Wire Size Calculator
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Minimum wire size
- Governing constraint
- voltage drop
- Minimum by ampacity
- 12 AWG (20 A rated)
- Minimum by voltage drop
- 8 AWG
- Design current
- 20 A
- Voltage drop at that size
- 3.06 V (2.55%)
- Distance where drop takes over
- 47 ft at 12 AWG
- Voltage drop governs here, not ampacity. 12 AWG carries 20 A legally, but over 100 ft it drops 6.43% — past your 3% limit. You need 8 AWG, which is 2 sizes larger. Beyond about 47 ft one way, distance decides the wire size rather than the current.
- Ampacity and voltage drop are independent. Ampacity is about the conductor not overheating and ignores distance entirely; voltage drop is about the load receiving usable voltage and is proportional to distance. The required conductor is whichever constraint demands more, and only one of the two is enforceable code.
- Table values here assume 75°C terminations, no more than three current-carrying conductors in a raceway, and a 30°C ambient. Bundling, high ambient temperature, conduit fill and termination ratings all derate ampacity, and the breaker must be sized for the conductor. This is a planning tool, not a substitute for the code book or an electrician.
Both constraints, every gauge
20 A over 100 ft one way on 120 V. A gauge must pass both columns to be usable.
| Gauge | Rated | Ampacity | Drop | Drop limit |
|---|---|---|---|---|
| 14 AWG | 15 A | Too small | 10.23% | Over |
| 12 AWG | 20 A | Pass | 6.43% | Over |
| 10 AWG | 30 A | Pass | 4.03% | Over |
| 8 AWG (required) | 50 A | Pass | 2.55% | Pass |
| 6 AWG | 65 A | Pass | 1.64% | Pass |
| 4 AWG | 85 A | Pass | 1.03% | Pass |
| 2 AWG | 115 A | Pass | 0.65% | Pass |
| 1/0 AWG | 150 A | Pass | 0.41% | Pass |
| 2/0 AWG | 175 A | Pass | 0.32% | Pass |
| 4/0 AWG | 230 A | Pass | 0.2% | Pass |
How the answer changes with distance
At short runs ampacity governs; past the crossover, voltage drop does. This is the whole point of checking both.
| One-way run | Minimum size | Governed by |
|---|---|---|
| 25 ft | 12 AWG | Ampacity |
| 50 ft | 10 AWG | Voltage drop |
| 100 ft (yours) | 8 AWG | Voltage drop |
| 150 ft | 6 AWG | Voltage drop |
| 250 ft | 4 AWG | Voltage drop |
| 400 ft | 2 AWG | Voltage drop |
How do you choose the right wire size?
Size the conductor for both ampacity and voltage drop, then use whichever is larger. A 20 amp load needs 12 AWG copper on ampacity, but over a 100 foot run the drop reaches 6.43%, so 8 AWG is required instead.
ampacity: rated amps ≥ load × (1.25 if continuous) drop: (2 × L × R × I) ÷ 1,000 ÷ V ≤ limit- rated amps
- NEC Table 310.16, with the 240.4(D) limits on 14, 12 and 10 AWG
- R
- Ohms per 1,000 ft from NEC Chapter 9, Table 8
- L
- One-way distance; doubled for single-phase, × √3 for three-phase
- 1.25
- Required for a load running three hours or more
Two independent constraints decide a wire size, and the common mistake is checking only the first. Ampacity is about the conductor not overheating: it depends on the current, the insulation temperature rating and the installation conditions, and it ignores distance completely. Voltage drop is about the load actually receiving usable voltage, and it is directly proportional to distance. The required conductor is whichever constraint demands more, which means the answer changes with the length of the run even though the current has not. On a 25 foot run of a 20 amp circuit, 12 AWG copper satisfies both. Push the same circuit out to 100 feet and 12 AWG still satisfies ampacity — it is entirely legal — while the drop reaches 6.43%, more than double the 3% recommendation, so the run needs 8 AWG. That is two sizes larger, driven by distance alone. The asymmetry worth remembering is that ampacity is enforceable code and voltage drop is an informational note, so an inspector will pass an undersized-for-distance conductor that performs badly. Dim lights, motors running hot and energy wasted as heat in the wall are the result.
- Multiply the load by 1.25 if it runs three hours or more.
- Find the smallest gauge whose ampacity meets that design current.
- Calculate the voltage drop for that gauge over your run length.
- If the drop exceeds your limit, step up until it passes.
- Use whichever of the two requirements is the larger conductor.
Worked example: A 20 A load 100 ft from the panel on 120 V single-phase copper, 3% drop limit
A 20 A load 100 ft from the panel on 120 V single-phase copper, 3% drop limit
On ampacity alone the answer is 12 AWG: NEC 240.4(D) limits 12 AWG copper to 20 A, which exactly matches the load, and distance plays no part in that judgement. Voltage drop tells a different story. Table 8 gives 12 AWG a resistance of 1.93 ohms per 1,000 ft, and the current travels out and back, so the drop is (2 × 100 × 1.93 × 20) ÷ 1,000 = 7.72 V — 6.43% of 120 V, more than double the 3% limit. Stepping up, 10 AWG gives 4.03% and still fails; 8 AWG gives 2.55% and passes. So the run needs 8 AWG, two sizes above what ampacity requires, and voltage drop is the governing constraint. The crossover is the useful figure to carry away: 12 AWG stays inside 3% out to about 47 ft one way, so below that length ampacity governs and above it distance does. At 25 ft this same circuit would need only 12 AWG.
What each wire size input means
- Load current
- The current the circuit will actually carry.
- One-way distance
- Panel to load, measured one way. This is what separates the two constraints — ampacity ignores distance entirely, voltage drop is proportional to it.
- System voltage
- 120 V or 240 V for most residential circuits.
- Conductor material
- Aluminium carries less current and has more resistance at the same gauge.
- Continuous load
- A load expected to run for three hours or more. The NEC requires the conductor and breaker to be sized at 125% of it.
- Maximum voltage drop
- The NEC suggests 3% on a branch circuit and 5% total as informational guidance, not as a requirement.
- Phase
- Single-phase doubles the distance; three-phase multiplies by √3.
What this wire size does not account for
- Ampacity and voltage drop are separate constraints and the larger one governs. Checking only ampacity gives an undersized conductor on any long run.
- Ampacity is enforceable code; the 3% and 5% voltage-drop figures are informational notes. An inspector can pass a conductor that performs poorly.
- NEC 240.4(D) limits 14, 12 and 10 AWG to 15, 20 and 30 A regardless of their insulation rating. These small-conductor limits override the table.
- Continuous loads running three hours or more must be sized at 125% for ampacity, but voltage drop uses the actual current, since that is what flows.
- Table values assume 75°C terminations, no more than three current-carrying conductors in a raceway and a 30°C ambient. Bundling, heat and conduit fill all derate ampacity.
- The breaker must match the conductor. Upsizing wire for voltage drop does not allow a larger breaker unless the whole circuit is designed for it.
- Aluminium carries less current and has more resistance at the same gauge, so it needs roughly two sizes larger, plus listed terminations and anti-oxidant compound.
- This is a planning tool. Final conductor selection belongs with the code book and a licensed electrician, particularly for services, feeders and motor circuits.
Frequently asked questions
What size wire do I need for 20 amps?
12 AWG copper on ampacity grounds, but only on a short run. Over 100 feet the voltage drop reaches 6.43%, so 8 AWG is needed. The crossover is around 47 feet.
Does wire size depend on distance?
Ampacity does not, but voltage drop does and is proportional to length. So the required conductor grows with distance even though the current has not changed.
What size wire for a 50 amp circuit?
8 AWG copper meets the 50 A ampacity rating. Over a long run voltage drop may require 6 or 4 AWG instead, so the distance has to be checked separately.
Can I use a bigger breaker if I use bigger wire?
No, not on its own. The breaker protects the circuit and must suit the whole design, including the load and the terminations. Upsizing wire for voltage drop does not license a larger breaker.
Is aluminium wire acceptable?
Yes, with listed terminations and anti-oxidant compound, and it is common for larger feeders on cost grounds. It needs roughly two sizes larger than copper for the same duty.
What is the 125% rule for continuous loads?
A load running three hours or more must have its conductor and breaker sized at 125% of it. A 16 A continuous load is treated as 20 A for ampacity purposes.
Sources and review
- NFPA 70, National Electrical Code — Table 310.16 and Chapter 9, Table 8 — National Fire Protection Association
Last reviewed .