Current density
J = I / A
Current density divides branch current by total parallel conductor cross-sectional area.
Wire Size Calculator
Screen AWG conductor sizes against a maximum voltage-drop percentage and maximum current-density target. The calculation supports copper/aluminum, conductor temperature, parallel conductors, and DC or AC circuit models.
Access: Free to use, no installation, and No account required. Calculations run in your browser.
Design inputs
Important: current density is not code ampacity. This calculator is a preliminary electrical/thermal screening tool, not an NEC/CEC conductor-selection approval.
Recommendation
How the recommendation is selected
The engine checks 32 AWG through 4/0 AWG and returns the smallest conductor that simultaneously stays below your maximum voltage-drop percentage and maximum current-density value. Temperature-adjusted conductor resistance is included.
Why ampacity still has to be checked separately
A conductor can pass voltage-drop and current-density screens and still be unsuitable for the installation. Final selection depends on insulation temperature rating, raceway/cable configuration, ambient correction, conductor bundling, terminal temperature limits, continuous-load rules, overcurrent protection, fault duty, and the locally adopted electrical code.
Engineering reference
Wire Size Calculator: background and worked detail
Use the Wire Size Calculator to screen AWG conductors from 32 AWG through 4/0 against user-selected voltage-drop and current-density limits using the same conductor model as the Electrical Design Workbench.
Sizing a conductor by resistance and heat
The calculator evaluates each supported AWG size with the same voltage-drop engine, then selects the smallest listed conductor that passes both the maximum voltage-drop percentage and maximum current-density constraints.
Current density is useful as an engineering screening metric, but it is not a substitute for code ampacity tables. The selected gauge should be treated as a starting point for a full conductor and protection design.
Code ampacity is a separate decision
Assumptions
- The AWG geometric formula is used for conductor area.
- All parallel conductors are assumed identical and equally current-sharing.
- The chosen current-density limit is supplied by the user rather than inferred from an installation standard.
Limitations
- Not a code-compliant ampacity calculator.
- Does not evaluate voltage-drop limits mandated by a specific jurisdiction, conductor insulation, conduit fill, short-circuit withstand, or termination temperature ratings.
Equations used by this calculator
DC screening drop
ΔV = 2IR
For DC, each candidate conductor is checked using the temperature-adjusted loop resistance.
Selection rule
pass = (% drop ≤ limit) AND (J ≤ limit)
The first passing gauge in the supported AWG sequence is returned as the recommendation.
48 V, 40 A DC feeder
For a 5 m one-way copper feeder at 30 °C, set a 3% maximum voltage drop and 4 A/mm² maximum current density.
- Each AWG candidate is evaluated from 32 AWG toward larger conductors.
- The voltage-drop and current-density thresholds are applied independently.
- With the default assumptions, 7 AWG is the first size that passes both screening limits.
Result: The result is a preliminary 7 AWG screening recommendation; a real installation still needs code ampacity, protection, insulation, and termination checks.
Sanity cases
Relaxed limits
Case: Increase both limits substantially.
Expected: The recommended conductor should stay the same size or become smaller, never larger solely because the limits were relaxed.
Tighter limits
Case: Reduce either allowable drop or allowable current density.
Expected: The recommended conductor should stay the same size or become larger, or no supported gauge may pass.
Wire Size Calculator FAQ
Is the recommended wire size code compliant?
Not automatically. The result is an engineering screen based on voltage drop and current density; final sizing must use the applicable electrical code and installation conditions.
Why can voltage drop and current density select different gauges?
They constrain different physics. Voltage drop depends strongly on length and system voltage, while current density depends on current divided by conductor area.
Can I use parallel conductors?
Yes for the electrical calculation. Whether a real installation may parallel a given conductor size and how it must be installed is governed by the applicable code and equipment requirements.
Where this calculation comes from
The AWG geometry table and resistance model are shared with the voltage-drop route and the Electrical Design Workbench.
Open the source workbench →Read calculation and source methodology →