Main Wire Sizing Inputs
Example Data Table
| Scenario | Load | Voltage | Distance | Material | Common target |
|---|---|---|---|---|---|
| Small residence feeder | 100 A | 240 V | 60 ft | Copper | 3% drop or less |
| Detached garage main | 60 A | 240 V | 120 ft | Aluminum | Upsize for distance |
| Shop service panel | 150 A | 240 V | 90 ft | Copper | Breaker and derating check |
| Three-phase equipment main | 50 kW | 480 V | 180 ft | Copper | Power factor included |
Formula Used
Single-phase current: Amps = Watts ÷ (Volts × Power Factor)
Three-phase current: Amps = Watts ÷ (√3 × Volts × Power Factor)
Design current: Operating Amps × Continuous Factor × Service Factor × Reserve Factor
Derated ampacity: Table Ampacity × Parallel Runs × Ambient Factor × Conductor Count Factor × Custom Factor
Single-phase voltage drop: Drop = 2 × Amps × Resistance × One-way Feet ÷ 1000
Three-phase voltage drop: Drop = √3 × Amps × Resistance × One-way Feet ÷ 1000
The ampacity table is a planning reference. Local rules, terminal ratings, conductor insulation, raceway fill, grounding, neutral loading, and equipment labels can change final sizing.
How to Use This Calculator
- Enter the load in watts, kilowatts, or amps.
- Select single-phase, three-phase, or DC wiring.
- Add voltage, power factor, and one-way distance.
- Choose copper or aluminum and the temperature column.
- Set voltage drop, derating, reserve, and breaker values.
- Press the calculate button and review the result above the form.
- Download the CSV or PDF for project notes.
Main Wire Planning Guide
Why Main Wire Size Matters
Main conductors carry power into a panel, feeder, shop, garage, or equipment group. A weak estimate can cause heat, nuisance trips, wasted voltage, and expensive rework. This calculator helps you compare load current, breaker rating, derating, and voltage drop in one place. It is not a permit design, but it gives a strong planning start.
Load and Current
The first step is finding current. If you enter watts, the tool converts watts into amps. Single-phase loads use voltage and power factor. Three-phase loads also use the square root of three. If you already know the service amperage, choose amps and enter that value directly.
Continuous Loads and Reserve
Many main feeders serve loads that may run for long periods. The continuous load option applies a 125 percent factor. This adds margin before wire selection. The reserve field adds future capacity. It is useful for panels that may later serve chargers, tools, pumps, heating, or added circuits.
Derating and Installation Conditions
Wire does not always carry its table ampacity. Heat, crowded raceways, insulation limits, and custom site conditions can reduce safe capacity. The calculator multiplies the listed ampacity by derating factors. A lower factor usually forces a larger conductor. This is important for mains in conduit, hot rooms, attics, service spaces, and bundled runs.
Voltage Drop Review
Long runs lose voltage. The calculator estimates drop from current, conductor resistance, distance, and phase type. A wire may pass ampacity but fail the voltage drop target. In that case, the result moves to a larger conductor when available. Lower drop helps motors start better, lights stay stable, and equipment run cooler.
Final Safety Check
Use the result as a comparison guide. Always verify final conductor size, breaker size, grounding, neutral sizing, conduit fill, temperature rating, and local electrical rules before installation. Main wiring is safety critical work. A licensed electrician or qualified designer should approve the final selection.
FAQs
1. Can this calculator size a residential main feeder?
Yes, it can estimate a residential main feeder. Enter the load, breaker, voltage, distance, material, and derating values. Final sizing must still follow local code and inspection rules.
2. Should I choose copper or aluminum?
Copper usually carries more current in a smaller size. Aluminum is often cheaper for larger mains. Use terminals rated for the chosen material and follow torque requirements.
3. What voltage drop target should I use?
Many planners use 3 percent for feeders or important branch circuits. Long runs may need larger wire. Sensitive equipment may need a tighter target.
4. Why does distance change wire size?
Longer conductors have more resistance. More resistance creates more voltage drop. The calculator may choose a larger wire even when ampacity already passes.
5. What is derating?
Derating reduces usable ampacity for heat, conductor count, or site conditions. It helps prevent overheating when conductors cannot cool normally.
6. Does the breaker size affect wire size?
Yes. Conductors usually need ampacity suitable for the overcurrent device. If you enter a main breaker, the calculator uses it as a sizing basis.
7. Why apply a 125 percent continuous factor?
Continuous loads can heat conductors for long periods. The 125 percent factor adds required planning margin for many continuous load situations.
8. Is this a replacement for an electrician?
No. Main wiring needs code review, permits, grounding checks, and safe installation. Use this calculator for planning before professional approval.