop, or a subpanel serving part of the house, doesn't qualify; neither does a 208Y/120 V or three-phase service.

What is a continuous load?

A load at its maximum current for three hours or more (Article 100), such as store lighting. EV charging is treated as continuous by 625.42. The conductor must carry 125% of a continuous load (210.19(A)(1), 215.2(A)(1)). Turn on "Continuous load" and the calculator checks the termination column at 125%.

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