Calculator
Electrical Wire Size Calculator
Size branch circuits and feeders per NEC — enter load amperage, voltage, run length, insulation rating, ambient temperature, and bundling to get recommended AWG/kcmil for both copper and aluminum, with an ampacity and voltage drop breakdown. Free, no signup.
Free
Common circuitsoptional
Pre-fills the load, voltage, run length, and voltage-drop fields below for a typical circuit — EV charger, range, subpanel feeder, and more. Adjust any field afterward.
Installation & derating
Motor sizing (HP)
Enter horsepower instead of amps — FLA is calculated automatically.
EV 32 A
Recommended wire size
Copper
8 AWG
1.9% voltage drop
Aluminum
6 AWG
2.0% voltage drop
Meets ampacity and voltage drop
Recommended breaker40 A
Sizing breakdown
Design current
40.0 AIncludes 125% factor
Adjusted ampacity (Cu)
50 A100% ambient · 100% bundling
Voltage drop (Cu)
4.67 VTarget ≤ 3% (7.2 V)
Governing constraint
AmpacityBoth constraints satisfied at same size
Equipment ground
10 AWG CuCopper EGC per NEC Table 250.122
NEC notes
- 125% multiplier applied to load current per NEC 210.19(A) / 430.22.
NEC ampacity reference
| AWG | 75°C Cu | Use |
|---|---|---|
| 14 | 20 A | 15 A branch |
| 12 | 25 A | 20 A branch |
| 10 | 35 A | 30 A circuit |
| 8 | 50 A | 40–50 A / EV 32 A |
| 6 | 65 A | 60 A / EV 48 A |
| 4 | 85 A | Large branch |
| 2 | 115 A | 100 A subpanel |
| 1/0 | 150 A | 125–150 A feeder |
| 3/0 | 200 A | 200 A service (Cu) |
| 250 kcmil | 255 A | 200–250 A service |
What you'll get
Enter load and circuit details
Set amperage or motor HP, voltage, phase, run length, and maximum voltage drop — or use an optional quick-start preset for common circuits like EV chargers, ranges, or subpanel feeders.
Set installation conditions
Set insulation and termination temperature ratings, ambient temperature, and number of current-carrying conductors for NEC derating.
Review copper and aluminum sizes
Get the governing AWG/kcmil size for both copper and aluminum from ampacity and voltage drop constraints, adjusted ampacity after derating, recommended OCPD, and copper EGC per Table 250.122.
How it works
- 1
Enter load and circuit details
Set amperage or motor HP, voltage, phase, run length, and maximum voltage drop — or use an optional quick-start preset for common circuits like EV chargers, ranges, or subpanel feeders.
- 2
Set installation conditions
Set insulation and termination temperature ratings, ambient temperature, and number of current-carrying conductors for NEC derating.
- 3
Review copper and aluminum sizes
Get the governing AWG/kcmil size for both copper and aluminum from ampacity and voltage drop constraints, adjusted ampacity after derating, recommended OCPD, and copper EGC per Table 250.122.
Why electricians size conductors for both ampacity and voltage drop
Wire sizing is a dual-constraint problem. The conductor must carry the load current without exceeding its ampacity after temperature and bundling derating — otherwise it overheats. It must also deliver acceptable voltage at the load end over the run length — otherwise motors fail to start, lights dim, and equipment underperforms. NEC Table 310.16 provides base ampacity; NEC 310.15(B) and (C) reduce it for hot ambient and crowded raceways; NEC 210.19(A) Informational Note recommends 3% voltage drop for branch circuits.
This calculator implements the full screening procedure used in the field: 125% multiplier for continuous and motor loads, Table 310.16 ampacity lookup at your insulation and termination ratings, ambient and bundling correction factors, voltage drop using NEC Chapter 9 Table 8 resistance values, NEC 240.4(D) small-conductor OCPD limits, and equipment grounding conductor sizing from Table 250.122. Use it to quote jobs, verify plans, and pull the right wire before you open the conduit.
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View statesWire Size Calculator FAQs
What inputs does a wire size calculator need?
At minimum: load current (amps), system voltage, one-way run length in feet, and insulation temperature rating. This calculator sizes copper and aluminum side by side, so you do not pick a conductor material. Industry tools also ask for phase (single or three), ambient temperature, number of current-carrying conductors in the raceway, continuous load status, and maximum allowable voltage drop percentage.
What NEC tables does this calculator use?
NEC Table 310.16 for base ampacity, Table 310.15(B)(1) for ambient temperature correction, Table 310.15(C)(1) for bundling adjustment when more than three current-carrying conductors share a raceway, Chapter 9 Table 8 for voltage drop resistance, NEC 240.4(D) for small-conductor OCPD limits, and Table 250.122 for equipment grounding conductor sizing.
When does voltage drop govern wire size instead of ampacity?
On long runs — typically over 75–100 feet at 120 V or 150+ feet at 240 V — voltage drop often requires a larger conductor than ampacity alone. The calculator compares both constraints and recommends the larger size. EV charger circuits, detached garage feeders, and rooftop equipment are common cases where voltage drop governs.
What is the 125% continuous load factor?
NEC 210.19(A) requires conductors for continuous loads — expected to run three hours or more — to be sized at 125% of the load current. Motor branch circuits use the same 125% factor per NEC 430.22. A 32 A EV charger on a continuous load requires conductors sized for 40 A.
