Electrical
Wire Size Calculator
Compare the conductor suggested by a simplified ampacity table with the larger size a chosen voltage-drop target may require.
Result
Electrical work is life-safety work governed by the NEC or your local code, and must be designed and installed by a licensed electrician. Conductor and overcurrent sizing depends on ambient temperature, conductor bundling, termination ratings, continuous-load rules and more. This is an educational estimate and must never be used as the basis for an installation. This uses a simplified 75°C conductor table with small-conductor protection limits, not a free-air rating. Real installations require derating for ambient temperature, conduit fill and bundling, and terminations are often limited to 60°C on smaller conductors. All of which can require a larger conductor than shown.
Get a Free Business AuditWhat moves the number
- The current and duration selection establish the planning load.
- Long routes can make voltage drop govern before table ampacity does.
- The maximum-drop input lets you examine a project-specific target.
Where this one goes wrong
The table-based result cannot account for every cable construction, termination temperature, ambient condition or grouping arrangement. An apparently adequate conductor can still fail the installation requirements. The largest size available in a short lookup table is not proof that a high load is supported.
Questions
About this calculator
What does the ampacity-only result help with?
It shows whether the route-length constraint is causing an upsize. That helps distinguish a thermal sizing issue from a voltage-performance issue.
Does selecting aluminium preserve the same gauge?
No equivalence should be assumed. Material changes the table and resistance inputs, and compatible terminations remain a separate design requirement.
Can I order cable from this recommendation?
Use it to prepare a planning comparison. Final conductor selection is code-governed and must be checked against the installation conditions and manufacturer ratings.
Put this calculator on your site
Free to embed. Paste this where you want it to appear. It stays up to date automatically because it loads from BDEVY.
Please keep the attribution line. It is the only thing we ask in return.
How to use this calculator
- Load current (A). Enter running load current. The duration selection below applies the model's allowance.
- Load type. Enter your own figure; the page starts at Continuous (3+ hours).
- Conductor material. Enter your own figure; the page starts at Copper.
- One-way run length (ft). One-way installed route length, including rises and detours. Do not double it.
- System voltage (V). Enter your own figure; the page starts at 240.
- Maximum voltage drop (%). Your planning target for this run. Check the complete supply path and project requirements separately.
The result updates as you type, so you can try a figure, see what it does to recommended conductor, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Load current (A) | 40 |
| Load type | Continuous (3+ hours) |
| Conductor material | Copper |
| One-way run length (ft) | 80 |
| System voltage (V) | 240 |
| Maximum voltage drop (%) | 3 |
What comes out
| Result | Value |
|---|---|
| Recommended conductor main result | 8 AWG copper |
| Design current | 50.0 |
| Size by ampacity alone | 8 AWG |
| Voltage drop at the recommended size | 2.08% |
Planning selection: 8 AWG copper for 40 A over 80 ft. The ampacity-table selection is 8 AWG; the selected conductor's estimated drop is 2.08%. Verify installation adjustments and equipment limits before specifying it.
How sensitive the answer is
Holding everything else steady and changing only load current (a), this is what happens to recommended conductor. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Load current (A) | Recommended conductor |
|---|---|
| 20 | 10 AWG copper |
| 30 | 8 AWG copper |
| 40 example above | 8 AWG copper |
| 60 | 4 AWG copper |
| 80 | 3 AWG copper |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Load current (A)
- Enter running load current. The duration selection below applies the model's allowance.
- Load type
- Choose from Non-continuous, Continuous (3+ hours).
- Conductor material
- Choose from Copper, Aluminium.
- One-way run length (ft)
- One-way installed route length, including rises and detours. Do not double it.
- System voltage (V)
- A number, between 0 and 100000, starting at 240.
- Maximum voltage drop (%)
- Your planning target for this run. Check the complete supply path and project requirements separately.
What the results tell you
| Result | What it is |
|---|---|
| Recommended conductor | First supported conductor meeting this model's current and voltage-drop checks. Installation conditions still need verification. |
| Design current | Entered running current with the selected duration allowance applied. |
| Size by ampacity alone | First table entry meeting design current and the small-conductor protection limits used here. |
| Voltage drop at the recommended size | Estimated voltage lost as a share of supply voltage at the final planning selection. |
Before you rely on this number
Conductor sizing, voltage drop and load figures are governed by code, and the code takes precedence over any calculator. Ambient temperature, conduit fill, continuous load factors and the specific installation all change the answer. Treat this as a first pass, not a design.
Keep going
Related calculators
Voltage Drop Calculator
Compare conductor voltage loss across single-phase and three-phase runs using the actual route length and expected load.
Open calculatorBreaker Size Calculator
Compare a watts-based load estimate with a standard breaker lookup while separating operating current from the tool's design allowance.
Open calculatorElectrical Load Calculator
Build an early dwelling-load estimate from floor area and major appliances, then identify what a formal service calculation still needs.
Open calculatorEstimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.